CALCIUM 219 



2. Methods 



CALCIUM 



Sendroy (1942b) adapted iodometric reactions, previously used 

 in titrimetric measurements of calcium, to colorimetry. Using an 

 Evelyn macro photoelectric colorimeter, the method was applied 

 to volumes of serum down to 20 /xl. (about 2 fig. calcium). Further 

 refinement could be obtained if the colorimetry were carried out with 

 smaller volumes in microcuvettes. The calcium is precipitated as 

 the oxalate; the latter is washed, dried, dissolved in acid, and reacted 

 with an excess of eerie sulfate. The excess of eerie ion is made to 

 liberate iodine from potassium iodide and the yellow color thus 

 developed is measured. A precision of ±2^0 has been reported. An 

 alternative method is measurement of the blue color formed when 

 starch is added to the iodine solution. However, the many factors 

 which influence the blue color make it a less desirable choice even 

 though the relative color intensity of the blue is about 100 times that 

 of the yellow (Sendroy and Alving, 1942). The method to be de- 

 scribed was developed for serum, but it can be adapted to other 

 fluids. A thorough study of different procedures for the determination 

 of serum calcium was made by Sendroy (1944). For titrimetric 

 methods see page 272. 



Sendroy Method for Calcium 



SPECIAL REAGENTS 



Saturated Ammonium Oxalate (about 3.5%). Prepare at room 

 temperature using analytical reagent grade of the salt. 



2% Ammonium Hydroxide. Dilute 2 ml. cone, ammonium hy- 

 droxide (26% analytical reagent grade) to 100 ml. 



Water-Alcohol-Ether Mixture. Mix equal vol. distilled water, 

 absolute ethyl alcohol, or redistilled 95% alcohol, and ethyl ether 

 (analytical reagent grade, or absolute, or redistilled U.S.P. grade). 



1 N Sulfuric Acid (approx.). Dilute 27 ml. cone, acid, sp. gr. 1.84, 

 analytical reagent grade, to 1 1. 



0.2 N and 0.1 N Sidfuric Acid (approx.). Prepare from the 1 N 

 soln. 



