Background: In the course of developing some bicarbonate-based toothpastes with silica, several prototype formulas were found to have abrasivity lower than would be expected on the basis of their silica content. Objectives: These experiments were undertaken to find the reason. Methods: The titration of different types of silica with alkaline reagents was followed by turbidimetry, which gives an indication of the silica particle size on the nano-scale. Similar titrations were carried out on silica extracted from experimental toothpaste formulas, and on silicas which had received prior treatment with sodium bicarbonate. Results: Silodent 756 required a full equivalent of NaOH to reduce the particle size below nephelometric detection, whereas Silodent 700 required less than 1/3 of an equivalent, and gave significant turbidity decrease with excess sodium bicarbonate. The same effect was found to occur in toothpaste formulas. Increasing concentrations of bicarbonate in a Silodent 700 toothpaste gave increasing reductions of RDA:

0 % bicarbonate RDA 50 to 80 10 % bicarbonate RDA 48 to 68 25 % bicarbonate RDA 39 to 55

and the WeAr profilometry index was reduced from 20.9 +/- 5.0 to 17.7 +/- 4.1, which is in the same direction but not significant at the 95% level. Conclusions: Base-promoted particle size reduction may account for the unusually low abrasivity and high cleaning power of these formulas.

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