ISO 4281:1977 Preview
Sodium hexafluorosilicate for industrial use -- Determination of free acidity and total hexafluorosilicate content -- Titrimetric method
This standard was last reviewed and confirmed in 2011. Therefore this version remains current.
With respect to the free acidity, applicable to products having contents, expressed as H2 SiF6 , equal to or more than 0,1 % (m/m). First, determination of the free acid contents by titration of an ice-cooled test portion with a standard volumetric sodium hydroxide solution in the presence of potassium nitrate and using bromothymol blue as indicator, then, followed by titration at near boiling temperature with standard volumetric sodium hydroxide solution to the appearance of a permanent blue colour.