The sample and acid mixture having been put in the combustion flask the latter is shaken, at intervals, for an hour, and the contents cooled.
The conversion of the nitrates into nitro-phenol compounds is finished in this time, and the next step consists in reducing these bodies to the amido-phenol group. This is accomplished in the cold by nascent hydrogen produced by the addition of zinc dust to the mixture. From one to three grams of the dust are to be used in proportion to the quantity of nitrates originally present.
The flask should be placed in a cooling mixture and the zinc dust added in small portions to prevent a too violent evolution of hydrogen. After the reduction is ended the flask is allowed to stand for two hours, after which the combustion, distillation, and titration are accomplished in the usual way. On cooling, after the end of the combustion, the contents of the flask become solid. They may be brought again into liquid state by shaking and gentle warming.
193. The Official Kjeldahl Method for Nitric Nitrogen.—As has already been stated, the presence of certain organic compounds, rich in hydrocarbons, permits the reduction of nitric nitrogen to ammonia by combustion with sulfuric acid. Benzol, phenol, and salicylic acid have all been used for this purpose. The official chemists have adopted for their method the salicylic acid process first proposed by Scovell.[164]
Besides the reagents and apparatus given under the Kjeldahl method there will be needed:
(1) Zinc dust: This should be an impalpable powder; granulated zinc or zinc filings will not answer.
(2) Sodium thiosulfate:
(3) Commercial salicylic acid:
It is found most convenient to prepare a solution of 33.3 grams of salicylic acid in one liter of the strongest sulfuric acid, and keep it for use rather than to mix it for each combustion. We prefer the thiosulfate process first mentioned below. In the zinc dust method there has been noticed a tendency for the distillation flask to break just at the end of the process.
The Manipulation.—Place from seven-tenths to three and five-tenths grams of the substance to be analyzed in a kjeldahl digesting flask, add sixty cubic centimeters of sulfuric acid containing one gram of salicylic acid, and shake until thoroughly mixed, then add five grams of crystallized sodium thiosulfate; or add to the substance thirty cubic centimeters of sulfuric acid containing two grams of salicylic acid, then add gradually two grams of zinc dust, shaking the contents of the flask at the same time. Finally place the flask on the stand for holding the digestion flasks, where it is heated over a low flame until all danger from frothing has passed. The heat is then raised until the acid boils briskly and the boiling continued until white fumes no longer pour out of the flask. This requires about five or ten minutes. Add now approximately seven-tenths gram of mercuric oxid or its equivalent in metallic mercury, and continue the boiling until the liquid in the flask is colorless or nearly so. In case the contents of the flask are likely to become solid before this point is reached add ten cubic centimeters more of sulfuric acid. Complete the oxidation with a little potassium permanganate in the usual way, and proceed with the distillation as described in the kjeldahl method. The reagents should be tested by blank experiments.