The volume of the aqueous solution is completed with water to half a liter. One hundred cubic centimeters of this solution are used for the determination of total nitrogen, and another 100 cubic centimeters for the determination of ammoniacal nitrogen by distillation with barium carbonate. A part of the ammonia may have escaped during the preliminary distillation of the alcohol and therefore the amount found may not represent the whole amount originally present. The use of the above determination is principally to ascertain the correction to be made in the amount of total nitrogen found in the first 100 cubic centimeters of the solution.
422. Treatment of the Residue Insoluble in Alcohol.—The residue insoluble in alcohol is washed from the filter into the beaker in which the first solution was made. The aqueous mixture is warmed on a water-bath until the alcohol adhering to the precipitate is completely evaporated, when the contents of the beaker are poured upon a filter free of nitrogen. A small part of the albumose, by reason of the treatment with alcohol, tends to remain undissolved, and it is advisable to collect this albumose upon a filter, wash it well with hot water and estimate the nitrogen therein. The quantity of nitrogen thus found is to be added to the albumose nitrogen determined as described later on.
The total filtrate obtained from the last filtration is made up to a volume of half a liter, of which fifty cubic centimeters are used for the determination of total nitrogen, fifty cubic centimeters for the determination of gelatin, albumose and peptone, and 100 cubic centimeters for the residual peptones. The albumose, together with the gelatin and peptones carried down with it, is precipitated with zinc or ammonium sulfate solution, and its per cent calculated from the amount of nitrogen found in the precipitate. The true peptone is determined by subtracting the quantity of nitrogen determined as albumose from the total nitrogen in solution.
The rest of the liquid, viz., 300 cubic centimeters, is evaporated to a small volume and tested qualitively for true peptones as follows:
To separate the albumose and gelatin a concentrated liquor is treated with an excess of finely divided ammonium sulfate so that a part of the salt remains undissolved. The separated albumose, gelatin and undissolved ammonium salts are collected on a filter, the filtrate mixed with a few drops of dilute copper sulfate solution and a considerable quantity of concentrated soda or potash lye added. Care should be taken that the quantity of copper is not too great, otherwise the peculiar red coloration will be obscured by the blue color of the copper solution.
423. Pancreas Peptone.—The filtrate obtained as described above, by treating the portion of the material insoluble in alcohol with warm water, contains in addition to the albumose and gelatin the whole of the pancreas peptone which may be present. To separate this peptone, 100 cubic centimeters of the aqueous solution are evaporated in a porcelain dish until the volume does not exceed ten cubic centimeters. When cool, at least 100 cubic centimeters of a saturated cooled solution of ammonium sulfate solution are added, the mixture thoroughly stirred, the precipitate collected upon a filter and washed with a cold saturated solution of ammonium sulfate. The contents of the filter are dissolved in boiling water, the filter thoroughly washed and the filtrate and washings evaporated in a porcelain dish with the addition of barium carbonate until, on the addition of new quantities of barium carbonate, no further trace of ammonia can be discovered. The residue is extracted with water, the barium sulfate and carbonate present separated by filtration, well washed and the nitrogen determined in the evaporated filtrate and washings in the usual way and multiplied by 6.25 to determine the quantity of pancreas peptone.
424. Albumose Peptone.—A part of the albumose peptone which may be present is determined in conjunction with the other bodies mentioned above. The chief quantity is found in the solution of the residue insoluble in alcohol in the following manner:
Fifty cubic centimeters of the solution of this residue in hot water are mixed with an equal volume of dilute sulfuric acid, one volume of acid to three of water, in the cold, and a solution of sodium phosphotungstate added until it produces no further precipitate. The precipitate is washed with dilute sulfuric acid and the nitrogen determined therein. The nitrogen thus found is derived from the albumose, pancreas peptone and gelatin. The quantity of nitrogen in the pancreas peptone and gelatin, as above described, is subtracted from the total quantity found in the phosphotungstic acid precipitated, and the remainder represents the nitrogen due to the albumose.
425. Nitrogen in the Form of Flesh Bases Insoluble in Alcohol.—This is determined by subtracting the quantity of nitrogen, determined by the phosphotungstic acid method already described, from the total quantity of nitrogen found in the precipitate insoluble in alcohol and soluble in water.