§231. ESTIMATION. 239 



genous substances accompany the albumen in aqueous solution he 

 advises the determination first of the total nitrogen ; a part of the 

 original substance is then to be boiled with water for an hour, 

 made distinctly acid with lactic acid, mixed with acetate of 

 lead, and filtered ; the insoluble residue is dried and the nitrogen 

 in it estimated. He thus assumes that all the nitrogen not 

 present in the form of albuminoids passes into aqueous solution, 

 and the nitrogen in the insoluble residue after precipitation with 

 lead indicates the total albumen, both soluble and insoluble. 



In addition to the foregoing precipitants, some of the group 

 reagents for alkaloids — phosphomolybdic, phosphotungstic acid, 

 potassiomercuric iodide, etc. — also throw down albuminous sub- 

 stances (§ 63). Phosphotungstic acid precipitates ]jeptones, and 

 might therefore be used for their estimation in vegetable infusions 

 previously freed from albuminous substances by coagulation or 

 precipitation with lead.^ 



^ See Schulze and Barbieri, Landwirthsch. Versuchsst. xxvi. 213, 230, 234, 

 1881 (Journ. Chem. Soc. xl. 312) ; Cheni. Centralblatt, 714, 731, 747, 761, 

 1881 ; Defresne, Repert. de Pharm, viii. 453, 1881 ; Hofmeister, Zeitschr. f. 

 phys. Chem. iv. 253, 1880. 



From the results recently obtauied by Schulze and Barbieri, it appears pro- 

 bable that peptones are of far more frequent occurrence than could have been 

 anticipated. As plants contain peptonizing ferments, the possibility must not 

 be ignored of peptones being jiroduced during the preparation of aqueous infu- 

 sions ; they are also occasionally found ready formed in plants. The following 

 are the more important properties of peptones : They yield with water solutions 

 from which they are precipitated by alcohol, and redissolved by the addition 

 of water. Estimation, however, by precipitation with alcohol is said to yield 

 unsatisfactory results. In aqueous solution they are not coagulated by warm- 

 ing, nor are they thrown down by nitric acid, alum, ferrocyanide of potassium, 

 or acetate of lead, but they are precipitated by tannin ; and in the presence of 

 neutral salts (sulphate of magnesia, etc.) the separation is often very complete. 

 Peptones are {)recipitated, as above stated, by phosphotungstic acid, and this 

 takes place in an acetic acid solution ; a property that enables us to separate 

 them from other nitrogenous substances thrown down by the same reagent 

 from solutions containing a mineral acid. The most important reaction of 

 peptones is the so-called biuret reaction. An aqueous solution of a peptone 

 assumes a pure red colour on the addition of caustic soda and very dilute 

 solution of sulphate of copper (avoiding excess) ; Pehling's solution produces 

 the same effect. The following might temporarily be recommended as a 

 suitable method for the detection of peptones : The (fresh) material to be 

 examined is triturated with sand and water, strained, washed with water and 

 pressed. The liquors are united, acidified with acetic acid, warmed and 

 filtered from the coagulum. From the filtrate any albuminoids remaining in 

 solution are precipitated by the addition of acetate of lead, or, better, by warm- 

 ing with basic acetate and hydrate of lead, and filtered off. The clear liquid 

 is then rendered strongly acid with sulphuric acid, and the peptone precipitated 



