286 



PHYSIOLOGICAL CHEMISTRY 



about 5 c.c. of concentrated nitric acid (sp. gr. 1.42), mix, and let stand for 5 

 minutes, to permit the flocking out of the silver chloride. Then add with a 

 spatula an abundant amount of powdered ferric ammonium sulfate (about 0.3 gm.) 

 and titrate the excess of silver nitrate with the standard sulfocyanate solution 

 until the definite salmon-red (not yellow) color of the ferric sulfocyanate persists 

 in spite of stirring for at least 15 seconds. 



Calculation. 5.00 (cc.AgNO 3 used) - x(cc.KCNS used) = mg. of Cl per 

 c.c. of blood (or plasma). To express as NaCl multiply Cl value by 1.65. 



Interpretation. Whole blood normally contains from 0.45 to 0.50 

 per cent of chlorides expressed as sodium chloride, and the plasma 

 from 0.57 to 0.62 per cent. Higher values are obtained in nephritis, 

 and this determination may aid in deciding whether or not salt should 

 be restricted in the diet. There may be a decrease of chloride in 

 diabetes and fevers as well as in pneumonia with chloride retention. 



FIG. 90. ASPIRATION APPARATUS FOR UREA DETERMINATION. 



(Myers: "Practical Chemical Analysis of the Blood," C. V. Mosby Co., St. Louis, 

 1921. Myers suggests the use of test tubes within the cylinders as illustrated for ease in 

 manipulation). & 



METHODS CONTINUED 



i. Urea, The Urease Method. Van Slyke and Cullen's 1 Modification of 

 Marshall's Method. 2 



Principle. See Urease Method, Chapter XXVII. 



Procedure. Run 3 c.c. of fresh blood (carefully measured with an accurate 

 pipette) Into a 100 c.c. test-tube containing i c.c. of a 3 per cent solution of potas- 

 sium citrate (to prevent clotting). Add 0.5 c.c. of the urease solution 3 and 2 or 3 

 drops of caprylic alcohol (to prevent foaming). 4 After ten minutes add 15 c.c. 



be dissolved in a liter of water. By titration under the conditions specified under "Pro- 

 cedure" and by proper dilution prepare a standard such that 5 c.c. are equivalent to 5 c.c 

 of the silver nitrate solution. 



The solid ferric alum is used rather than a solution, in order to insure a very high con- 

 centration in the mixture to be titrated. It is powdered in order to facilitate its solution. 



1 Van Slyke and Cullen: /. Am. Med. Ass'n, 62, 1558, 1914. 



2 Marshall: Jour. Biol. Chem., 15, 487, 1913. 



3 The enzyme solution is prepared as described under "Reagents and Solutions," p. 646. 



4 Lee (St. Luke's Hosp. Med. and Surg. Rep., 4, 1917) suggests the use of a mixture con- 

 taining 70 per cent phenyl ether and 30 per cent amyl alcohol as a substitute for caprylic 

 alcohol, while Hammett (Jour. Biol. Chem., 33, 381, 1918) uses a mixture of equal parts 

 of amyl alcohol, toluene and ethyl alcohol. 



