Calcium fluoride precipitation and deposition from 12 mmol/L fluoride solutions with different calcium addition rates

Abstract

Effects of different Ca-addition rates on calcium fluoride (Ca2) precipitation and deposition were investigated in 12 mmol/L sodium fluoride solutions to which 0.1 mol/L calcium chloride solution was continuously added at average rates of (5, 7.5, 10, 12.5, 15 or 20) mmol L-1 min The changes in ionic fluoride and calcium concentrations, as well as turbidity, were continuously recorded by F and Ca electrodes, and a fiber optic based spectrophotometer, respectively. The F concentration decreased and turbidity increased with time indicating precipitation of CaF2. For the systems with Ca-addition rates of (5, 7.5, 10, 12.5, 15, and 20) mmol L-1 min 1, the 1 min CaF2 depositions in the model substrate (cellulose filter paper, pores 0.2 |lm) expressed as mean ± SD of deposited F per substrate surface area were (3.78 ± 0.31, 11.45 ± 0.89, 9.31 ± 0.68, 8.20 ± 0.56, 6.63 ± 0.43, and 2.09 ± 0.28) |lg/cm2, respectively (n = 10 for each group). The 1-min F depositions did not show positive correlation to Ca-addition rates. The lowest 1-min F deposition was obtained in the systems with the highest Ca-addition rate of 20 mmol L-1 min 1 for which CaF2 precipitation rate reached the maximum value of 0.31 mmol L s almost immediately after beginning of reaction (6 s). The largest 1-min F depositions were obtained from the systems with Ca addition rates of (7.5 to 12.5) mmol L 1 min 1 in which CaF2 precipitation rates continuously increased reaching the maximum values of (0.13 to 0.20) mmol L 1 s 1 after (18 to 29) s, respectively. The 1-min F depositions were greatly enhanced in comparison with the control F solutions that did not have continuous Ca-addition. This indicates that continuous Ca addition that controls the rate of CaF2 formation could be a critical factor for larger F depositions from F solutions. The efficacy of conventional F mouthrinses could be improved with addition of a substance that continuously releases Ca.