Acid diffusion analysis of different forms of fluoride in human dental plaque

Fluoride concentration was determined by: (a) extraction with diphenylsilanediol after combustion of the plaque in an oxygen bomb; (b) acid diffusion from 0.5 M HClO4 for 16 h at room temperature; (c) acid diffusion from 4.6 M HClO4 for 16 h at room temperature; and (d) acid diffusion from 18 M H2SO4 for 16 h at 60 degrees C. The total fluoride was determined by all the diffusion procedures, and there was no evidence of a large proportion of the plaque fluoride being released only after treatment with strong acid (18 M H2SO4). When approx. 10 mg of plaque was extracted three times with 0.1 ml 0.5 M HClO4, 81 per cent of the fluoride was released by one 5 min extraction. After three extractions no further fluoride was detected when the residue was diffused from 4.6 M HClO4 for 16 h at room temperature. When larger plaque samples (21-66 mg) were extracted four times with 1 ml 0.5 M HClO4, 85 per cent of the fluoride was in the first extract, and none was detected in the fourth. Treatment of the residue with 18 M H2SO4 for 16 h at 60 degrees C released a further, small amount which may constitute up to 3 per cent of the total plaque fluoride. Thus the amount of tightly-bound plaque fluoride, released only by treatment with strong acid, is much smaller than previously believed.