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.
(a) Gravimetric. - This method is made up of four operations: (I) a weighed quantity of the substance is dissolved in a suitable solvent; (2) a particular reagent is added which precipitates the substance it is desired to estimate; (3) the precipitate is filtered, washed and dried; (4) the filter paper containing the precipitate is weighed either as a tared filter, or incinerated and ignited either in air or in any other gas, and then weighed.