This International Standard specifies an alizarin sulphonate photometric method for the determination of insoluble zirconium in magnesium alloys containing zirconium as an alloying element. Rare earths, thorium and silver do not interfere. The method is applicable to alloys having insoluble zirconium contents between 0,02 and 0,3 % (m/m). NOTE — Hafnium which is normally present in conjunction with zirconium (up to 1 % zirconium) will be determined with zirconium.