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 HClO 4 for 16 h at room temperature; (c) acid diffusion from 4.6 M HClO 4 for 16 h at room temperature; and (d) acid diffusion from 18 M H 2 SO 4 for 16 h at 60°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 H 2 SO 4 ). When approx. 10 mg of plaque was extracted three times with 0.1 ml 0.5 M HClO 4 , 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 HClO 4 for 16 h at room temperature. When larger plaque samples (21-66 mg) were extracted four times with 1 ml 0.5 M HClO 4 , 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 H 2 SO 4 for 16 h at 60°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. © 1987 Pergamon Journals Limited.
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