II. Make up a description of any element you like.

Unit 6. ANALYTICAL CHEMISTRY

METHODS OF ANALYSIS

The analysis of a complex material usually involves four steps, sampling, dissolving the sample, separating mutually interfering substances, and determining the constituents of interest. The first step, sampling can be a significant problem, particularly in industrial applications.

Sampling is complete when the subdivision is small enough to permit analysis.

The second step is the dissolving of a sample. If we know the nature of the sample we use a suitable reagent.

I/Gravimetric methods involve a weighing operation as the final measurement.

Gravimetric analysis have been developed for almost every­thing from A(luminium) to Z(irconium).

Gravimetric procedures may be done in various ways: by precipitating, by dissolving, by removing as a volatile com­pound^.

Volumetric methods involve measurement of that volume of a solution of known concentration which reacts with a known amount of the sample. Such a solution is called a standard so­lution.

Volumetric techniques are now applicable to most of the elements and to many specific inorganic and organic com­pounds. They are widely used in all phases of chemistry, in medicine, and in many allied sciences.

Physico-chemical methods depend upon the measurement of physical properties other than mass and volume. Such me­thods are important when the simpler methods of analysis are inadequate.

 

METHODS OF SEPARATION

Methods of separating a solid and a liquid are built around two processes, filtration and centrifugation.

Filtration is the process of passing the suspension of solid and liquified through a porous barrier which will trap the solid. The barrier may be filter paper, sintered glass, asbestos matting, glass wool and others.

Centrifugation is mechanized setting (or floating) and de­pends upon the difference between the densities of the solid and the solution. Gravitational setting is usually inadequate. A centrifuge can be used to enhance the gravitational force moving the particles. Most centrifuges operate at hundreds of revolutions per minute. Extremely difficult separations require speeds of tens of thousands of revolutions per minute.

 

NOTES AND COMMENTARY

are built around two processes – базируются на двух процессах the process of passing … through – прцесс пропускания … через

mechanized setting - механическое осаждение depends upon – зависит revolutions per minute – обороты в минуту

ION EXCHANGE METHODS IN ANALYTICAL CHEMISTRY

Ion exchange is now one of the recognized processes of chemical engineering. It has been applied to the separation processes of quantitative analysis.

General Principles. By ion exchange we mean the exchange of ions of like sign between a solution and a solid insoluble body in contact with it. For such an exchange to be possible, the solid must contain ions of its own. The solid (called the ion exchanger) must have an open, permeable molecular struc­ture, so that ions and solvent molecules can move freely in and out. Many substances, both natural and artificial, have ion exchanging properties.

In analytical work we are primarily interested in the syn­thetic organic exchangers. These have a high capacity for hold­ing ions and they are not broken down by acids or alkalies, they have a relatively simple composition.

 

NOTES AND COMMENTARY

by "ion exchange" we mean -под ионным обменом мы подразумеваем of like sign -одноименные по знаку for such exchange to be possible –чтобы осуществить этот обмен

of it own -свои собственные can move freely in and out -могут свободно входить и выходить

CHROMATOGRAPHY AND ION EXCHANGE TECHNIQUE

Chromatography is a method of chemical analysis based upon the selective absorption and partial fractionation of vari­ous substances by certain suitable materials. The method is simple and requires a minimum of special equipment. The tech­nique consists of pouring a solution through a column con­taining a suitable adsorbing material. A selective developing agent is then passed through the column and the different substances in the solution are spread down the column into layers visibly separated from one another, provided the sub­stances are colored. In the case of colorless substances, the layers of the different substances may be located by the use of ultra-violet light or by chemical tests.

This method was first described by the Russian botanist Tswett, in 1906. Tswett was engaged in the extraction and purifictaion of plant pigments.

Methods of chromatography have been applied to the sepa­ration of the rare earths and a number of procedures, based on chromatography techniques, have been developed for the separation of the inorganic cations and anions.

NOTES AND COMMENTARY

are spread down … into layers -оседают пластами

provided -при условии, что was engaged in -занимался

CHROMATOGRAPHY TECHNIQUES

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