Transfer total filtrate (with washings) to a weighed evaporating dish and evaporate to dryness on a steam bath or in a drying over. If necessary, add successive portions to the same dish after evaporation. Dry evaporated sample for at least 1 h in an over at 180 + 2°c, cool in a desiccator to balance temperature,and weigh. Repeat drying cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained or until weight change is less than 4% of previous weight or 0.5 mg, whichever is less.