GPM 1.2.2.2.0014.15 Sulphated ash

Ignite a porcelain, quartz or platinum crucible at a temperature of 550 – 650 °C for 30 minutes, cool it in a desiccator over silica gel or another suitable desiccant and accurately weigh at the end of each calcination.

Place an accurately weighed sample of test solutions (usually, 1-2 g) into the preignited crucible, moisten with 1 mL of concentrated sulphuric acid and carefully (avoiding excessive foaming) heat over fire, in a sand bath or on an electrical stove with an enclosed heating element and a temperature controller until the sample is charred. Let it cool down, moisten the residue with 1 mL of concentrated sulphuric acid and carefully heat until white vapours are no longer evolved. Place the crucible into a muffle furnace and ignite at a temperature of 550 – 650 °C until the residue is completely incinerated. Avoid flaming, fusion and sintering of the ash. After ignition, cool the crucible in a desiccator, weigh accurately and calculate the percentage of residue.

If the results exceeds the limit indicated in the general monograph, repeat the moistening with sulphuric acid, heating and ignition as before, using a 30 – minute ignition period, until two consecutive weightings of the residue do not differ by more than 0.5 mg or until the percentage of residue complies with the limit in the individual monograph.

Amount of test substance is selected in such manner that at an indicated limit the weight of residue (usually about 1 mg) can be measured with satisfactory accuracy.

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