A method of estimating the amount of calcium bound to the metallochromic indicator arsenazo III.

As a metallochromic indicator for ionized calcium, arsenazo III is approximately 50 times more sensitive than murexide. However, because of the high binding constant for calcium, the following problems may occur: (a) a considerable amount of calcium is bound to arsenazo III, thereby causing an error in estimating the concentration of ionized calcium; (b) the amount of bound calcium varies with the concentrations of calcium;, arsenazo III, magnesium ion and monovalent cations; (c) the amount also varies with pH, (d) the relationship between the absorbance change and the concentration of ionized calcium is nonlinear; and (e) the binding constant of arsenazo III for calcium cannot be determined by the conventional double reciprocal plot. A new experimental and theoretical method is presented which copes with these problems.