Gasometric Determination of Blood Calcium.

Abstract

The calcium is precipitated and washed in a centrifuge tube as in the Kramer-Tisdall method, and is redissolved in 2 cc. of 1 N sulfuric acid. The solution is transferred to the manometric blood-gas apparatus of Van Slyke and Neill, 1 using sufficient water for washing to bring the volume up to 6 cc. Dissolved air and CO₂ are removed by shaking the solution 2 minutes in the evacuated chamber and ejecting the extracted gases. One cc. of 0.1 N potassium permanganate, previously acidified with 1/20 volume of 0.1 N sulfuric acid, to decompose any carbonate present as contamination, is added. The excess permanganate oxidizes the oxalic acid to CO_2. The chamber is evacuated and shaken 3 minutes. The gas pressure P₁ is read on the manometer with the gas volume at 0.5 cc. if the calcium is being determined in a 1 cc. serum sample, at 2 cc. volume if a 3 cc. sample has been used. After reading p₁ the CO₂ is absorbed with 1 cc. of 5 N sodium hydroxide, and p₂ is read with the same gas volume in the chamber. The pressure due to the CO₂ from the oxalate is p₁-p₂-c, where c is the value of p₁-p₂ determined in a control analysis of the reagents. The CO₂ in millimols is estimated from Table III of Van Slyke and Neill. If a 1 cc. sample of serum is used, the factor in the column for “Sample = 2 cc., s = 7.0, a = 2.0, i = 1.014” is multiplied by 2 × 1.03/1.014 × 2.0/0.5 = 0.508, to obtain the factor giving milliequivalents of Ca per liter; since in the present analysis the sample is 1/2 as much, i is changed to 1.03, and a is changed from 2.0 to 0.5, only s being the same as on the table.