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Stannous fluoride
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Tin(II) fluoride, also known as stannous fluoride, is the chemical compound with the formula SnF2. This colourless solid is a common ingredient in toothpaste. In the enamel, it converts apatite into fluorapatite, which is more resistant to attack by acids generated by bacteria. Sodium fluoride and sodium fluorophosphate perform similarly. Stannous fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive combined with calcium.
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Encyclopedia
Tin(II) fluoride, also known as stannous fluoride, is the chemical compound with the formula SnF2. This colourless solid is a common ingredient in toothpaste. In the enamel, it converts apatite into fluorapatite, which is more resistant to attack by acids generated by bacteria. Sodium fluoride and sodium fluorophosphate perform similarly. Stannous fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive combined with calcium. It has also been shown to be more effective than sodium fluoride in controlling gingivitis.
Stannous fluoride was used (under the trade name Fluoristan) in the original formulation of the toothpaste Crest., though it was later replaced with sodium monofluorophosphate. However it is the active ingredient in Crest Pro Health brand toothpaste according to the label on the box and the tube. Crest Pro Health also issues a warning on the tube that stannous fluoride may cause staining; and that by proper brushing this can be avoided; and that their particular formulation is resistant to staining.
Used in combination with abrasives that contain calcium, sodium fluoride is ineffective; stannous fluoride remains effective when used with such abrasives. Stannous fluoride can cause surface staining of teeth, but these stains are not permanent.
SnF2 can be prepared by evaporating a solution of SnO in 40% HF.
Aqueous solutions
Readily soluble in water SnF2 is hydrolysed forming at low concentration species such as SnOH+, Sn(OH)2 and Sn(OH)3- and at higher concentrations, predominantly polynuclear species, 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.
Lewis acidity
SnF2 is a Lewis acid forming, for example, a 1:1 complex (CH3)3NSnF2 and 2:1 complex [(CH3)3N]2SnF2 with trimethylamine, and a 1:1 complex with dimethylsulfoxide, (CH3)2SO.SnF2.
In solutions containing fluoride ion, F- it forms fluoride complexes SnF3-, Sn2F5-, SnF2(OH2). Crystallization from an aqueous solution containing NaF produces compounds containing polynuclear anions, e.g. NaSn2F5 or Na4Sn3F10 depending on the reaction conditions, rather than NaSnF3. The compound NaSnF3 containing the pyramidal SnF3- anion can however be produced from a pyridine - water solution.
Other compounds containing the pyramidal SnF3- anion are known for example Ca(SnF3)2
Reducing properties
SnF2 is a reducing agent, with a standard reduction potential Eo (SnIV/ SnII) = +0.15V .Solutions in HF are readily oxidised by a range of oxidizing agents, O2, SO2 or F2, to form the mixed valence compound, Sn3F8 (containing SnII and SnIV and no Sn - Sn bonds).
Structure
The monoclinic form contains tetramers, Sn4F8, where there are two distinct coordination environments for the Sn atoms but in each case there are three nearest neighbours with Sn at the apex of a trigonal pyramid and the lone pair of electrons is sterically active. Other forms reported have the GeF2 and TeO2 structures.
Molecular SnF2
In the vapour phase SnF2 forms monomers as well as dimers and trimers. Monomeric SnF2 is a non-linear molecule with an Sn-F bond length of 206 pm.
Complexes of SnF2, sometimes called difluorostannylene, with an alkyne and aromatic compounds deposited in an argon matrix at 12 K have been reported
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