关于游离酸度，适用于含量等于或大于0.1%（m/m）的产品，以H2 SiF6表示。首先，通过在硝酸钾存在下用标准体积氢氧化钠溶液滴定冰冷却试验部分，并使用溴百里酚蓝作为指示剂，测定游离酸含量，然后，在接近沸腾温度下用标准体积氢氧化钠溶液滴定至永久蓝色。

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.