Chemistry 版 (精华区)
发信人: loafer (快乐的化身), 信区: Chemistry
标 题: Sci.chem FAQ - Part 5 of 7[转]
发信站: 哈工大紫丁香 (Tue Jul 1 10:01:01 2003)
发信人: nichloas (nil), 信区: Chemistry
标 题: Sci.chem FAQ - Part 5 of 7
发信站: 北大未名站 (2003年01月27日14:02:24 星期一), 转信
Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - MultiPage
Sci.chem FAQ - Part 5 of 7
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem,sci.answers,news.answers
Subject: Sci.chem FAQ - Part 5 of 7
Followup-To: sci.chem
Date: Sat, 27 Apr 2002 09:46:18 GMT
Organization: Industrial Research Limited
Expires: 27 May 2002 00:00:01 GMT
Message-ID: <3cca72d3.190558188@Newshost.comnet.co.nz>
Reply-To: B.Hamilton@irl.cri.nz
X-Trace: news.comnet.co.nz 1019901782 521 131.203.243.182 (27 Apr 2002 10:03:0
2 GMT)
X-Complaints-To: usenet@news.comnet.co.nz
NNTP-Posting-Date: 27 Apr 2002 10:03:02 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
Archive-name: sci/chem-faq/part5
Posting-Frequency: monthly
Last-modified: 22 October 1999
Version: 1.17
18.6 What is the most bitter compound?
Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
aminium benzoate [3734-33-6]. It is added to toxic chemicals ( such as
methylated spirits ) as a deterrent to accidental ingestion.
18.7 What is the sweetest compound?
Most scales use sucrose as a sweetness of 1, and compare the relative
sweetness of other sweeteners to sucrose.
Name Relative Sweetness Category
D-Glucose 0.46 Natural Food Product
Lactose 0.68 " " "
D-Fructose 0.84 " " "
Sucrose 1 " " "
Cyclamate 30 EC Permitted, USA Prohibited
Aspartame 200 EC, USA Permitted.
Saccharin 300 EC Permitted, USA Prohibited
Sucralose 650 Au, Ca Permitted, trials elsewhere
Alitame 2,000 Undergoing trials
Thaumatin 3,000 EC permitted, US chewing gum only.
Carrelame 160,000 Guanidine sweetener
Bernardame 200,000 " "
Sucrononate 200,000 " "
Lugduname 220,000 " "
The guanidine sweeteners are not expected to be approved for food use.
There are several other important attributes of sweeteners, such as
low toxicity, no after-taste, whether metabolised or excreted, etc.,
that must also be considered.
The potency scale is fairly flexible, and differing publications can
assign different values. The August 1995 copy of the Journal of Chemical
Education contained several papers from a symposium on sweeteners [3,4],
and an article in Chemistry and Industry also discusses sweeteners from
both natural and artificial sources [5], and Kirk Othmer has a monograph
on sweeteners.
The sweetener used in "diet" beverages is usually Aspartame, and they
are usually required to display a warning for phenylketurics that the
product contains a source of phenylalanine. As Aspartame slowly degrades
in acid solutions, such products also have a "use-by" date.
Although banned by the FDA in 1970 ( because a mixture of saccharin and
cyclamate caused tumours in test animals ), saccharin has been still
marketed under extensions of approval, Ironically, subsequent work
implicated the saccharin, and the cyclamate was found not to be the
tumour-causing agent, but it is still banned.
18.8 What salts change the colour of flames?.
Both Vogel ( qualitative inorganic ) and the Rubber Handbook list details of
flame tests for elements. The spectra of the alkaline earth compounds are
relatively complex, so using filters to view the flame can change the colour
observed as dominant lines are filtered out. In general, except for copper,
any compound of an element can be used, however toxic salts ( such as
cyanides ) should not be used. Halogen salts are usually readily available,
and are reasonably volatile. In all cases, perform experiments in a
well-ventilated area - preferably a fume hood. The emission spectra in the
visible region is the sum of several emission lines, with dominant lines
masking others. The visible spectrum is approximately :-
Red 800 - 620 nm
Orange 620 - 600 nm
Yellow 600 - 585 nm
Green 585 - 505 nm
Blue 505 - 445 nm
Violet 445 - 400 nm
There are also the various bead tests employing borax ( sodium tetraborate
Na2B4O7.10H2O ), Microcosmic salt ( NaNH4HPO4 ), or sodium carbonate
(Na2CO3), using both oxidising and reducing flames. The bead test procedures
are detailed in Vogel ( qualitative inorganic ), and similar texts.
Element Colour Some of the contributing lines, and comments.
Arsenic Light Blue 449.4 nm, 450.7 nm.
( Arsenic is highly toxic - only perform in fume hood under supervision )
Barium Green-Yellow 553.6 nm, 539.1 nm, 536.1nm, 614.2 nm.
Blue (faint) 455.4 nm, 493.4 nm.
Cesium Red-Violet 852.1 nm.
Calcium Orange 618.2 nm, 620.3 nm.
Yellow-Green 530.7 nm, 559.5 nm.
Violet (faint) 422.7 nm.
Greenish with blue glass.
Copper Emerald Green 521.8 nm, 529.2 nm, 515.3 nm.
Not chloride, or in presence of HCl
Azure Blue 465.1 nm.
Copper chloride, or HCl present
Lead Light Blue 500.5 nm.
( Lead is highly toxic - only perform in fume hood under supervision )
Lithium Carmine Red 670.78 nm, 670.79 nm.
Orange (faint) 610.1 nm.
Violet with blue glass
Potassium Red 766.5 nm, 769.9 nm.
Violet 404.4 nm, 404.7 nm.
Purple-red with blue glass
Rubidium Violet 780.0 nm, 794.8 nm.
Sodium Yellow 589.0 nm, 589.6 nm.
Invisible when viewed with blue glass
Strontium Scarlet Red 640.8 nm, 650.4 nm, 687.8 nm, 707.0 nm.
Violet 460.7 nm, 421.5 nm, 407.8 nm.
Violet with blue glass
Tellurium Green 557.6 nm, 564.9 nm, 566.6 nm, 570.8 nm.
( Tellurium is highly toxic - only perform in fume hood under supervision )
Thallium Green 535.0 nm.
( Thallium is highly toxic - only perform in fume hood under supervision )
Zinc Whitish Green Large number of peaks between 468.0-775.8 nm.
( Zinc fumes are toxic - only perform in a fume hood under supervision )
Impressive coloured flames have been obtained using chlorides and a methanol
flame in a petri dish [6]. Even more spectacular results have been obtained
by nitrating cellulose filter paper, and impregnating it with salts prior
to ignition [7].
18.9 What chemicals change colour with heat, light, or pressure?.
Compounds that visibly and reversibly change colour when subjected to a
change in their environment are known as chromogenic materials. There are
four major categories - electrochromic, photochromic, piezochromic, and
thermochromic, all of which are extensively discussed in a recent, well
referenced, monograph in Kirk Othmer [8].
Electrochromic materials exhibit a change in light transmittance or
reflectance induced by direct current at potentials of approximately one
volt. The change usually is an oxidation-reduction reaction, using either
inorganic or organic compounds, and the colour change can occur at either
the anode or the cathode - which are usually thin films. There are two major
classes, the ion-insertion/extraction type - such as tungsten trioxide, and
the noninsertion group - such as the viologens, a family of halides of
quaternary bases derived from 4,4'-bipyridinium. One viologen example is
1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
clear to bluish purple. The most common application of viologens has been
the electrochromic interior rearview mirrors available for cars since 1988.
These utilise a substituted viologen as the cathode colouring material, with
a compound like phenylene diamine as the anode colouring electrochromic
material. The mechanism details, along with a description of the ingenious
control system, are described in a recent comprehensive review of
electrochromic materials [9].
Photochromic materials undergo a reversible change in light absorption that
is induced by electromagnetic radiation, however most common applications
involve reversible changes in colour or transparency on exposure to visible
or ultraviolet light. This is often seen as a change in the visible spectrum
( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
of photochromic materials, inorganic and organic.
Examples of the inorganic type are the silver halides, which are suspensions
of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
has been slowly cooled. An alternative technique involves diffusion of the
silver halide into the surface of the glass. The cuprous ion can catalyse
both the photochromic darking and thermal fading reactions, and the colour
can be shifted from grey to brown by the addition of gold or palladium -
which may be added to the glass in trace amounts. The most popular current
application for glass containing silver halide is for prescription eyewear.
The organic photochromic systems can be subdivided according to the type of
reaction. Geometric isomerism can result in different optical properties,
eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the
cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
Cycloaddition can produce photochromism, such as the reversible formation of
the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red
parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride
[76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole
[63245-02-3] to form a red-purple free radical ), may also produce
photochromism.
UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N
[226-36-8] ), to their triplet state, which has a different absorption
spectrum. Viologens may undergo redox reactions and exhibit photochromic
behaviour when crystalline and subjected to UV. The most popular photochromic
materials utilise reversible electrocyclic reactions, and are often indolino
spiropyrans and indolino spiroxazines, however the mechanism also covers
fulgide, stilbene, and dihydroindolizine examples. Details and structures
are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
Education has published descriptions and preparation techniques for both
inorganic [10] and organic photochromic compounds and sunglasses [11].
Piezochromic materials change colour as they are compressed. There are three
common types:- organic molecules ( such as N-salicylidene-2-chloroaniline
[3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates,
(Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
compounds like ethylene diamine. They are still being researched, and
interested readers should investigate the references in the Kirk Othmer
monograph [8].
Thermochromic materials reversibly change colour as their temperature is
changed. There are a very large number of systems, but one common example
of thermochromic transitions in metal complexes is the transition between
the blue tetrahedral and pink octahedral coordinations of cobalt (II) when
cobalt chloride is added to anhydrous ethanol and the temperature changed.
Examples of thermochromic transitions in inorganic compounds include
Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large
changes occurring in the infra-red range, and are being considered for IR
imaging applications.
There are thousands of organic thermochromic compounds, with well known
examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced
heterolytic bond cleavage resulting in ring opening), poly(xylylviologen
dibromide [38815-69-9] ( charge transfer interactions resulting in hydration-
dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
induced transitions in the unsaturated backbone resulting in rearranged side
groups ). Information on photochromism in organic and polymeric compounds is
available in published reviews [12,13].
Subject: 19. Physical properties of chemicals
19.1 Rheological properties and terminology
Contributed by Jim Oliver
RHEOLOGY
What is RHEOLOGY ?
RHEOLOGY describes the deformation of a material under the influence of
stresses. Materials in this context can be solids, liquids or gases. In this
FAQ we will be concerned only with the rheological properties of liquids.[1]
Perry discusses the some aspects of the behaviour of gases, and Ullmann
discusses elastic solids.
When liquids are subjected to stress they will deform irreversibly and flow.
The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
are very few, whereas non-ideal examples abound. Ideal liquids are : water
and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
mixed with a little water. [2]
What is VISCOSITY ?
VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
conditions used to measure the VISCOSITY must be given. This is due to the
fact that non-ideal liquids have different values of VISCOSITY for different
test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
curve reveals the particular type of VISCOSITY for the liquid being studied.
[3]
What is a NEWTONIAN LIQUID ?
NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
are mineral oil, water and molasses. (Isaac NEWTON first described the laws
of viscosity) [1] All the other types are NON NEWTONIAN.
What does NON NEWTONIAN mean ?
a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
the origin again and curving up and along but falling under the straight
line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
increasing shear. Examples are toothpaste and whipped cream.
b. DILATANT liquids give a curve which curves under then upward and higher
than the straight line NEWTONIAN curve. (Like a square law curve) Such
liquids display increasing viscosity with increasing shear. Examples are
wet sand, and mixtures of starch powder with small amounts of water. A car
may be driven at speed over wet sand, but don't park on it, as the car may
sink out of sight due to the lower shear forces (compared to driving over)
the wet sand.
There are other terms used which include :
THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
What this means is the curve runs straight up the y axis for a short way then
curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
This YIELD POINT is time dependant. Some water based paints left overnight
develop a FALSE BODY which only breaks down to become useable after rapid
stirring. Also: the curve describing a THIXOTROPIC liquid will be different
on the way up (increasing shear rate) to the way down (decreasing shear rate).
The area inside these two lines is a measure of it's degree of THIXOTROPY.
This property is extremely important in industrial products, e.g to prevent
settling of dispersed solids on storage. [3]
A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
viscosity with a constant shear rate over time. Again, time dependant but in
this case _increasing_ viscosity.
Why do some liquids become solid ?
A few special liquids (dispersions usually) display extraordinary DILATANT
properties. A stiff paste slurry of maize or cornflour in water can appear to
be quite liquid when swirled around in a cup. However on pouring some out
onto a hard surface and applying extreme shear forces (hitting with a hammer)
can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
VISCOSITY can become so high as to make it appear solid. The "liquid" then
becomes very stiff for an instant and can shatter just like a solid material.
It should be noted that the study of viscosity and flow behaviour is
extremely complex. Some liquids can display more than one of the above
properties dependant on temperature, time and heat history.
What are Electrorheological Fluids? ( added by Bruce Hamilton )
Electrorheological (ER) fluids change their flow properties when an electric
field is applied, and are usually dispersions of polarizable particles in an
insulating base fluid [5]. Their apparent viscosity can change by orders of
magnitude in milliseconds when a fews watts of electrical power are applied.
The shear stress versus shear rate properties of ER fluids vary as a function
of the applied electric field, When an electric field is applied, the fluid
switches from a liquid to semisolid. The particles are usually irregularly-
shaped 0.5-100um and present at concentrations of 10-40% by mass. ER fluids
are dielectric particles in an insulating medium ( such as silicone oil ),
along with additives ( such as surfactants, dispersants, and possibly a
polar activator ). ER fluid effectively function as leaky capacitors. The
electric field can be either AC, pulsed DC, or DC, with AC producing less
electrophoresis of particles to electrodes.
There are two categories of ER particulate materials, extrinsically
polarizable materials ( which require a polar activator ), and intrinsically
polarizable materials. Extrinsically polarizable materials can be polar
nonionic compounds ( such as silica, alumina, or polysaccharides ), or polar
ionic materials ( such as the lithium salt of polymethacrylic acid ),
Intrinsically polarizable materials provide simpler systems - because a polar
activator is not required, and they have a lower thermal coefficient of
conductance. The most common examples are the ferroelectrics like barium
titanate (BaTiO3 ) and polyvinylidene difluoride, however their performance
has been poor, as has been that of metal powders ( such as iron and
aluminium - even when coated with an insulating layer ), and research is
concentrating on conducting polymers ( such as polyanilines and pyrolysed
hydrocarbons ) [5,6].
The ability to utilise computer-based electrical switching to control ER
fluid properties has resulted in vehicle suspension and industrial vibration
control as major target applications for ER fluids. Demonstration systems
have been built, and they match performance predictions, however cost and
durability issues still have to be solved [7].
19.2 Flammability properties and terminology
There are several properties of flammable materials that are frequently
reported. It should be remembered that most discussions concerning
flammable liquids usually consider air as the oxidant, but oxygen and
fluorine can also be used as oxidants for combustion, and they will result
in very different values.
The flammability limits in air are usually reported as the upper and lower
limits ( in volume percent at a certain temperature, usually 25C ), and
represent the concentration region that the vapour ( liquid HCs can not burn )
must be within to support combustion. Hydrocarbons have a fairly narrow range,
( n-hexane = 1.2 to 7.4 ), whereas hydrogen has a wide range ( 4.0 to 75 ).
The minimum ignition energy is the amount of energy ( usually electrical )
required to ignite the flammable mixture. Some mixtures only require a very
small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ),
whereas others require more (eg methanol = 0.14mJ, n-hexane = 0.29mJ,
diethyl ether = 0.20mJ, acetone = 1.15mJ, dichloromethane = 133mJ @ 88C ),
and some require significant amounts, (eg ammonia = >1000mJ ).
The flash point is the most common measure of flammability today, especially
in transportation of chemicals, mainly because most regulations use the flash
point to define different classes of flammable liquids. The flash point of a
liquid is the temperature at which the liquid will emit sufficient vapours
to ignite when a flame is applied. The test consists of placing the liquid
in a cup and warming it at a prescribed rate, and every few degrees applying
a small flame to the air above the liquid until a "flash" is seen as the
vapours burn. Note that the flame is not applied continuously, but is
provided at prescribed intervals - thus allowing the vapour to accumulate.
There are a range of procedures outlined in the standard methods for
measuring flash point ( ASTM, ISO, IP ) and they have differing cup
dimensions, liquid quantity, headspace volume, rate of heating, stirring
speed, etc., but the most significant distinction is whether the space above
the liquid is enclosed or open. If the space is enclosed, the vapours will be
contained, and so the flash point is several degrees lower than if it is
open. Most regulations specify closed-cup methods, either Pensky-Martens
Closed Cup or Abel Closed Cup. It is important to remember that these methods
are only intended for pure chemicals, if there is water or any other volatile
non-flammable compounds present, their vapours can extinguish or mask the
flash. For used lubricants, this may be partially overcome by using the TAG
open cup procedure - which is slightly more tolerant of non-flammable
vapours. A material can be flammable, but may not have a flash point if other
non-flammable volatile compounds are present. For alkane hydrocarbons, flash
point increases with molecular weight.
There is an older measure, called the fire point, which is the temperature
at which the liquid emits sufficient vapours to sustain combustion. The fire
point is usually several degrees above the flash point for hydrocarbons.
The minimum autoignition temperature is the temperature at which a material
will autoignite when it contacts a surface at that temperature. The procedure
consists of heating a glass flask and squirting small quantities of sample
into it at various temperatures until the vapours autoignite. The only
source of ignition is the heat of the surface. For the smaller hydrocarbons
the autoignition temperature is inversely related to molecular weight, but it
also increases with carbon chain branching. Autoignition temperature also
correlates with gasoline octane ratings ( refer to Gasoline FAQ available in
rec.autos.tech, which lists octane ratings and autoignition temperatures for
a range of hydrocarbons.)
Flash Point Autoignition Flammable Limits
Temperature Lower Upper
( C ) ( C ) ( vol % at 25C)
methane -188 630 5.0 15.0
ethane -135 515 3.0 12.4
propane -104 450 2.1 9.5
n-butane -74 370 1.8 8.4
n-pentane -49 260 1.4 7.8
n-hexane -23 225 1.2 7.4
n-heptane -3 225 1.1 6.7
n-octane 14 220 0.95 6.5
n-nonane 31 205 0.85 -
n-decane 46 210 0.75 5.6
n-dodecane 74 204 0.60 -
n-tetradecane 99 200 0.50 -
19.3 Supercritical properties and terminology?
Supercritical fluids have some very unusual properties. When a compound is
subjected to conditions around the critical point ( which is defined as
the temperature at which the gas will not revert to a liquid regardless how
much pressure is applied ), the properties of the supercritical fluid become
very different to the liquid or the gas phases. In particular, the solubility
behaviour changes. The behaviour is neither that of the liquid or that of the
gas. The transition between liquid and gas can be completely smooth.
The pressure-dependant densities and corresponding Hildebrand solubility
parameters show no break on continuity as the supercritical boundary is
crossed. Physical properties fall between those of a liquid and a gas.
Diffusivities are approximately an order of magnitude higher than the
corresponding liquid, while viscosities are an order of magnitude lower.
These properties ( along with low surface tension ) allow SCFs to have
liquid-like solvating power with the mass transport characteristics of
a gas.
Potential Supercritical Fluids
Compound Critical Critical Density
Temperature Pressure
( C ) ( bar ) (g cm^-3)
Ammonia 132.4 112.8 0.235
Carbon dioxide 30.99 73.75 0.468
CFC-12 111.8 41.25 0.558
Dimethyl ether 126.9 52.7 0.271
Ethane 32.4 49.1 0.212
HCFC-22 96.15 49.90 0.524
HCFC-123 183.68 36.62 0.550
HFC-116 19.7 29.8 0.608
HFC-134a 101.03 40.57 0.508
Methanol 240.1 83.1
Nitrous oxide 36.4 72.54 0.453
Propane 96.8 42.66 0.225
Water 374.4 227.1
Xenon 16.6 58.38 1.105
Nitrous oxide is seldom used because early researchers reported explosions.
Note that using liquid CO2 at pressure ( as for the commercial extraction
of hops ) is still just liquid CO2 extraction, not supercritical CO2
extraction. There are several good general introductions to supercritical
fluids [8,9,10]
19.4 Formation of gaseous bubbles in liquids
Discussions about the behaviour of dissolved gases in liquids, especially
when discussing carbonated beverages, are usually more appropriate in
sci.physics and/or sci.mech.fluids, and there is a good text available [11].
Section 23.9 of this FAQ lists the change in solubility with temperature
for common atmospheric gases in water at near-ambient pressure. As the
temperature increases, the solubility decreases, creating a supersaturated
solution that can result in bubble formation. A similar effect occurs if the
pressure is reduced. The formation of bubbles can be understood in
thermodynamic terms using the Gibbs free energy of the bubble.
Gibbs free energy = -n * R * T ln(C/Cs) + gamma * A
A = Surface area of the bubble.
C = Concentration of gas in the liquid,
Cs = Concentration of gas in the liquid at saturation,
gamma = Interfacial tension between the gas and the liquid
n = Number of moles of gas in the bubble
= (P*V)/(R*T), where P = pressure, and V = volume of a sphere.
R = Gas Constant
T = Temperature
After inserting the expressions for the surface area of a sphere (r = radius)
and number of moles, and differentiating, then we obtain:-
r(mininum) = 2 * gamma / ( P * ln(C/Cs))
This describes the size of a bubble that would continue to grow under the
existing conditions, rather than redissolve. Of course, the expression
assumes homogeneous precipitation of the bubble, and in real life most
bubbles are created heterogeneously. Statistics and kinetics are also
required to determine the rate of formation of bubbles, and predict the
effect of changing parameters such as temperature. As the liquid is warmed,
bubbles may be created faster, as the higher temperatures overcome the
activation barrier - which is the difference between the Gibbs free energy
when r is less than r(minimum), and the Gibbs free energy at r(minimum).
The formation of a bubble also dramatically perturbs the system, even
causing secondary bubbles to form. Secondary bubble formation may be
implicated in the production of copious quantities of froth from shaken,
quickly-opened, carbonated drink containers. The sites for gaseous bubble
formation in supersaturated drinks are typically small particles, or minor
flaws on the smooth surface of the container.
19.5 Why is Mercury a liquid at room temperature?.
First, let's look at the melting points of some of the elements surrounding
mercury in the periodic table ( in degrees C ) :-
Period IB IIB IIIA
4s3d4p Cu 1083 Zn 419.5 Ga 29.8
5s4d5p Ag 960.8 Cd 320.9 In 157
6s(4f)5d6p Au 1063 Hg -38.4 Tl 304
The interesting comparison is between Hg and Au, as their properties differ
dramatically, although their electron structures are similar:-
14 10 1
Au(g) : Xe | 4f , 5d , 6s
79 54
14 10 2
Hg(g) : Xe | 4f , 5d , 6s
80 54
Very few chemistry textbooks discuss relativistic effects on chemical
properties, despite the availability of a comprehensive review by P.Pyykko
[12]. There several good introductory articles on the derivation and
calculation of various relativistic effects in molecules and atoms, so I'm
not going to include details [13,14,15]. Suffice to say, that whilst
smaller elements can treated simply, larger elements need treatment based
on the Dirac equation, which shows that the s electrons are approaching
the speed of light, consequently relativistic effects are important.
If we take the relativistic mass of mercury (m);-
Mo where
m = -------------------- c (speed of light) = ~137 atomic units
_____________ v = Z = 80
/ ( v ) 2 Mo = rest mass
/ 1 - ( - )
\/ ( c )
The masses of the 1s electrons are increased by approximately 20% over
their rest masses, which means that the radius is decreased by 20% - since
mass appears in the denominator of Bohr radius calculations. All the other
s shells also contract, with the 6s contracting ~14%, because their electron
speeds near the nucleus are comparable, and the contraction of the inner part
of the wave function also pulls in the outer tails. The p orbitals also
contract a similar amount, and these contractions also results in increases
the screening for d and f orbitals, which may then expand - about 3% for the
5d orbital of mercury.
In mercury, the relativistically-contracted 6s2 orbital is full, thus the
the two electrons do not contribute much to the metal-metal bond, which is
not the situation for gold. The bonding in mercury is believed to be mainly
van der Waals forces with a contribution from 6p orbital interaction. The
relativistic contraction of the filled 6s2 orbital, when added to the
contraction across the sixth row of the periodic table, results in relatively
weak Hg-Hg bonds that are responsible for mercury being a liquid at room
temperature. For those curious to know more, a recent article in J.Chem.Ed.
provides much more detail and several good references [16]. Relativistic
effects are also responsible for the colour of gold ( partially explained
by the 5d -> 6s transition in gold requiring less energy than the 4d -> 5s
transition in silver, resulting in a smaller d-s gap ) [12,14,16].
Subject: 20. Optical properties of chemicals
20.1 Refractive Index properties and terminology
When light passes between media of different density, the direction of the
beam is changed as it passes through the surface, and this is called
refraction. In the first medium, the angle between the light ray and the
perpendicular is called the angle of incidence (i), and the corresponding
angle in the second medium is called the angle of refraction (r). The
ratio sine i / sine r is called the index of refraction, and usually the
assumption is that the light is travelling from the less dense (air) to more
dense, giving an index of refraction that is greater than 1. Although the
theoretical reference is a vacuum, air ( 0.03% different ) is usually used.
The refractive index of a compound decreases with increasing wavelength
( dispersion ), except where absorption occurs, thus the wavelength should
be reported. The D lines of sodium are commonly used.
The refractive index of a liquid varies with temperature and pressure, but
the specific refraction ( Lorentz and Lorentz equation ) does not. The molar
refraction is the specific refraction multiplied by the molecular weight,
and is approximately an additive property of the groups or elements
comprising the compound. Tables of atomic refractions are available in the
literature, as are descriptions of the common types of refractometers [1].
20.2 Polarimetry properties and terminology
Supplied by: Vince Hamner <vinny@vt.edu>
Polarimetry is a method of chemical analysis that is concerned
with the extent to which a beam of linearly polarised light is rotated
during its transmission through a medium containing an optically active
species.[2] Helpful discussions regarding polarised light may be found
elsewhere.[3,4] In general, a compound is optically active if it has
no plane of symmetry and is not superimposable on its mirror image.
Such compounds are referred to as being "chiral". Sucrose, nicotine,
and the amino acids are only a few of these substances that exhibit
an optical rotary power.
A simple polarimeter instrument would consist of:
1). a light source -- typically set to 589 nm (the sodium "D" line)
2). a primary fixed linear polarising lens (customarily called the
"polariser")
3). a glass sample cell (in the form of a long tube)
4). a secondary linear polarising lens (customarily called the
"analyser") and
5). a photodetector.[5]
Biot is credited with the determination of the basic equation
of polarimetry.[6,7] The specific rotation of a substance (at a given
wavelength and temperature) is equivalent to the observed rotation (in
degrees) divided by the path length of the sample cell (in decimeters)
multiplied by the concentration of the sample (for a pure liquid,
-density- replaces concentration). Influences of temperature,
concentration, and wavelength must always be taken into consideration.
If necessary, it is possible to apply corrections for each of these
variables.[8] A few early contributors to our understanding of optical
activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel,
Fresnel, and Pasteur.
Subject: 21. Molecular and Structural Modelling
Supplied by: Dave Young (young@slater.cem.msu.edu)
21.1 What hardware do I need to run modelling programs?
There are two types of programs that are referred to as molecular
modeling programs. This first is a program which graphically displays
molecular structures as Lewis structures, ball & stick, etc. The second
is a program which does a calculation to tell you something about the
molecule, such as it's energy, dipole moment, spectra, etc.
For an introductory description of various types of computations,
see http://www.cem.msu.edu/~young/topics/contents.html
There are many programs of both sorts available for a large range
of machines. The speed, memory, graphics and disk space on the machine
will determine the size of molecules that can be modelled, how accurate
the model is, and how good the images will look. There are a few programs
that will run on a 286 PC with Windows. There are some fairly nice things
that can be done on a 386 with about 8 MB of RAM and Windows. The
professional computational chemists are generally using work stations and
larger machines.
Currently many computational chemists are using machines made by
Silicon Graphics (SGI) ranging from the $5,000 Indy to the $1,000,000
power challenge machines. These are all running Irix, which is SGI's
adaptation of Unix. SGI is popular for two reasons; first that the power
is very good for the price, second that SGIs run the largest range
of chemical software. However, you will find some computational chemistry
software that can run on almost any machine.
As far as graphics quality, the SGI Onyx (about $250,000) is about
the top of the line. Even if you find a machine that claims to have better
graphics than this, chances are you won't find and chemistry software that
can utilise it.
For chemical calculations there is no limit to the computing
power necessary. There are some calculations that can only be done
on the biggest Crays or massively-parallel machines in the world. There
are also many calculations which are too difficult for any existing
machine and will just have to wait a few years or a few centuries.
21.2 Where can I find a free modelling program?
The single best place for public domain modelling software
is probably the anonymous FTP server at ccl.osc.edu in the directory
pub/chemistry/software. "ccl" stands for "computational chemistry
list server" and is a list frequented mostly by professional
computational chemistry researchers. This machine contains their
archives with quite a bit of information as well as software.
For work stations and larger, the program GAMESS (General Atomic
and Molecular Electronic Structure System) can be obtained as source
code from Mike Schmidt at mike@si.fi.ameslab.gov GAMESS is a quantum
mechanics, ab initio and semi-empirical program. It is powerful. but
not trivial to learn how to use.
The COLUMBUS program for work stations and larger can be obtained
by anonymous FTP from ftp.itc.univie.ac.at It is a HF, MCSCF and
multi-reference CI program. This is probably the most difficult program
to use that is in use today since it requires the user to input EVERY
detail manually. However, because you control everything there are some
calculations that can only be done with COLUMBUS.
CACAO is an extended Huckel program available by anonymous FTP
at cacao.issecc.fi.cnr.it
21.3 Where can I find structural databanks?
21.4 Where can I find ChemDraw or ChemWindows
For ChemDraw (Macintosh, Windows, UNIX)
CambridgeSoft Corporation
875 Massachusetts Avenue
Cambridge, MA 02139
Phone: (800) 315-7300 or (617) 491-2200
Fax: (617) 491-7203
Internet: info@camsci.com
http://www.camsci.com
For ChemIntosh or ChemWindows
SoftShell
1600 Ute Avenue
Grand Junction, CO 81501
Phone: (970) 242-7502
Fax: (970) 242-6469
Internet: info@softshell.com
http://www.softshell.com
Subject: 22. Spectroscopic Techniques
All of these are covered in texts on instrumental Analysis [1-4], and I
will eventually include a paragraph about each.
22.1 Ultra-Violet/Visible properties and terminology
22.3 Nuclear Magnetic Resonance properties and terminology
22.4 Mass Spectrometry properties and terminology
22.5 X-Ray Fluorescence properties and terminology
22.6 X-Ray Diffraction properties and terminology
22.7 Fluorescence/Phosphorescence properties and terminology
Subject: 23. Chromatographic Techniques
There are chromatography mailing lists and WWW sites available that provide
comprehensive introductions and access to chromatography experts. The
following are simple introductions to popular techniques.
23.1 What is Paper Chromatography?
Paper chromatography was the first analytical chromatographic technique
developed, allegedly using papyrus (Pliny). It was first published by Runge
in 1855, and consists of a solvent moving along filter or blotting paper.
The interaction between the components of the sample, the solvent, and the
paper, results in separation of the components. Most modern paper
chromatography is partition chromatography, where the cellulose of the
paper is the inert support, and the water adsorbed ( hydrogen bonded ) from
air onto the hydroxyl groups of the cellulose becomes the stationary phase.
If the mobile phase is not saturated with water, then some of the stationary
phase water may be removed from the cellulose - resulting in a separation
that is a mixture of partition and adsorption. Paper chromatography remains
the method of choice for a wide range of coloured compounds, and is used
extensively in both natural and artificial pigment research. The technique
is suitable for any molecules that are significantly less volatile than the
solvent, and many examples and references are provided in Heftmann [1].
23.2 What is Thin Layer Chromatography?
Thin layer chromatography involves the use of a particulate sorbant on an
inert sheet of glass, plastic, or metal. The solvent is allowed to travel
up the plate with the sample spotted on the sorbant just above the solvent.
Depending on the sorbant, the separation can be either partition or
adsorption chromatography ( cellulose, silica gel and alumina are commonly
used ). The technique came to prominence during the late 1930s, however it
did not become popular until Merck and Desaga developed commercial plates
that provided reproducible separations. The major advantage of TLC is the
disposable nature of the plates. Samples do not have to undergo the
extensive clean-up steps required for HPLC. The other major advantage is the
ability to detect a wide range of compounds cheaply, using very reactive
reagents ( iodine vapours, sulfuric acid ) or indicators. Non-destructive
detection ( fluorescent indicators in the plates, examination under a UV
lamp ) also means that purified samples can be scraped off the plate and
be analysed by other techniques. There are special plates for such
preparative separations, and there are also high-performance plates that can
approach HPLC resolution. The technique is described in detail in Stahl [2]
and Kirchner [3].
23.3 What is Gas Chromatography?
Gas chromatography is the use of a carrier gas to convey the sample ( as a
vapour ) through a column consisting of an inert support and a stationary
phase that interacts with sample components, thus it is usually partition
chromatography. There are also a range of materials, especially for permanent
gas and light hydrocarbon analysis that utilise adsorption. The simplest
partition systems consisted of a steel tube filled with crushed brick that
had been coated with a hydrocarbon that had a high boiling point, eg
squalane. Today, the technique uses very narrow fused silica tubes ( 0.1 to
0.3mm ID ) that have sophisticated stationary phase films ( 0.1 to 5um )
bonded to the surface and also cross-linked to increase thermal stability.
The ability of the film to retard specific compounds is used to ascertain
the "polarity" of the column. If benzene elutes between normal alkanes
where it is expected by boiling point ( midway between n-hexane and
n-heptane ), then the column is "non-polar" eg squalane and methyl silicones.
If the benzene is retarded until it elutes after n-dodecane, then the column
is "polar" eg OV-275 ( dicyanoallyl silicone ) and 1,2,3-tris (2-cyanoethoxy)
propane. In general, polar columns are less tolerant of oxygen and reactive
sample components, but the ability to select different polarity columns to
obtain satisfactory peak resolution is what has made GC so popular.
The column is placed in an oven that has exceptional temperature control,
and the column can be slowly heated up to 350-450C ( sometimes starting at
-50C to enhance resolution of volatile compounds ) to provide separation of
wide-boiling range compounds. The carrier gas is usually hydrogen or helium,
and the eluting compounds can be detected several ways, including flames
( flame ionisation detector ), by changes in properties of the carrier
( thermal conductivity detector ), or by mass spectrometry. The availability
of "universal" detectors such as the FID and MS, makes GC a popular tool in
laboratories handling organic compounds. There are also columns that have a
layer of 5-10 um porous particulate material (such as molecular sieve or
alumina ) bonded to the inner walls ( PLOT = Porous layer open tubular ),
and these are used for the separation of permanent gases and light
hydrocarbons. GC is restricted to molecules ( or derivatives ) that
are sufficiently stable and volatile to pass through the GC intact at the
temperatures required for the separation. Specialist books on the production
of derivatives for GC are available [4,5].
There are several manufacturers of GC instruments whose catalogues and
brochures provide good introduction to the technique. (eg Hewlett Packard,
Perkin Elmer, Carlo Erba ). The catalogues of suppliers of chromatography
consumables also contain explanations of the criteria for selection of the
correct columns and conditions for analyses, and they provide an excellent
indication of the range of applications available. Well-known suppliers
include Alltech Associates, Supelco, Chrompack, J&W, and Restek. They also
sell most of the standard GC texts, as do the instrument manufacturers.
Popular GC texts include "Basic Gas Chromatography" [6], "High-Resolution
Gas Chromatography" [7], and "Open Tubular Column Gas Chromatography" [8].
There are Standard Retention Index Libraries available [9], however they
really only complement unambiguous identification by mass spec. or
dual-column analysis.
23.4 What is Column Chromatography?
Column chromatography consists of a column of particulate material such as
silica or alumina that has a solvent passed through it at atmospheric or low
pressure. The separation can be liquid/solid (adsorption) or liquid/liquid
(partition). The columns are usually glass or plastic with sinter frits to
hold the packing. Most systems rely on gravity to push the solvent through.
The sample is dissolved in solvent and applied to the front of the
column. The solvent elutes the sample though the column, allowing the
components to separate based on adsorption ( alumina, hydroxyapatite) or
partition ( cellulose, diatomaceous earth ). The mechanism for silica
depends on the hydration. Traditionally, the solvent was non-polar and the
surface polar, although today there are a wide range of packings including
bonded phase systems. Bonded phase systems usually utilise partition
mechanisms rather than adsorption. The solvent is usually changed stepwise,
and fractions are collected according to the separation required, with the
eluted solvent usually monitored by TLC.
The technique is not efficient, with relatively large volumes of solvent
being used, and particle size is constrained by the need to have a flow of
several mls/min. The major advantage is that no pumps or expensive equipment
are required, and the technique can be scaled up to handle sample sizes
approaching a gram in the laboratory. The technique is discussed in detail
in Heftmann [1].
23.5 What is High Pressure Liquid Chromatography?
HPLC is a development of column chromatography. it was long realised that
using particles with a small particle size ( 3, 5, 10um ) with a very narrow
size distribution would greatly improve resolution, especially if the flow
rate and column dimensions could be adjusted to minimise band-broadening.
Pumps were developed that could handle both the chemicals and pressures
required. Traditional column chromatography ( nonpolar solvent and
polar surface ) is described as "normal" and, as well as silica, there are
columns with amino, diol, and cyano groups. If the system uses a polar
solvent ( water, methanol, acetonitrile etc. ) and a non-polar surface it
is described as "reverse-phase". Common surface treatments of silica include
octadecylsilane ( aka ODS or C18), and it has been the development of
reverse-phase HPLC that has experienced explosive growth. Reverse-phase HPLC
is the method of choice for larger non-volatile biomolecules, however it is
only recently that a replacement "universal" detector ( evaporative
light-scattering ) has emerged. The most popular detector (UV), places
constraints on the solvents that can be used, and the refractive index
detector can not easily be used with solvent gradients. There are several
excellent books introducing HPLC, including the classic "Introduction to
Modern Liquid Chromatography" [10]. HPLCs can be a pain to operate, and
novices should borrow "Troubleshooting LC Systems" by Dolan and Snyder [11].
There is also a handy basic primer on developing HPLC methods by Snyder and
Kirkland [12], however, unlike GC, you also need to search the journals
( Journal of Chromatography, Journal of Liquid Chromatography ) to find
relevant examples to assist with method development.
23.6 What is Ion Chromatography?
Ion chromatography has become the method of choice for measuring anions
( eg Cl-, SO4=, NO3- ) in aqueous solutions. It is effectively a development
from ion-exchange systems ( which were extensively developed to deionise
water and soften aqueous process streams ), and brings them down to HPLC
size. IC uses pellicular polymeric resins that are compatible with a wide pH
range. The sample is eluted through an ion-exchange column using a dilute
sodium hydroxide solution. The eluant is passed through self-regenerating
suppressors that neutralise eluant conductance, ensuring electrochemical
detectors ( conductivity or pulsed amperometric ) can detect the ions down
to sub-ppm concentrations. The major manufacturer of such systems is Dionex,
who hold several patents on column, suppression, and detection technology.
There are several books covering various aspects of the technique [13,14].
23.7 What is Gel Permeation Chromatography?
Gel Permeation chromatography ( aka Size Exclusion chromatography ) is based
on the ability of molecules to move through a column of gel that has pores of
clearly-defined sizes. The larger molecules can not enter the pores, thus
they pass quickly through the column and elute first. Slightly smaller
molecules can enter some pores, and so take longer to elute, and small
molecules can be delayed further. The great advantage of the technique is
simplicity, it is isocratic ( single solvent - no gradient programming ),
and large molecules rapidly elute. The technique can be used to determine
the molecular weight of large biomolecules and polymers, as well as
separating them from salts and small molecules. The columns are very
expensive and sensitive to contamination, consequently they are mainly used
in applications where alternative separation techniques are not available,
and sample are fairly clean. The best known columns are the Shodex
cross-linked polystyrene-divinylbenzene columns for use with organic solvents,
and polyhydroxymethacrylate gel filtration columns for use with aqueous
solvents. "Modern Size Exclusion Chromatography" [15], and Heftmann [1],
provide good overviews, and there are some good introductory booklets from
Pharmacia.
23.8 What is Capillary Electrophoresis?
Capillary electrophoresis uses a small fused silica capillary that has been
coated with a hydrophilic or hydrophobic phase to separate biomolecules,
pharmaceuticals and small inorganic ions. A voltage is applied and the
analytes migrate and separate according to their charge under the specific
pH conditions, as also happens for electrophoresis. The capillary can also
be used for isoelectric focusing of proteins. The use of salt or vacuum
mobilisation is no longer required.
23.9 How do I degas chromatographic solvents?
One major problem with pressurising chromatography systems using liquid
solvents is that pressure reductions can cause dissolved gases to come out
of solution. The two locations where this occurs are the suction side of the
pump ( which is not self-priming, consequently a gas bubble can sit in the
pump and flow is reduced ), and at the column outlet ( where the bubbles
then pass through the detector causing spurious signals). Note that the
problem is usually restricted to solvents that have relatively high gas
solubilities - usually involving an aqueous component, especially if a
gradient is involved where the water/organic solvent ratio is changing.
As water usually has a higher dissolved gas content, then a gradient
programme may cause the gases to come out of solution as the mobile phase
components mix.
There are three traditional strategies used to remove problem dissolved
gases from chromatographic eluants. Often they are used in combination to
lower the dissolved gases.
a. Subject the solvent to vacuum for 5-10 mins. to remove the gases.
b. Subject the solvent to ultrasonics for 10-15 mins. to remove the gases.
c. Sparge the solvent with a gas that has a very low solubility compared
to the oxygen and nitrogen from the atmosphere. Helium is the preferred
choice - 5 minutes of gentle bubbling from a 7um sinter is usually
sufficient, although maintaining a positive He pressure is even better.
Note that most aqueous-based solvents usually have to be degassed every
24 hours. Also remember that solubility of gases increases as temperature
decreases, so ensure eluants are at instrument temperature prior to
degassing. Helium is preferred as the degassing solvent because it has
relatively low solubility in water, and the solubility is less affected by
temperature.
The following data is from Kaye and Laby, 13th edition, and the units are
the number of cm3 of gas at 0C and 760 mmHg which dissolve in 1 cm3 of water
at the temperature stated ( when the gas is at 760 mmHg pressure and in
equilibrium with the water ).
Temp.(C) 0 10 20 30 40 50 60
Helium 0.0098 0.0091 0.0086 0.0084 0.0084 0.0086 0.0090
Hydrogen 0.0214 0.0195 0.0182 0.0170 0.0164 0.0161 0.0160
Nitrogen 0.0230 0.0185 0.0152 0.0133 0.0119 0.0108 0.0100
Oxygen 0.047 0.037 0.030 0.026 0.022 0.020 0.019
Argon 0.054 0.041 0.032 0.028 0.025 0.024 0.023
CO2 1.676 1.163 0.848 0.652 0.518 0.424 0.360
I've no explanation for the aberrant trend for helium at higher temperatures,
but I assume it's real - but it's irrelevant for HPLC solvents that are
usually stored at ambient temperature. Points to note - the lower solubility
of helium over the range of concern, *and* the lower rate of change of
decreasing solubility with increasing temperature. There is heat generated
in the compression of the solvent, along with friction in HPLC pump heads
and, more importantly, HPLC columns are often heated - thus the solvent
could outgas and form bubbles in UV detector cells that are at ambient.
By using helium, there is less chance of that happening. For example, if the
temperature increased from 10C to 40C, the undissolved gas volume would be
0.0007 cm3 for helium, and 0.0066 cm3 for nitrogen.
Modern HPLCs are sold with a "solvent degassing module" that removes
undissolved gases in the solvent automatically. These usually consist of
a tube made from gas-permeable membrane that passes through a vacuum
chamber.
23.10 What is chromatographic solvent "polarity"?
There are four major intermolecular interactions between sample and solvent
molecules in liquid chromatography, dispersion, dipole, hydrogen-bonding,
and dielectric. Dispersion interactions are the attraction between each pair
of adjacent molecules, and are stronger for sample and solvent molecules
with large refractive indices. Strong dipole interactions occur when both
sample and solvent have permanent dipole moments that are aligned. Strong
hydrogen-bonding interactions occur between proton donors and proton
acceptors. Dielectric interactions favour the dissolution of ionic
molecules in polar solvents. The total interaction of the solvent and
sample is the sum of the four interactions. The total interaction for a
sample or solvent molecule in all four ways is known as the "polarity" of
the molecule. Polar solvents dissolve polar molecules and, for normal
phase partition chromatography, solvent strength increases with solvent
polarity, whereas solvent strength decreases with increasing polarity
in reverse-phase systems. The subject is discussed in detail in Snyder
and Kirkland [10].
------------------------------
Subject: 24. Extraction Techniques
24.1 What is Solvent Extraction?
Solvent extraction is usually used to recover a component from either a solid
or liquid. The sample is contacted with a solvent that will dissolve the
solutes of interest. Solvent extraction is of major commercial importance
to the chemical and biochemical industries, as it is often the most efficient
method of separation of valuable products from complex feedstocks or
reaction products. Some extraction techniques in involve partition between two
immiscible liquids, others involve either continuous extractions or batch
extractions. Because of environmental concerns, many common liquid/liquid
processes have been modified to either utilise benign solvents, or move to
more frugal processes such as solid phase extraction. The solvent can be a
vapour, supercritical fluid, or liquid, and the sample can be a gas, liquid
or solid. There are a wide range of techniques used, and details can be found
in Organic Vogel, Perry, and most textbooks on unit operations.
24.2 What is Solid Phase Extraction?
Solid Phase Extraction (SPE) is an alternative to liquid/liquid extraction,
and has become the method of choice for the separation and purification of
a wide range of samples in the laboratory. The sample is usually dissolved
in an appropriate solvent and passed through a small bed of adsorbent of
very consistent particle size and shape to maximise separation efficiency.
The compounds are eluted with step changes of small volumes of solvents.
The major advantage is that solvent volumes are greatly reduced. There is
a newer, modified technique that is used in analytical laboratories, called
Solid Phase Micro Extraction. This immerses a fused silica fibre coated with
a stationary phase into the sample solution for several minutes, The analytes
adsorb onto the stationary phase, which is subsequently pushed into a hot GC
injector to rapidly desorb the sample for analysis.
24.3 What is Supercritical Fluid Extraction?
Refer to Section 19.3 for some critical data on common supercritical fluids.
Supercritical fluids have been investigated since last century, with the
strongest commercial interest initially focusing on the use of supercritical
toluene in petroleum and shale oil refining during the 1970s. Supercritical
water is also being investigated as a means of destroying toxic wastes, and
as an unusual synthesis medium [1]. The biggest interest for the last decade
has been the applications of supercritical carbon dioxide, because it has
a near-ambient critical temperature (31C), thus biological materials can
be processed at temperatures around 35C. The density of the supercritical
CO2 at around 200bar pressure is close to that of hexane, and the solvation
characteristics are also similar to hexane, thus it acts as a non-polar
solvent. Around the supercritical region CO2 can dissolve triglycerides at
concentrations up to 1% mass. The major advantage is that a small reduction
in temperature, or a slightly larger reduction in pressure, will result in
almost all of the solute precipitating out as the supercritical conditions
are changed or made subcritical. Supercritical fluids can produce a
product with no solvent residues. Examples of pilot and production scale
products include decaffeinated coffee, cholesterol-free butter, low-fat meat,
evening primrose oil, squalene from shark liver oil. The solvation
characteristics of supercritical CO2 can be modified by the addition of an
entrainer, such as ethanol, however some entrainer remains as a solvent
residue in the product, negating some of the advantages of the "residue-free"
extraction.
There are other near-ambient temperature supercritical fluids, including
nitrous oxide and propane, however there are safety issues with some of them.
There are several introductory texts on supercritical fluid extraction,
including some the ACS Symposium series [2-4]. There are also a large
number of articles on applications of the technique, including processing [5],
extraction of natural products [6], and chemical synthesis [7]. The major
concentration of information occurs in the various proceedings of the
International Symposium on Supercritical Fluids [8]. There is also a Journal
of Supercritical Fluids.
24.4 What traditional process extracted perfume from flower petals?
The traditional cold-fat extraction process is known as " enfleurage".
It is a very interesting, historical process used to obtain the essential
oils and perfume components from rose, jasmine, and other flowers. The
rose and jasmine flowers continue to produce perfume during the long
process. Thus the technique can obtain more perfume from those flowers than
if they were just macerated and extracted by hot fat, solvent or steam
when they were picked - as happens to many other plant perfume sources.
The process uses a fat comprised of 40 parts of beef tallow and 60 parts
of lard. The two fats are melted together, and repeatedly beaten under
cold water and alum solutions to purify them. Benzoin is added to the
fat mixture to prevent biological degradation.
The fat is spread about 4mm thick on both sides of 0.5 x 0.5 metre glass
plates in wooden frames. Flowers are pressed into the fat on one side of
the frame only, and the frames stacked vertically so that the flowers are
very close to the layer of fat on the frame above. After 1-3 days, the
flowers are stripped off and fresh flowers added to the other layer of fat
that had not been used, and the frame are again stacked. The cycle is
repeated about 30 - 35 times, or until the fat is saturated with perfume.
The saturated fat is known as "pomade". The fat is removed from the frames
and extracted with alcohol to collect the perfume. the alcohol is cooled
and filtered to remove most of the dissolved fat. The alcohol solution
is called the "extract", and the residue after evaporation of the solvent
is known as the "enfleurage absolute".
Subject: 25. Radiochemical Techniques
25.1 What is radiochemistry?
------------------------------
Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - MultiPage
[ By Archive-name | By Author | By Category | By Newsgroup ]
[ Home | Latest Updates | Archive Stats | Search | Usenet References | Help ]
Send corrections/additions to the FAQ Maintainer:
B.Hamilton@irl.cri.nz
Last Update January 01 2003 @ 00:56 AM
--
大家好，我叫nil，就读于海淀乡巴沟村万柳走读研修学校。
※ 来源:·北大未名站 bbs.pku.edu.cn·[FROM: 162.105.216.79]
------------------------------------------------------------------------------
--
.. 返回
--
---- 归零 ----
※ 来源:．哈工大紫丁香 bbs.hit.edu.cn [FROM: 218.9.122.29]
Powered by KBS BBS 2.0 (http://dev.kcn.cn)
页面执行时间：832.325毫秒