THE WORLD OF SCIENCE

Waipa Post, Volume 51, Issue 3645, 19 July 1935, Page 9

THE WORLD OF SCIENCE

DELICATE METHODS OF ANALYSIS.

HOW THE COLORIMETER t WORKS.

In a previous article I drew attention to the possibility of the occurrence of minute amounts of lead in foodstuffs, drugs, and other materials, and emphasised the desirability of control, on account of the cumulative toxic nature of lead compounds. Since the ordinary methods of analysis are inapplicable to the estimation of extremely small amounts of chemical compounds, delicate colorimetric methods have come into prominence, and in recent years have been greatly extended. The principles involved may be illustrated by reference to the case of lead and a few other selected substances. Solutions of lead salts of ordinary concentrations give a black precipitate of lead sulphide on the addition of a solution of sodium sulphide. If, however, the reacting solutions are dilute, the molecules of the sulphide do not clump together to form large aggregates, which are precipitated out, but, instead, they remain suspended in the liquid almost indefinitely, giving what is called a 1 collodial solution of lead sulphate. This behaviour is, indeed, very general, and by working with sufficiently dilute solutions and by careful control of the conditions of reaction, it is possible to prepare collodial solutions of the majority of substances which are ordinarily insoluble. In the case of coloured compounds, the collodial solutions also are coloured and lend themselves to colorimetric methods of estimation.

STANDARD OF COMPARISON. In such methods it is necessary to prepare a solution of a pure salt of the substance being investigated. This is done by direct weighing, and serves as a standard of comparison. Thus in the determination of the amount of contamination of a drug by lead a solution of lead nitrate conaining, say, 1 per cent of lead, is first prepared. By dilution of this a range of weaker solutions may be made, containing one part of lead per 1000, or per 10,000, or so on. A known weight of the drug is then dissolved in a fixed weight of water, and a drop of sodium sulphide solution is added. This produces a brown colouration due to the formation of lead sulphide, the depth of colour depending on the amount of lead present in the drug. This colour is then compared with that obtained adding sodium sulphide to a solution of lead nitrate prepared by dilution of the standard and containing say, one part of lead per million of water. If, on looking through a layer of this of the same thickness ifs the layer of the drug solution, the colour is not found to be sufficiently intense, a further trial is made with a solution containing two parts of lead per million, and the trial is continued until a solution is obtained which gives a colour match with the sulphided solution of the drug. The amount of lead in the two cases is obviously then the same, since equal amounts of lead sulphide are responsible for the similar depths of colour produced in the two solutions. The amount of lead in the standard solution is known and so the parts per million of the drug may be computed. The presence of other metals, such as copper, would interfere with the test, for they also give black sulphides, but this difficulty is easily overcome, as the addition of a little potassium cyanide and ammonia prevents the formation of such sulphides.

COLORIMETERS DEVISED. It will be evident that the method as described is tedious, because of the number of trials that have to be made. Also, there is a possibility of error, since it is exceedingly difficult to judge exactly when the colours are matched, especially when looking with the naked eye at two tubes side by side. Various forms of colorimeters have, therefore, been devised to render these tests less laborious and more accurate, and these are indeed very convenient for such work. The general construction and the working principles of such, instruments are easy to understan'd. Daylight, or artificial light, is passed light, or artificial light, is passed through two glass cells, which are fitted with movable plungers, and by means of a system of prisms the light is passed to an eye-piece. ' One half of the field seen in the eye-piece represents light from the left-hand cell, and the remainder of the field is illuminated by the light from the other cell. It is easy, therefore, to detect any difference in intensity. of light coming from the cells. . In an estimation the standard solution is placed in one cell, and the solution to be assayed in the other. The plunger in the latter is raised or lowered to increase or decrease the thickness of the layer or solution through which the light passes'. When ■ illumination in the eye-piece is uniform over the entire field the colours are matched and the amounts of lead in the two solutions are inversely proportional to

the thicknesses of the layers through which the light passes. These thicknesses are read off directly on a scale attached to the side of the instrument.

A similar process may be applied in a number of other cases. Flour, for example, is often bleached by the miller in order that he can offer to the public an attractive white product which is apparently preferred to the somewhat cream-coloured untreated natural product. Chemical bleaching of flour is on the whole injurious, and it is contended by many that any form of bleaching is undesirable, since even if harmful chemicals are not introduced in the process nevertheless the nutritive value of the flour is likely to be reduced.

BLEACHING AGENT. In one modern process bleaching is accomplished by circulating the flour during milling through a chamber in which it is subject to the action of oxides of nitrogen, which are obtained by passing an electric discharge through air. The colouring matter is thus discharged, but the flour absorbs some of the bleaching agent, which is fixed as nitrite. In many countries the bleaching of flour is prohibited by the food and drug regulations, but in others the above process is permitted—a definite limit to the amount of nitrite permissible in the flour being prescribed to control the extent of the bleaching treatment.

The presence of two parts of nitrite per million may be regarded as evidence of such bleaching, and the estimation of such , small amounts again rests on a colorimetric method. Under certain conditions nitrites react with aromatic organic bases to form dye-stuffs, aniline being the simplest compound of this type. In the estimation of nitrites aniline itself is not employed, since more complex derivatives give much more intensely-col-oured dyes.

An equeous extract of the flour is made, which contains the nitrite originally present in the material, and this solution is treated with a little sulphuric acid and the organic base. On standing, the dye is formed, and a colour develops, the intensity of which depends on the amount of nitrite present. A standard solution of the dye is prepared by adding a definite weight of pure potassium nitrite to an acidified solution of the organic base, and this is compared with the colour of the flour extract.—iA.K.M. in the Melbourne Argus.