Quantitative Analysis of Substituent Groups

It will seldom be necessary in this course to resort to methods of ultimate analysis, and it is for this reason that combustion methods for carbon, hydrogen, and nitrogen are omitted from this chapter. This is true also of the Carius determination for halogens and the fusion methods for sulfur, arsenic, and phosphorus. In dealing with compounds of unusual difficulty, the methods of ultimate analysis may have to be employed, but imder such circumstances the student is directed to other som-ces where directions will be found in more detail than would be justifiable here. 

Several of the qualitative methods and particularly the estimation of certain reactive groups, however, are of considerable value in connection with identification work, not merely in the first stages of an analysis but also in connection with confirmatory tests when the preparation of derivatives is not feasible. jMore-over, a considerable number of such tests involve simple volumetric methods and require comparatively little time wffien the standardized solutions arc available. Some of the more adaptable methods are, therefore, given here but the student is encouraged to become familiar T ith more advanced treatments of the subject - that will supply a greater variety of methods together with valuable references to the original articles. 

Determination of Nitrogen by the Kjeldahl Method.— Most organic compounds in which nitrogen is present in non-oxidized form are decomposed when digested with sulfuric acid with the resultant formation of ammonium sulfate. The ammonia may 

The oxidized mixture is allowed to cool, diluted carefully with 250 cc. of distilled water and a few chips of porous plate added. A 40 g. portion of solid stick NaOH c.p. is then carefully added to the cool solution and the flask immediately connected with the condensing apparatus, the receiving flask of which must be in place. After the caustic has dissolved, the solution is slowly distilled until at least 100 cc. of distillate has been collected. This should require about one-half hour. 

The receiving flask consists of a 250-cc. Erlcnmcyer fla.sk containing 30 cc. of standard 0.2 N. sulfuric acid and a few drops of Congo red (or methyl orange) indicator. The tip of the exit tube should be immersed in the standard acid. After the distillation is complete, the excess acid is titrated w ith 0.2 N alkali. 

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