2020-05-21
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The techniques of carrying out a chromatographic investigation are very simple. The basic apparatus is the adsorption column. The adsorption column may be constructed of soft glass or in special cases of quartz. The diameter аnd length of the column are determined by the quantity of material to be
absorbed.
No universal adsorbent has been found. A good adsorbent should satisfy the following criteria: it should hold relatively large quantities of the materials to be resolved; the resolved materials must be eluted from the adsorbent by polar solvents; the size of the particles of adsorbent should be such as will allow rapid and uniform percolation; the adsorbents must not react with either the materials to be resolved nor the materials to be used as solvent or color developer; the adsorbent should not be porous and should, if possible, be colorless.
The chromatograph is made as follows: a solution of the material to be adsorbed is poured into the adsorption column and allowed to percolate through the adsorbent. The column is washed with additional portions of the original solvent from which the compound was adsorbed. The sides of the column are washed with small portions of the solvent and then larger quantities are added to the column. The passage of the solvent through the column causes the adsorbed materials to move at different rates and thus produce the chromatogram.
NOTES AND COMMENTARY
| should be such as will allow -должен бы быть таким, чтобы позволить | rapid and uniform percolation -быстрое и равномерное просачивание |
PAPER CHROMATOGRAPHY, APPLICATIONS AND PROCEDURE
In paper chromatography-the absorption column is replaced by strips of paper. The absorbent or ion exchanger is precipitated into the pores of the paper. One end of the prepared paper is dipped into distilled water and allowed to stand until the water has climbed about a centimeter along the paper. It is then removed and dipped into a solution of the materials to be separated. After the unknown solution has climbed about 2 cm, the paper is removed from the unknown solution and returned to the distilled water. After the water has climbed to about 12 to 16 cm, the strip is removed and dried between filter paper. Brushing the dried paper strip with the proper developing agent will produce bands similar to those produced in the adsorption column.
Numerous studies have been made of the paper-strip method for separating cations, anions and metal complexes. The procedure is similar to that of column chromatography.
The paper-strip method has the advantage that the developing reagent does not pass through the adsorbent as it is required in column chromatography. The strip method requires a minimum of test solution, about 0.1 mm, several developers may be applied to the same strip.
The paper-strip method has been applied to quantitative determination of the inorganic cations and to many organic materials.
GAS ANALYSIS
Special techniques are usually employed in the analysis of the gases. Since the analysis of a gas, or gas mixture usually involves the measurement of a volume and only very rarely the weighing of a sample, the results are most frequently reported in per cent by volume rather than per cent by weight.
It must be remembered that the volume of a gas is greatly dependent upon both the temperature and the pressure and it is necessary to adjust each measurement to standard conditions of temperature and pressure. It is obvious then that these conditions must remain constant over the course of the analysis.
NOTES AND COMMENTARY
| the results are... reported in per cent by volume rather than per cent by weight -результаты даются в процентах относительно объема, а не относительно веса | over the course of the analysis -в течение всего процесса анализа |
SOME PHYSICAL METHODS USED IN GAS ANALYSIS
The relative proportions of various components of gas mixtures can be determined by merely measuring some physical constants of the mixture: the density, the viscosity, the thermal conductivity, heat of combustion, ionization potential.
Condensation methods are often applicable in the separation of complex mixtures of gases. This method has been applied to the gases of the argon group and of natural gas mixtures.
The application of the methods of mass spectrometry to gas analysis has been extensive. The use of a mass spectrometer in analysis enables one to determine the components of mixtures of hydrocarbons, fuel gases, rare gases, etc.
Thermal conductivity applied to gas analysis is rapid, simple to carry out and adaptable to continuous operation.and process control.
Some attempts to apply the methods of emission and absorption spectroscopy to gas analysis have been made.
Other miscellaneous methods include magnetic susceptibility, micro-wave analysis, acoustical method based on the principle that the velocity of sound in a gas is a function of the molecular weight of the gas, inferometric methods, diffusion methods and others.
NOTES AND COMMENTARY
| enables one - обеспечивает simple to carry out - прост в поизводстве | based on - основан на |
ANALYSIS OF MIXTURES
Many problems of quantitative chemistry are more complex than determining the amount of a pure substance or the composition of an aqueous solution of a pure compound. Often the problem arises simply because the compound or solution has an unknown or complex composition.
There are three fundamental schemes than can be used in the problem at hand.
1. Phase separation: The metal ion, A, can be determined
without interference front B if we separate A from B. We do
this by preparing a two-phase system such that all of A is in
one phase and all of B is in the other phase.
2. Selective determination: The metal ion, A, can be deter
mined in the presence of B if we can find a determination
which is selective toward A, ignoring B.
3. Combined determination: The two metal ions, A and B,
can be determined together. This type of measurement com
bined with another independent measurement gives the amount
of each ion.
NOTES AND COMMENTARY
| are more complex than - более сложны чем | the problem at hand - рассматриваемая проблема |
EXTRACTION
Liquid-liquid phase separations are possible when a metal forms a compound soluble in two immiscible liquids. The distribution of the compound between the two liquids can be considered to be a solubility contest. Practical considerations dictate that one of the liquids must be water. Among the liquids other contestants are: carbon tetrachloride, chloroform, carbon disulfide, ethers, paraffin hydrocarbons, and aromatic hydrocarbons. Alcohols cannot be added to this list.
Most inorganic compounds just are not interested in the organic solvents which are immiscible with water. Sometimes, however, a complexing agent can be found which will coach an inorganic substance into an organic solution. Cupric, lead, zinc, silver, mercuric, and cadmium salts, for example, will dissolve, in either chloroform or carbon tetrachloride if it contains some dithizone.
PRECIPITATION
The most generally useful technique for accomplishing a phase separation is the solid-liquid separation, obtained in a precipitation.
To have wide applicability a precipitant should form compounds with many metal ions, and these compounds should have a wide range of solubility. To obtain proper conditions, the concentration of the precipitant should be controlled easilly.
What sort of precipitant is most desirable depends upon many variables: how many samples must be determined, what constituents are present, what reagents are at hand, what time is available, what accuracy is desired, etc.
ELECTROLYSIS
Another type of solid-liquid phase separation is furnished by electrolytic techniques. Two electrodes are placed in the solution of interest, and a current is passed through the solution at a voltage sufficient to reduce some but not all of the metals present. If the current and concentrations are adjusted properly, the metals which are reduced will plate out on the electrode in a pure metallic deposit which can be dried and weighed directly.
NOTES AND COMMENTARY
| the solution of interest - исследуемый раствор will plate out - отлагается | to reduce some but not all - для частичного удаления |
ION EXCHANGE
Another procedure utilizing the elution technique is the ion exchange separation. This time the solid (which is called the substrate) is a salt or compound with salt-forming capacity, something like a sulfonic acid group. When a solution containing metal ions is passed through such an acid substrate, the ions can replace the protons, forming salts. Further elution repeats many times the cycle of ion exchange, replacement of a proton by a salt ion, followed by replacement of the metal ion by proton. As in chromatography, the repetitious procedure magnifies small differences in saltforming capacity and permits separations which are extremely difficult by any other method.
Ion exchange substrates fall into two groups: cation exchangers and anion exchangers. Acidic functional groups are
effective as cation exchangers. These groups include sulfonic acids,— SO3H; carboxylic acid,— COOH; phenols or alcohols,— OH; and mercaptans,— SH. These interact only with cations and by an exchange reaction of the following sort:
— SO3H + M+ = — SO2M + H+.
Most anion exchangers are amines, depending upon one of the functional groups — NH2,— NHR, and NR2. These groups form ammonium type salts, and the anion can be displaced:
— NH2 • HC1 + X = — NH2 • HX + Cl.
NOTES AND COMMENTARY
| saltforming capacity - способность солеобразования something like - нечто вроде | by any other method - любым другим методом fall into two groups - разделяются на две разные группы |
