Carius method

Carius method The quantitative determination of S and halogens in covalent (organic) compounds by complete oxidation of the compound with cone, nitric acid and subsequent estimation of precipitated AgX or BaS04. 

Carius tube A thick-walled glass lube originally used in the Carius method but used in any reaction involving volatile materials. 

For a description of the Carius Method on a semi-micro scale, see pp. 502. et seq. 

Two simple methods for the semi-micro estimation of halogens are available, (a) the Carius method, and (b) the Parr bomb method. It should be emphasised that there are other methods available for these estimations on the true micro scale, but they do not lend themselves, by virtue of the balances, apparatus and manipulation required, to semi-micro work, or to the intermittent usage which class-work necessarily entails. 

Salamon has recommended the following method, and appended are the results obtained compared with those yielded by the Carius method and the obviously inadequate lime method. 

No. Lime Method, Chlorine, per Cent. Sulphuric Acid Method, per Cent. Nitro-sulphuric Acid Method, per Cent, Carius Method, per Ceut,... 

Nitric acid is also useful as an oxidant for the formation of sulphate from sulphones. Two such methods have been developed, firstly the established AOAC method which involves oxidation with a mixture of nitric acid and bromine221, and secondly the Carius method222. The latter is probably the oldest method used for the determination of sulphones as sulphate. This oxidation procedure involves heating the sulphone with concentrated nitric acid and sodium chloride at 280-300 °C in a sealed tube. The traditional method as described is prone to explosions. This problem may be alleviated by using less nitric acid whilst employing an oxygen atmosphere2 23,224. The Carius method is slower than the other oxidation methods described above but it usually yields the best results. 

The Carius method is trustworthy but time-consuming. It is perferableto determine iodine by Leipert s method (p. 76) and chlorine and bromine as follows. 

The determination of sulphur by the Carius method is carried out in the same way as is the determination of halogen, but, in place of the silver nitrate, anhydrous barium chloride is here used. 

If halogen and sulphur have to be determined simultaneously in one substance, first determine the halogen by the Carius method. Collect the filtrate from the silver halide in a carefully cleaned filter flask, transfer to a beaker of heat-resistant glass, dilute to 120-150 c.c., and precipitate the sulphuric acid from the boiling solution with 1 per cent barium nitrate solution absolutely free from halogen. 

Analysis of the Benzyl Chloride.—The quantitative determination of halogen in substances containing halogen in aliphatic combination is not carried out in a sealed tube by the Carius method (cf. p. 69), but by hydrolysis with standard alcoholic potassium hydroxide solution. Since this method is very often used, a check on the purity of the present preparation may be combined with practice in this method of analysis. 

The importance of the determination of sulphuric acid and sulphates lies very largely in the fact that sulphur in various forms of combination, and also free sulphur, is frequently determined quantitatively by primary conversion into sulphuric acid or sulphate, followed by actual estimation in the latter form. For example, a convenient volumetric process applicable to the Carius method for determining sulphur in organic compounds is to neutralise the solution of sulphuric acid after removal of the excess of nitric acid by evaporation, and then add silver nitrate, by which the sulphate is converted into silver sulphate. This is separated from the excess of silver nitrate by means of its insolubility in alcohol, and is then estimated by dissolving in dilute nitric acid and titrating the silver with standard thiocyanate solution.3... 

The Carius method can also be adapted for the estimation of selenium in organic compounds.8 About 0-2 gram of the substance is treated with nitric acid (sp. gr.=l-4) and about 0-5 gram of silver nitrate. The contents of the tube are transferred to a basin, evaporated to dryness, washed with alcohol and then dissolved in 20 per cent, nitric acid. 

The Carius method can be worked on a micro (see Pregl, loc. tit.) or hemi-macro scale. Halogens can be estimated by other methods (see, inter alia, Bobranski and Sucharda, loc. tit.) in which the substance is completely oxidised in the dry state, Cl and Br being trapped in solid BaC03,1 in Na2S03 in a special apparatus. 

Halogens by the Carius Method.— The halogens, chlorine bromim iodinej are also determined by the Carius method. In this case, crystalline silver nitratej nitric acid and the compound under examination are all introduced into the Carius tube. The nitric acid must be kept separate from the compound until after the tube is sealed. After sealing the tube as before, the contents are mixed and the tube heated in the bomb oven. The halogen of the organic compound is converted into silver halide which, when the tube is opened, is carefully washed out, filtered, and weighed. 

TABLE 7. Comparison of percentage arsenic found by the micro Carius method at different temperatures, with and without potassium chloride ... 

In quantitative, as in qualitative, analysis, covalently bonded halogen must be converted into halide ion. The organic compound is heated either (a) in a bomb with sodium peroxide or (b) in a sealed tube with nitric acid (Carius method), The halide ion thus formed is converted into silver halide, which can be weighed. 

Problem 2.7 Calculate the percentage composition and then the empirical formula for each of the following compounds (a) Combustion of a 3.02-mg sample of a compound gave 8.86 mg of carbon dioxide and 5.43 mg of water, (b) Combustion of an 8.23-mg sample of a compound gave 9.62 mg of carbon dioxide and 3.94 mg of water. Analysis of a 5.32-mg sample of the same compound by the Carius method gave 13.49 mg of silver chloride. 

Sulfur in an organic compound is converted into sulfate ion by the methods used in halogen analysis (Sec. 2.26) treatment with sodium peroxide or with nitric acid (Carius method). This is then converted into barium sulfate, which is weighed. 

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