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5.3 DETERMINATION OF SULFATED ASH

 ​            Use Method I unless otherwise directed.

Method I

 ​            Heat a platinum dish to redness for 10 minutes, allow to cool in a desiccator and weigh. Place about 1 g of the substance, or the quantity specified in the monograph, accurately weighed, in the dish, moisten with sulfuric acid, heat gently until white fumes are no longer evolved, again moisten with sulfuric acid and ignite at 800º±25º until the carbon is consumed. Allow the crucible to cool in a desiccator over self-indicating silica gel, weigh it again and calculate the weight of the residue. If the weight of residue so obtained exceeds the prescribed limit, repeat the moistening with sulfuric acid and ignition, as previously, to constant weight, unless otherwise prescribed.

Method II

 ​            Ignite a suitable crucible (silica, platinum, porcelain or quartz) at 600º±50º for 30 minutes, allow to cool in a desiccator over self-indicating silica gel and weigh. Place the prescribed amount of the substance in the crucible and weigh. Moisten the substance with a small amount of sulfuric acid (usually 1 ml) and heat gently at temperature as low as practicable until the sample is thoroughly charred. After cooling, moisten the residue with a small amount of sulfuric acid, heat gently until white fumes are no longer evolved and ignite at 600º±50º until the residue is completely incinerated. Ensure that flames are not produced at any time during the procedure. Allow the crucible to cool in a desiccator over selfindicating silica gel, weigh it again and calculate the weight of the residue. If the weight of residue so obtained exceeds the prescribed limit, repeat the moistening with sulfuric acid and ignition, as previously, to constant weight, unless otherwise prescribed.