In published reports, the values of stability constants of 1:1 complex of Ca(2+) and the dye ammonium purpurate (murexide) were not determined under controlled conditions and were not properly corrected for the binding of Ca(2+) with ions of buffer used to maintain pH and that of the background electrolyte used to maintain ionic strength. We report the molar absorptivities (epsilon) of murexide at pH 7.0, 7.5, 8.0, as well as the differential molar absorptivities (Deltaepsilon). Using these, we calculate the stability constants of the Ca-murexide complex at pH 5.0, 6.0, 6.5, 7.0, 7.5 and 8.0 at 15, 25 and 35 degrees C and 0.100 M ionic strength using KCl as background electrolyte. No buffer was used and the complication arising from buffer binding is thus avoided. These values are compared with those determined in the presence of buffers that bind metal ions negligibly (Tris at pH 7.5 and 8.0) or whose binding constant to Ca(2+) is reported and therefore can be corrected for (acetate at pH 5.0, Bistris at pH 6.5). Agreement is obtained within errors of measurement. The reported values are not true stability constants but can be used to calculate the concentration of free Ca(2+) ion in a metal-ligand mixture with high precision and accuracy. The effect of K(+) binding to murexide is considered and is found not to alter the calculated value of free calcium concentration in a mixture.