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Sodium fluoride for industrial use -- Determination of fluorine content -- Modified Willard-Winter method
The principle consists in separation of the fluorine from a test portion by distillation with sulphuric acid or perchloric acid.
Titration with thorium nitrate solution using sodium alizarinsulphonate and methylene blue as indicators.
Alternatively, the thorium nitrate titration may be carried out using sodium alizarinsulphonate alone as indicator, the end-point being determined spectrophotometrically under carefully defined conditions.
The absorbance at a wavelength of 525 nm shall reach the arbitrary value of 0.6.
Has also been approved by the IUPAC.
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