Abstract
A semiautomated method for the determination of ionic calcium in plasma and urine with little preliminary sample treatment is reported. Separation of ionic calcium from interfering materials is achieved by dialysis under “physiological” conditions for plasma and absorption on Norit for urines. The activity of ionic calcium is determined colorimetrically using a metal complexing dye (tetramethylmurexide) which is not pH dependent. Only sodium interfered with the readings but this interference was easily removed. In 18 normal subjects the plasma ionized calcium concentration was 4.00 ± 0.40 mg%. The precision was a relative S.D. of 0.20%. The urinary ionic calcium concentration range of eight normal subjects was 42.2 to 146.1 mg/24 h with a relative S.D. of 33.84%. The proportion of plasma ionized and bound calcium as a percentage of total calcium was 41.17 ± 2.06% and 58.83 ± 2.06%, respectively. For urine these values were 50.01 ± 6.67% and 49.93 ± 6.67%. Reproducibility and recovery data are presented. The calculation of the dissociation constant of calcium tetramethylmurexide complex is also shown.
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