Calcium complexes determined by polarography

A d.c. polarographic technique has been developed which allows measurement of the concentration of calcium ions in aqueous solutions. By applying a potential scan at a rate of 25 mV/sec to a 1.5 sec/drop dropping mercury electrode, peak-shaped waves are obtained for calcium at mM concentrations in tetrabutylammonium hydroxide supporting electrolyte. The square root of the peak height is directly proportional to the calcium concentration over the range 0.1-1.0 x 10(-3)M. In the presence of the sequestrants ethylenediaminetetra-acetic acid (EDTA), nitrilotriacetic acid (NTA) and tripolyphosphate (TP), indications of distinct calcium complexes were seen by noting the positions of slope changes in plots of peak height vs. mole ratio of calcium to sequestrant. The species found were CaEDTA, Ca(3)(NTA)(2), Ca(2)(TP)(3) and Ca(3)TP. All of these are readily acceptable complexes at the high pH of the experimental conditions.