If the substance to be weighed changes in composition on strong heating, it is necessary to employ a tared filter, i.e.
In the case of a tared filter it is weighed repeatedly until the weight suffers no change; then knowing the weight of the filter paper, the weight of the precipitate is obtained by subtraction.
It may also be precipitated as zinc ammonium phosphate, NH4ZnP04, which is weighed on a filter tared at 100°.
In the form of a chromate, it may be determined by precipitation, in acetic acid solution, with lead acetate; the lead chromate precipitate collected on a tared filter paper, well washed, dried at loo° C. and weighed; or the chromate may be reduced by means of sulphur dioxide to the condition of a chromic salt, the excess of sulphur dioxide expelled by boiling, and the estimation carried out as above.
The heavy old-fashioned country-made packages are rapidly being replaced by light-tared Boxes made from several thicknesses of veneer pressed closely together, most of which come from Russia.