PROTEINS. 99 



9. Sodium Chloride and Acetic Acid Test. Mix two volumes 

 of albumin solution and i volume of a saturated solution of sodium 

 chloride in a test-tube, acidify with acetic acid and heat to boiling. 

 The production of a cloudiness or the formation of a precipitate 

 indicates the presence of albumin. 



10. Acetic Acid and Potassium Ferrocyanide Test. To 5 c.c. 

 of dilute egg albumin solution in a test-tube add 5-10 drops of acetic 

 acid. Mix well, and add potassium ferrocyanide, drop by drop, 

 until a precipitate forms. This test is very delicate. 



Schmiedl claims that a precipitate of Fe(Cn) 6 K 2 Zn or Fe(Cn) 6 - 

 Zn 2 , is formed when solutions containing zinc are subjected to this 

 test, and that this precipitate resembles the precipitate secured with 

 protein solutions. In the case of human urine a reaction was 

 obtained when 0.000022 gram of zinc per cubic centimeter was pres- 

 ent. Schmiedl further found that the urine collected from rabbits 

 housed in zinc-lined cages possessed a zinc content which was suffi- 

 cient to yield a ready response to the test. Zinc is the only in- 

 terfering substance so far reported. 



11. Salting-out Experiments. (a) To 25 c.c. of egg albumin 

 solution in a small beaker add solid ammonium sulphate to the point 

 of saturation, keeping the temperature of the solution below 40 C. 

 Filter, test the precipitate by Millon's reaction and the filtrate by the 

 biuret test. What are your conclusions? (b) Repeat the above ex- 

 periment making the saturation with solid sodium chloride. How 

 does this result differ from the result of the saturation with am- 

 monium sulphate? Add 2-3 drops of acetic acid. What occurs? 

 All proteins except peptones are precipitated by saturating their so- 

 lutions with ammonium sulphate. Globulins are the only proteins 

 precipitated by saturating with sodium chloride (see Globulins, page 

 102), unless the saturated solution is subsequently acidified, in which 

 event all proteins except peptones are precipitated. 



Soaps may be salted-out in a similar manner (see p. 137). 



12. Coagulation or Boiling Test. Heat 25 c.c. of dilute egg 

 albumin solution to the boiling-point in a small evaporating dish. 

 The albumin coagulates. Complete coagulation may be obtained by 

 acidifying the solution with 3-5 drops of acetic acid 1 at the boiling- 

 point. Test the coagulum by Millon's reaction. The acid is added 

 to neutralize any possible alkalinity of the solution, and to dissolve 

 any substances which are not albumin (see further discussion on 

 page 316). 



1 Nitric acid is often used in place of acetic acid in this test. In case nitric 

 acid is used, ordinarily 1-2 drops is sufficient. 



