Drying the Precipitate
Finally, after separating the precipitate from its super- natant solution the precipitate is dried to remove any residual traces of rinse solu- tion and any volatile impurities. The temperature and method of drying depend on the method of filtration, and the precipitate’s desired chemical form. A temperature of 110 °C is usually sufficient when removing water and other easily volatilized im- purities. A conventional laboratory oven is sufficient for this purpose. Higher tem- peratures require the use of a muffle furnace, or a Bunsen or Meker burner, and are necessary when the precipitate must be thermally decomposed before weighing or when using filter paper. To ensure that drying is complete the precipitate is repeat- edly dried and weighed until a constant weight is obtained.
Filter paper’s ability to absorb moisture makes its removal necessary before weighing the precipitate. This is accomplished by folding the filter paper over the precipitate and transferring both the filter paper and the precipitate to a porcelain or platinum crucible. Gentle heating is used to first dry and then to char the filter paper. Once the paper begins to char, the temperature is slowly increased. Although the paper will often show traces of smoke, it is not allowed to catch fire as any pre- cipitate retained by soot particles will be lost. After the paper is completely charred the temperature is slowly raised to a higher temperature. At this stage any carbon left after charring is oxidized to CO2.
Fritted glass crucibles cannot withstand high temperatures and, therefore, should only be dried in an oven at temperatures below 200 °C. The glass fiber mats used in Gooch crucibles can be heated to a maximum temperature of approxi- mately 500 °C.