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

Abstract

The activity of Ca in plaque fluid is needed to calc. the satn. level of that fluid relative to the tooth mineral. One method to det. the Ca activity in very small plaque fluid samples is by micro ion-selective electrode (ISE). Two com. available Ca ionophores, a neutral-carrier and a charged-carrier, were evaluated in micro ISEs and compared to a com. available macro ISE using saliva as a model for plaque fluid. The neutral-carrier contg. ISEs gave results consistent with those of the macro ISE. Ca activity measurements made with micro ISEs that contained the neutral ion-carrier of whole plaque samples and plaque fluid samples obtained by centrifugation of whole plaque showed that the activities did not change due to centrifugation. Ests. of the satn. with respect to hydroxyapatite were made from these measurements. A simplified calcn. method is presented to est. the ion activity product (IAP) of the Ca-phosphate minerals. The method is based on the relative abundance of some of the possible Ca-binding species and a fixed ionic strength for plaque fluid. Calcns. show that within a normal pH range for plaque fluid (5.0 to 7.5) the differences in the IAP calcns. for hydroxyapatite using the simplified method are less than those estd. from propagation of uncertainty calcns. [on SciFinder (R)]