QUANTITATIVE ANALYSIS 483 of the generator nearly filled so that there is as little air space left as possible. The acid to be used is made up from equal parts of concentrated hydrochloric acid (A.R. standard) and air-free water. Before the acid is transferred to the generator, a number of de-aerated marble chips are dissolved in it in order to expel any residual dissolved air and also to saturate it with carbon dioxide. The top portion of the generator A is then lifted several times in order to cause a rapid evolution of carbon dioxide so as to sweep out the air from the middle bulb. A number of pieces of marble, of such a size that they will not slip down the acid funnel, are then added to the acid in the upper bulb in order to sweep out any residual air in this part of the vessel. The stopper of this top bulb is provided with a non-return valve (a thin layer of mercury on a sintered FIG. 87 (not to scale). glass plate in a small funnel) to allow the escape of any excess of carbon dioxide
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