A brief outline of one of the most important processes is given below: Determination of Carbon and Hydrogen.—A weighed quantity (3 to 5 milligrams) of the substance, placed in a small platinum boat, is burned in a current of oxygen (dried by passage through the tubes B and D) in the apparatus shown in the diagram. The hard-glass tube, C, is 4o cm. in length, and contains a filling of chemicals (as indicated in the diagram) designed to arrest the escape of nitrogen, sulphur, halogens, and other products which might vitiate the analysis. The remaining gaseous products of the combustion, water and carbon dioxide, are swept over with air, and are retained, respectively, in the tubes and A2, which are weighed on the microbalance both before and after the experi ment. From the gains in weight of these tubes, the amounts of hydrogen and carbon in the original material are calculated. The gas pressures at different points of the apparatus are carefully regulated, and the temperature of the lead peroxide is maintained at 18o° C by means of the external heating device shown at H. Practically all classes of organic substances may be analysed with great accuracy in this apparatus.
Reference may be made also to the semi-micro-analytical methods, requiring 5o milligrams and upwards of material, of H. Ter Meulen, who has employed manganese dioxide in the com bustion tube. Especially worthy of notice is his method of deter mining oxygen in organic compounds by heating in hydrogen in the presence of a nickel catalyst. (H. Ter Meulen and J. Heslinga, Recueil des Travaux chimiques des Pays-Bas, 1922 et seq.) Some of the following methods of qualitative micro-analysis, may be applied quantitatively also, e.g., spectroscopic analysis. (See A. B. P. Leme, Comptes rendus, 1918, 166, p. 465, and Zeit schrift fur das Gesamtgebiet der Mikrochemie and Mikrophysik, W. Muller, Vienna; 1923 et seq.) Qualitative Microanalysis.—Since each chemical produces its own characteristic spectrum, an examination, by means of the spectroscope (q.v.), of the flame or spark spectrum of a substance reveals the identities of the elements present. The delicacy of the
method far transcends that of the most sensitive chemical reac tions. Thus, the presence of one hundred-thousandth part of a milligram of lithium may be revealed by means of the spectro scope; or, again, five hundred-thousandths of a milligram of the rare gas, neon, present in the atmosphere, may be detected. In this manner the metallic elements gallium, rubidium, indium, caesium and thallium were discovered. The absorption spectra (q.v.) of many inorganic and organic substances are also char acteristic, thus enabling the presence of these substances to be recognised. Examples are : copper sulphate ; the blood pigments (haemoglobin and oxyhaemoglobin) ; and the porphyrins.
Far more widely employed, however, are the purely chemical methods which depend upon the production, on the addition of reagents to the sample or to its solution, of colorations or of colour changes ; of precipitates having characteristic odour, colour, or crystalline form ; or of gases or liquids which can be distilled over and identified. These tests, many of which are very delicate, may be specific for particular compounds, radicals, or elements; or for groups of these. Thus, the presence of i part of copper in 'co million parts of water is shown by the blue colour produced on the successive addition of alcoholic hydrogen peroxide and guaiacum resin in pyridine. The presence of I part of gold in 20 million parts of solution is revealed by the yellow coloration given with o tolidine. Prussic acid (I part in 2 millions of air) may be detected by means of the blue colour which it gives upon a test paper moistened with a solution containing o-tolidine, acetic acid, and copper acetate. There are many hundreds of such tests, some of which are also quantitative.
Methods of manipulating minute quantities of liquids and pre cipitates have been worked out, particularly by F. Emich of Graz. Thus two drops of a mixture of two organic liquids may be frac tionated and the boiling points of each ascertained with accuracy.