PROTEINS 105 



solutions. In the case of human urine a reaction was obtained when 

 0.000022 gram of zinc per cubic centimeter was present. Schmiedl 

 further found that the urine collected from rabbits housed in zinc-lined 

 cages possessed a zinc content which was sufficient to yield a ready re- 

 sponse to the test. Zinc is the only interfering substance so far 

 reported. 



10. 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 4oC. Filter, test the precipitate by 

 Millon's reaction and the filtrate by the biuret test. What are your conclu- 

 sions? (b) Repeat the above experiment, making the saturation with solid 

 sodium chloride. How does this result differ from the result of the saturation 

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



All proteins except peptones are precipitated by saturating their 

 solutions with ammonium sulphate. Globulins are the only proteins 

 precipitated by saturating with sodium chloride (see Globulins, page 

 1 08), 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 page 181). 



11. 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. Com- 

 plete 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 solu- 

 tion, to dissolve any substances which are not albumin and to facilitate 

 coagulation (see further discussion on pages 117 and 424). 



12. Coagulation Temperature. Prepare four test-tubes each containing 5 c.c 

 of neutral egg albumin solution. To the first add i drop of 0.2 per cent hydro- 

 chloric acid, to the second add i drop of 0.5 per cent sodium carbonate solution, 

 to the third add i drop of 10 per cent sodium chloride solution and leave the 

 fourth neutral in reaction. Partly fill a beaker of medium size with water and 

 place it within a second larger beaker which also contains water, the two vessels 

 being separated by pieces of cork. Fasten the four test-tubes compactly together 

 by means of a rubber band, lower them into the water of the inner beaker and 

 suspend them, by means of a clamp attached to one of the tubes, in such a manner 

 that the albumin solutions shall be midway between the upper and lower sur- 

 faces of the water. In one of the tubes place a thermometer with its bulb entirely 

 beneath the surface of the albumin solution (Fig. 36). Gently heat the water in 

 the beakers, noting carefully any changes which may occur in the albumin solu- 

 tions and record the exact temperature at which these changes occur. The 

 first appearance of an opacity in an albumin solution indicates the commencement 

 of coagulation and the temperature at which this occurs should be recorded as 

 the coagulation temperature for that particular albumin solution. 



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

 ordinarily 1-2 drops are sufficient. 



