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 

 glycogen 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 filtrate 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 filtrate. 



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

 original solution with magnesium sulphate, or half saturate it with ammo- 

 nium 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 filtrate after acidulation 

 with acetic acid. A precipitate = albumin. 



(b) Half saturate the filtrate 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 filtrate from (b) with ammonium sulphate crystals. A 

 precipitate = secondary proteoses. Filter. 



(d) To the filtrate 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) (b), (c), and (d). 



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

 coagulable 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 sul- 

 phuric 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 

 precipitated by basic lead acetate, dextrin is not (p. 608). Both are 

 changed into reducing sugar by boiling with dilute acid. 



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

 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 shows that cane-sugar was originally 

 present, and has been inverted. 



