378. Qualitive Tests for Albumin.—Albumin is one of the chief proteids and exists in both animal and vegetable substances. It is soluble in cold water and may therefore be separated from many of its nearly related bodies which are insoluble in that menstruum. In aqueous solutions its presence may be determined by the general reactions for proteid matters given above or by the following tests:
Precipitation by Heat.—Albumin is coagulated by heat. Vegetable albumins become solid at about 65° and those of animal origin at a somewhat higher temperature (75°). Some forms of animal albumin, however, as for instance that contained in the serum, coagulate at a lower temperature.
Precipitation by Acids.—Dilute acids also precipitate albumins especially with the aid of heat. Practically all the albumins are thrown out of solution by application of heat in the presence of dilute acids.
Mercuric Salts.—Acid mercuric nitrate and a mixture of mercuric chlorid, potassium iodid and acetic acid completely precipitate all albuminous matters.[344]
The yellow or red color produced on heating albumin with the mercuric nitrate is known as Million’s reaction.
379. Qualitive Test for Peptones and Albuminates.—When peptones and albuminates are dissolved in an excess of glacial acetic acid and the solution treated with sulfuric acid a violet color is produced and also a faint fluorescence.
Separation of Peptones and Albumoses.—In a solution of peptones and albumoses the latter may be precipitated by saturating the solution with finely powdered zinc or ammonium sulfate.
Action of Phosphotungstic Acid.—All proteid matters in aqueous, alkaline or acid solutions, are precipitated by sodium phosphotungstate in a strongly acid solution. Acetic, phosphoric, or sulfuric acid may be used for producing the required acidity, preference being given to the latter.
Action of Trichloracetic Acid.—In the precipitation of albumin by trichloracetic acid, there is formed a compound of the two bodies which to 100 parts of albumin has 26.8 parts of the trichloracetic acid.
The different albuminoid bodies obtained by precipitation behave in a similar manner. There are formed flocculent precipitates insoluble both in dilute and concentrated acids in the cold and also at a high temperature, with the exception of the hemialbumose compound.[345]