COMPARISON BETWEEN MEASURED AND CALCULATED CONCENTRATIONS OF CALCIUM-IONS IN BUFFERS

Concentrations of ionic calcium found by careful calculation from stability constants (corrected for temperature and ionic strength) and by direct measurement with a calciumselective electrode are compared. The two methods give almost identical results in several calcium buffers in the range pCa 3.5-7.2. In series of calcium buffers at given pH values, two calcium-chelatingligands(EGTA and NTA) were used. In these calcium buffers, which were adapted for use in biological experiments, the two methods led to almost identical pCa values, even in the presence of magnesium and adenosine triphosphate. The greatest discrepancies between results from the two methods were observed at calcium/ligand concentration ratios of 0.9-1.0. Under these conditions calculations, in contrast to electrode measurements, led to calcium ion concentrations which were up to 7 times too low, judging from measurements of the enzyme activities of a calcium-sensitive ATPase. The results demonstrate that the calcium electrode is essential for reliable determination of the actual calcium ion concentration in complex calcium buffers. © 1979.