BLOOD ANALYSIS 287 



iodide. 1 The titration is performed in the presence of nitrous acid and starch, so 

 that the first drop of iodide in excess of the silver present is changed to free iodine 

 and gives the blue starch-iodine color. The optimum acidity for the end point is 

 fixed by the addition of trisodium citrate in amount equivalent (% mol.) to the 

 free nitric acid present. Under these conditions i drop of excess N/SO iodide 

 gives a color preceptible in 150 c.c. of solution. 



In detail, the titration of the protein-free plasma nitrate is carried out as 

 follows: Either 10 c.c. of the filtrate, containing the chlorides of i c.c. of plasma, 

 are taken in a pipette for titration or the filtrate is collected directly in a certified 

 25 c.c. graduated cylinder, where it is measured, so that the entire amount may 

 be taken for titration. In this way 13 to 14 c.c. of filtrate, corresponding to 1.3 

 to 1.4 c.c. of plasma, may be obtained for titration. 



After either measuring 10 c.c. of the filtrate into a 25 c.c. volumetric flask or 

 recording the amount of filtrate obtained in the cylinder, 5 c.c. of the acidified 

 If/29.25 silver nitrate solution (Solution I) are added, and the whole is made to the 

 25 c.c. mark with water. This will precipitate up to 10 mg. of NaCl. In samples 

 with high percentage of chloride, only enough filtrate is taken to keep within this 

 limit of 10 mg. Two drops of octyl (caprylic) alcohol are added, and the vessel is 

 stoppered and shaken gently by inverting it several times. Immediate coagulation 

 of the silver chloride occurs. After allowing five minutes for complete precipitation 

 to occur, the solution is filtered through a dry folded filter, and a perfectly clear and 

 colorless filtrate again obtained. An aliquot part of the filtrate (20 c.c.) is now 

 taken with a pipette for titration. 



Just before titration with the potassium iodide solution (Solution II) one adds 

 a volume of the citrate solution (Solution III) equal to the volume of Solution I 

 represented in the filtrate to be titrated. If, as is usually the case, one has used 

 5 c.c. of Solution I diluted to 25 c.c., and taken 20 c.c. of the filtrate for titration 

 one has the equivalent of 4 c.c. of Solution I present, and must accordingly add 



1 The following solutions are required; 



I. An acid 3/729.25 solution of silver nitrate, i c.c. of which is equivalent to 2 mg. of 

 NaCL. 



AgNO 3 5.812 grams 



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



Water to 1000 c.c. 



II. A solution of Jlf 758.5 potassium iodide, i c.c. of which is equivalent to i mg. of 

 NaCL. 



KI 3.0 grams 



Water to 1000 c.c. 



This solution is standardized against the silver solution by adding 5 c.c. of the latter 

 to 5 c.c. of Solution III, and titrating with the iodide solution to the blue end-point. The 

 iodide solution is then diluted to such a degree that 10 c.c. are exactly equivalent to 5 c.c. 

 of the silver solution. 



III. A solution, for use in the final titration, containing sodium citrate, sodium nitrate, 

 and starch, which substances respectively regulate the acidity, provide an oxidizing agent 

 for the iodide, and serve as indicator. 



Sodium citrate (NasCsHeOy+S i/2H 2 O) 446 grams 



Sodium nitrite 20 grams 



Soluble starch 2.5 grams 



Water to 1000 c.c. 



The starch is first dissolved with the aid of heat in about 500 c.c. of water. The citrate 

 and nitrite are then added, and the mixture is heated until all is dissolved. The solution, 

 while still hot is filtered through cotton, the filter washed with hot water, the filtrate 

 allowed to cool, and made up to 1000 c.c. Filtration removes insoluble substances occur- 

 ring chiefly in the nitrite, and cotton niters more rapidly than filter paper. The solution 

 keeps indefinitely. It becomes cloudy on standing, but its efficacy is not impaired. 



