PRACTICAL EXERCISES 13 



SCHEME FOR TESTING A SOLUTION FOR THE MORE COMMON 

 PROTEINS AND PROTEIN-DERIVATIVES, AND FOR CARBO- 

 HYDRATES 



1. Note the reaction, and whether the liquid is coloured or colourless, clear 

 or opalescent. A reddish colour suggests blood ; opalescence suggests glyco- 

 gen or starch. Try one or more of the general protein tests (e.g., the xantho- 

 proteic or biuret) . If the result is positive, proceed as in 2 ; if negative, pass 

 to 3. 



2. Test for Proteins. (i) If the reaction is acid or alkaline, neutralize with 

 very dilute sodium carbonate or sulphuric acid. A precipitate = acid- or 

 alkali-albumin, according as the original reaction is acid or alkaline. If the 

 original reaction is neutral, no acid- or alkali-albumin can be present in 

 solution. Filter off the precipitate, if any. 



(2) Boil some of the nitrate from (i) (or some of the original solution if 

 it is neutral), acidulating slightly with dilute acetic acid. A precipitate = 

 albumin or globulin. Filter, and keep the nitrate. 



(3) If a precipitate has been obtained in (2), (a) saturate some of the original 

 solution with magnesium sulphate, or half saturate it with ammonium sulphate 

 (i.e., add to it an equal volume of saturated ammonium sulphate solution). 

 If there is no precipitate, globulin is absent, and therefore the' precipitate 

 obtained in (2) must be albumin. A precipitate = globulin. But albumin 

 may also be present in the solution. To see whether this is so, filter off the 

 globulin and boil the nitrate after acidulation with acetic acid. A precipitate 

 = albumin. 



(b) Half saturate the nitrate from (2) with ammonium sulphate (i.e., add its 

 own volume of a saturated solution of the salt). A precipitate = primary 

 proteoses. Filter. 



(c) Saturate the nitrate from (b) with ammonium sulphate crystals. A 

 precipitate = secondary proteoses. Filter. 



(d) To the nitrate from (c) add excess of solid sodium hydroxide in small 

 pieces at a time. Much ammonia is given off. Allow the test-tube to stand 

 fifteen minutes, shaking it at intervals. Then add dilute cupric sulphate, 

 and if much of the sodium sulphate formed remains undissolved, add water 

 to dissolve it. A well-marked rose colour = peptone. 



(4) If no precipitate has been obtained in (2), the solution contains neither 

 albumin nor globulin. To test whether primary or secondary proteose or 

 peptone is present, apply (3) (6), (c), and (d). 



3. Test for Carbo- Hydrates. Use the original solution, freed from coagu- 

 lable proteins, if such have been found, by acidulation and boiling. 



(1) Add iodine. If the solution is alkaline neutralize it before adding the 

 iodine. A blue colour = starch. Confirm by boiling with dilute sulphuric 

 acid and testing for reducing sugar. A reddish-brown colour with iodine = 

 glycogen or dextrin. 



Glycogen gives an opalescent, dextrin a clear, solution. Glycogen is pre- 

 cipitated by basic lead acetate, dextrin is not (p. 689). Both are changed 

 into reducing sugar by boiling with dilute acid. 



(2) Add to some of the original solution cupric sulphate and excess oi 

 sodium hydroxide, and boil. Yellow or red precipitate = reducing sugar. 



(3) If (i) and (2) are negative, boil some of the liquid with one- twentieth 

 of its volume of strong hydrochloric acid for fifteen minutes, and test as in (2). 

 A red or yellow precipitate indicates that a disaccharide like cane-sugar was 

 originally present, and has been inverted. 



