574 PHYSIOLOGICAL CHEMISTRY 



(If a red tint is produced when the first drop of thiocyanate is added an addi- 

 tional 10 c.c. of the standard silver nitrate solution must be introduced. The 

 titration should then proceed as above described and proper allowance made in 

 the calculation for the extra volume of silver nitrate employed.) 



Calculation. Since 2 c.c. of the ammonium thiocyanate solution is equivalent 

 to i c.c. of the silver nitrate solution, divide the burette reading by 2 and sub- 

 tract the quotient from 10 c.c., the quantity of silver nitrate solution taken. 

 This value is the number of cubic centimeters of silver nitrate solution actually 

 used in the precipitation of the chlorides. As i c.c. of the silver nitrate solution 

 is equivalent to o.oi gram of sodium chloride, the number of cubic centimeters 

 of silver nitrate solution used multiplied by o.oi gram will give the weight of 

 sodium chloride in the 5 c.c. portion of urine used. The weight of chlorine may 

 be computed by using the factor 0.006 instead of o.oi. 



Calculate the weight of sodium chloride and chlorine in the 24-hour urine 

 specimen. 



A "short cut" method of calculating the 24-hour output of sodium chloride 

 consists in subtracting the burette reading from 20 c.c., multiplying this value 

 by the total urine volume and pointing off three places. 



Interpretation. See above. 



Calcium and Magnesium 



McCrudden's Methods. 1 -Principle. Urine contains magnesium, 

 phosphates and a small amount of iron, each of which will interfere 

 with the accurate determination of its calcium content if proper con- 

 ditions of acidity are not maintained during the precipitation. In the 

 following method the proper acidity is attained through the use of 

 sodium acetate and hydrochloric acid, and this with slow addition of 

 the ammonium oxalate reduces the danger of occlusion of magnesium 

 oxalate, calcium phosphate, or ferric phosphate in the calcium oxalate 

 precipitate. 



The calcium oxalate precipitate is either ignited and weighed as 

 CaO or determined volumetrically by titration with potassium per- 

 manganate. Magnesium is determined in the filtrate from the calcium 

 determination after destruction of the organic matter. It is determined 

 in the usual way by ignition of the magnesium ammonium phosphate 

 precipitate and weighing as the pyrophosphate. 



Lyman has suggested a nephelometric method for the determination 

 of calcium in urine and feces. 2 



Procedure for Calcium. If the urine is alkaline make it neutral or slightly 

 acid and filter. Take 200 c.c. of the filtered urine for analysis. If it is only 

 faintly acid to litmus paper add 10 drops of concentrated hydrochloric acid (sp. 

 gr. i .20) . If the urine is strongly acid it may be made just alkaline with ammonia 



1 McCrudden: Jour. Biol. Chem., 7, 83, 1910; 10, 187, 1911. 



2 Lyman: Jour. Biol. Chem., 21, 551, 1915. 



