PLASMA CHLORIDES 203 



AgNOs ............... ................. 5.812 gms. 



HNO 3 (sp. gr. 1.42) ..................... 250 c.c. 



Picric acid ............................. 7.5 gms. 



Water to .............................. 1000 c.c. 



II. Starch Solution. A solution, for use in the final titration, 

 containing sodium citrate, sodium nitrite, and starch, which 

 substances respectively regulate the acidity, provide an oxidizing 

 agent for the iodide, and serve as indicator. 



Sodium citrate (NasCeHeOr+S^O) ..... 446 gms. 



Sodium nitrite ......................... 20 gms. 



Soluble starch .......................... 2.5 gms. 



Water to .............................. 1000 c.c. 



The starch is first dissolved in about 500 c.c. of boiling water, 

 to which the citrate and nitrite are then added, the solution being 

 finally made up to 1 liter. It should be emphasized that when 

 the starch is dissolved the solution must be not merely heated but 

 boiled for several minutes, or it will not give a satisfactory end- 

 point. If starch other than the soluble variety is used the boiling 

 should continue for one hour. Four c.c. of this solution contains 

 sufficient citrate to react with the acid in 1 c.c. of nitric acid of 

 1.42 specific gravity, the resulting solution having the optimum 

 acidity for the production of the blue starch-iodine end-point. 



III. Potassium Iodide Solution. A solution of M/73.1 

 potassium iodide of which 1 c.c. is equivalent to 0.8 mgm. of NaCl. 



KI ................................... 2. 27 gms. 



Water to .............................. 1000 c.c. 



To standardize this solution proceed as follows: The KI 

 solution is made up to contain 2.4 gms. KI per liter and diluted 

 to the extent indicated by a preliminary titration. Five c.c. of 

 the silver nitrate solution, measured with a pipette which has an 

 error not greater than 0.01 c.c. (delivers 4.97 gms. of water at 20), 

 are mixed with 5 c.c. of the starch solution and 5 c.c. of water, and 

 iodide is run in from a burette to a permanent blue end-point. 

 The amount required should be 12.65 c.c., 12.50 c.c. being required 

 to precipitate the standard silver solution, and 0.15 c.c. additional 

 to give the end-point. The fact that 0.15 c.c. of excess KI solution 

 is required to give the end-point does not detract from the accuracy 

 of the titration, because, with a given volume of the solution, 

 the required excess is constant and sharply defined. 



