STABILITY OF MONOFLUOROPHOSPHATE AND FLUORIDE IONS IN DENTIFRICE 5 column temperature was 60øC and the carrier gas flow rate 15 ml min 4. A typical chromatogram is shown in Fig. 1. The molar ratio trimethyl- fiuorosilane and normal pentane was calculated and plotted against the peak height ratio of the same components (see Fig. 2). A good linearity was obtained (regression coefficient 0.9998). 6 5 4 3 Minutes Figure 1. GLC chromatogram of the benzene phase. Free fluoride determination The fluoride concentration in the aqueous phase was measured directly with a fluoride-selective electrode (Orion, model 94-09). About 1.0 g of dentifrice was diluted to 50 ml in a total ionic strength adjustment buffer (TISAB) (5). A linear correlation is obtained between the electrode potential and the logarithmic concentration of fluoride ions (see Fig. 3).

6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 4o 3,0 O.C L Peak height ratio (Me 3 SiF'/n-C 5 H•2) Figure 2. The correlation between the molar ratio of trimethylfluorosilane and normal pentane and the peak height ratio of the same components. mV I I I 50 I00 150 ß I I I I I I I I I I I I I I I I I I l -4O Figure 3. Fluoride electrode potential as a function of fluoride ion concentration in a TISAB-water solution. Monofiuorophosphate determination The monofluorophosphate concentration was then taken as the differ- ence between total fluorine and fluoride concentrations of the liquid phase. Note that the inactive calcium fluoride is not included in the total fluorine concentration.