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LESSON II 

 THE PROTEIDS 



1. Tests for Proteids. — The follo^-ing tests are to be tried with a mixture of 

 oue part of white of egg to ten of water. (Egg-white contains a mixture of 

 albumin and globulin.) 



(a) Heat Coagulation. — Faintly acidulate with a few drops of 2-per-cent. 

 acetic acid and boU. The proteid is rendered insoluble (coagulated proteid). 



(6) Precipitation with Nitric Acid. — The addition of strong nitric acid to 

 the original solution also produces a white precipitate. 



(c) Xanthoproteic Reaction. — On boiling the white precipitate produced by 

 nitric acid it turns yellow ; after cooling add anunonia ; the yellow becomes 

 orange. 



{d) Milton's Test. — Millon's reagent (which is a mixture of the nitrates of 

 mercury containing excess of nitric acid ; see p. 5) gives a white precipitate, 

 which turns brick-red on boiling. 



(e) After the addition of a few drops of 20-per-cent. acetic acid, potassium 

 ferrocvanide gives a white precipitate. 



(J) Piotroioski's test. — Add a drop of a 1-per-cent. solution of cupric 

 sulphate to the original solution and then caustic potash, and a violet solution 

 is obtained. 



Repeat experiment (/) with a solution of commercial peptone, and note 

 that a rose-red solution is obtained. This is called the biuret reaction. 



(g) Adaml-iewicz's Reaction (Hopkins's modification). — Add the albumin 

 solution to dUute glyoxyhc acid, and then strong sulphuric acid ; an intense 

 Anolet coloiu- is obtained. Most commercial specimens of glacial acetic acid 

 contain glyoxylic acid and may be used in place of pure glyoxyhc acid in this 

 test. 



2. Action of Neutral Salts.— (a) Saturate the solution of egg-white with 

 magnesium sulphate by adding crystals of the salt and grinding it up 

 thoroughly in a mortar. A white precipitate of egg-globulin is produced. 

 FUter. The filtrate contains egg-albimiin. The precipitate of the globulin 

 is very small. 



(b) Half saturate the solution of egg-white with ammonium sulphate. 

 This may be done by adding to the solution an equal volume of a saturated 

 solution of ammonium sulphate. The precipitate produced consists of the 

 globulin ; the albumin remains in solution. 



(c) Completely saturate another portion with ammonimn sulphate by 

 adding crystals of the salt and grinding in a mortar — a precipitate is pro- 

 duced of both the globuhn and albumin. Filter. The filtrate contains no 

 proteid. 



(d) Eepeat the last experiment (c) with a solution of commercial peptone. 

 A precipitate is produced of the albumoses or proteoses it contains. Filter. 

 The filtrate contains the true peptone. This gives the biuret reaction (see 

 above), but large excess of strong potash must be added on account of the 

 presence of ammonium sulphate. Ammonium sulphate precipitates all 

 proteids except peptone. 



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