55$ PHYSIOLOGICAL CHEMISTRY 



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

 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 o: 

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

 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. 



3. Dehn-Clark Method 1 . Principle. In this method any organic matte: 

 which may interfere with the titrimetric determination of the chlorides is destroyec 

 by oxidation with sodium peroxide. The chlorides are then determined by the Vol- 

 hard-Arnold titration procedure. 



Procedure. To 10 c.c. of urine in a 75-100 c.c. casserole, add 1.0-1.2 grams o: 

 sodium peroxide and evaporate the mixture to dryness on a boiling water-bath. Ii 

 case the residue is not pure white, thus indicating that insufficient sodium peroxide 

 has been added, the residue should be moistened with distilled water, additional 

 sodium peroxide added, and the mixture again evaporated to dryness. When the 

 oxidation is complete, treat the mass with 10-20 c.c. of distilled water and stii 

 until it has practically all been brought into solution. Then introduce a bit of lit- 

 mus paper and add dilute nitric acid (i: i) until the litmus paper turns red and al 

 effervescence ceases. Now place the casserole on a hot plate or on a gauze and heal 

 the contents almost to the boiling-point. 2 To the hot solution add a standard solu- 

 tion of silver nitrate (see page 556) in slight excess. 3 Filter off the silver chloride 

 while the solution is still hot and wash the precipitate thoroughly with distilled 

 water. To the nitrate, add i c.c. of a saturated .solution of ferric ammonium sul- 

 phate and then titrate with a standard solution of ammonium thiocyanate (see page 

 556) until the clear, slightly yellow fluid (or the opalescent, milky fluid, in case there 

 is much excess of silver nitrate) changes to a slight reddish-brown color. The coloi 

 of the end point varies with the individual. The exact end point reached is not sc 

 important as is the securing of the same end point in a series of determinations as 

 that obtained in the standardization of the standard solutions used. 



Calculation. The standard solution of silver nitrate should be made up so that 

 i c.c. equals o.oio gram of sodium chloride and i c.c. of the ammonium thiocyanate 

 should be equivalent to i c.c. of the silver nitrate solution. The calculation is then 

 exactly the same as in the Volhard- Arnold method (page 556) except that the 

 burette reading is not multiplied by 2. 



Interpretation. See page 557. 



4. Mohr's Method. Principle. The organic matter of the urine is destroyed 

 by ignition with potassium nitrate. To the solution of the ash potassium chromate 

 is added and then standard silver nitrate solution run in from a burette. The first 



1 Private communication to the author from Mr. S. C. Clark. 



2 If there is a slight precipitate, due to silicic acid from the casserole, this is filtered off 

 and the filtrate collected in a 200 c.c. beaker. 



3 This point is most easily recognized by keeping the solution hot and in constant agita- 

 tion while adding the silver nitrate so that the silver chloride formed coagulates and sinks, 

 leaving a clear, supernatant fluid. 



