CALCIUM 273 



found that the eerie sulfate procedure was superior to the direct 

 permanganate titration when the latter is used with setopaline C to 

 sharpen the end point. However, no comparison was made with the 

 iodometric permanganate method as used by Siwe. Siwe employed 

 his methods for measurements of the calcium in 50 ju,l. samples of 

 serum and reported a precision of ±3.5% for both procedures. 

 Lindner and Kirk stated that their method was adapted to the 

 determination of 0.5-12 /xg. calcium with a precision of less than 

 d=0.5% on all but the lowest amounts of calcium. 



The procedure followed by Sobel and Kaye ( 1940) , which substi- 

 tutes an iodometric for the acidimetric titration of excess acid in the 

 Siwe method, could be adapted to histochemical work. This involves 

 ignition of the calcium oxalate precipitate to convert it to the car- 

 bonate, solution of the carbonate in an excess of acid, and iodometric 

 determination of the excess acid by addition of an excess of potas- 

 sium iodate and potassium iodide, followed by titration of the liber- 

 ated iodine with thiosulfate. By employing very dilute thiosulfate 

 (0.0007 N) as little as 4 /xg. calcium could be measured using a 5 ml. 

 burette for the titration. By carrying out the oxalate precipitation 

 at pH 3.0 to 3.5, the authors minimized coprecipitation of magne- 

 sium. Conversion of the oxalate to carbonate was considered advan- 

 tageous since the oxalate precipitate could then be washed with am- 

 monium oxalate to reduce the loss during the washing, and any excess 

 of oxalate ion in the precipitate would be volatilized in the ignition. 

 See page 219 for a colorimetric method. 



Siwe Iodometric Permanganate Method for Calcium 



SPECIAL REAGENTS 



Ammonium Hydroxide (cone). 



5% Ammonium Oxalate. 



25% Nitric Acid. 



1% Potassium Iodide. 



0.2% Soluble Starch. 



0.01 N Potassium Permanganate. 



0.01 N Sodium Thiosulfate. 



PROCEDURE 



1. Place a soln. of the sample having a vol. of about 50 jA. into 



