Tin(II) fluoride  Tin(II) fluoride, commonly referred to commercially as stannous fluoride (from Latin "", 'tin'), is a chemical compound with the formula SnF2. It is a colourless solid used as an ingredient in toothpastes. Stannous fluoride was introduced as an alternative to sodium fluoride for the prevention of cavities. It was introduced for this purpose by Joseph Muhler and William Nebergall. In recognition for their innovation, these two individuals were inducted into the Inventor's Hall of Fame. Stannous fluoride converts the calcium mineral apatite into fluorapatite, which makes tooth enamel more resistant to bacteria-generated acid attacks. ; over time these react with sodium fluoride to form calcium fluoride, which is almost completely insoluble and thus ineffective for tooth protection. Stannous fluoride is a more stable ingredient and thus remains effective in strengthening tooth enamel upon longer storage. Stannous fluoride has been shown to be as effective as sodium fluoride in reducing the incidence of dental caries and controlling gingivitis. Stannous fluoride was used under the trade name Fluoristan in the original formulation of the toothpaste brand Crest, though it was later replaced with sodium monofluorophosphate under the trade name Fluoristat. Stannous fluoride is the active ingredient in Crest Pro Health brand toothpaste. Crest Pro Health issues a warning on the tube that stannous fluoride may cause staining, which can be avoided by proper brushing, and that its particular formulation is resistant to staining. However, any stannous fluoride staining that occurs due to improper brushing is not permanent. Stannous fluoride is also used in Oral-B Pro-Expert. Stannous fluoride is also readily available in over-the-counter rinses. SnF2 can be prepared by evaporating a solution of SnO in 40% HF. Readily soluble in water, SnF2 is hydrolysed. At low concentration, it forms species such as SnOH+, Sn(OH)2 and Sn(OH)3−. At higher concentrations, predominantly polynuclear species are formed, including Sn2(OH)22+ and Sn3(OH)42+. Aqueous solutions readily oxidise to form insoluble precipitates of SnIV, which are ineffective as a dental prophylactic. Studies of the oxidation using Mössbauer spectroscopy on frozen samples suggests that O2 is the oxidizing species.