WHE PROTEINS. 17 



Test for lead-blackening sulphur as follows: 



Place about 5 c.c. of dilute NaOH in a test-tube with a 

 small quantity of the substance, and add two drops of lead 

 acetate. Boil the mixture for a few minutes and note the 

 changes. The depth of color may be said to correspond 

 roughly to the amount of neutral sulphur present in the sub- 

 stance. 



Test for total sulphur as follows : 



Mix some of the substance with double its quantity of the 

 fusion mixture (Na 2 CO 3 + KN0 3 ). Place the whole in a 

 small porcelain crucible and warm cautiously until the mix- 

 ture becomes colorless. (If the fusion begins to sputter, 

 remove the flame.) The residue (fused mass) should be 

 nearly white. It is then dissolved in a small quantity of 

 water, filtered, and the filtrate, after the addition of a few 

 drops of HC1, is brought to boiling and treated with BaCl 3 

 solution. What is this white precipitate? Write the equa- 

 tion. 



Phosphorus. Heat some casein with the fusion mixture 

 as in the previous experiment. Dissolve the fused mass in 

 10 c.c. of water acidified with HN0 3 . Filter and add 5 c.c. 

 of ammonium molybdate solution. Warm some minutes at 

 about 80 C. What is the yellow precipitate? 



Iron. Incinerate a small quantity of haemoglobin in a 

 porcelain crucible. The ash should be red. Dissolve out 

 the ash with 10 c.c. of dilute HC1. Filter. Test this solu- 

 tion for Fe with potassium sulphocyanide or potassium 

 ferrocyanide. 



Color Reactions. 



The following tests are based upon reactions taking place 

 between the reagent employed and certain more or less well- 

 defined groups or atomic complexes of the protein molecule. 

 These groups are present in varying proportions in the dif- 

 ferent proteins and in some cases even certain complexes 



