A safer but slower method is to place the funnel and filter paper, or the filter paper alone resting upon a clock glass, inside a drying oven. Several types of centrifuge are available for semimicro analysis. The speed of filtration depends inter alia upon the length of the water column. This operation is usually difficult, particularly for centrifuge tubes. They are then heated in the lower oxidizing flame, allowed to cool and the colour observed again.
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