Measurement of calcium activity in oral fluids by ion selective electrode:Method evaluation and simplified calculation of ion activity products

The activity of calcium in prague fluid is needed to calculate the saturati on level of that fluid relative to the tooth mineral. One method to determi ne the calcium activity in very small plaque fluid samples is by micro ion- selective electrode (ISE). Two commercially available calcium ionophores, a neutral-carrier and a charged-carrier, were evaluated in micro ISEs and co mpared to a commercially available macro ISE using saliva as a model for pl aque fluid. The neutral-carrier containing ISEs gave results consistent wit h those of the macro ISE. Calcium activity measurements made with micro ISE s that contained the neutral ion-carrier of whole plaque samples and plaque fluid samples obtained by centrifugation of whole plaque showed that the a ctivities did not change due to centrifugation. Estimates of the saturation with respect to hydroxyapatite were made from these measurements. A simpli fied calculation method is presented to estimate the ion activity product ( IAP) of the calcium-phosphate minerals. The method is based on the relative abundance of some of the possible calcium-binding species and a fixed ioni c strength for plaque fluid. Calculations show that within a normal pH rang e for plaque fluid (5.0 to 7.5) the differences in the IAP calculations for hydroxyapatite using the simplified method are less than those estimated f rom propagation of uncertainty calculations.