Weight of Flask.—Use a small pyknometer of from twenty-five to thirty cubic centimeters capacity. The stopper should be beveled to a fine edge on top and the lower end should be slightly concave to avoid any trapping of air. The flask is to be thoroughly washed with hot water, alcohol and ether, and then dried for some time at 100°. After cooling in a desiccator the weight of the flask and stopper is accurately determined.[27]
Figure 28. Bath for Pyknometers.
Weight of Water.—The flask in an appropriate holder, [Fig. 28], conveniently made of galvanized iron, is filled with freshly boiled and hot distilled water and placed in a bath of pure, very hot distilled water, in such a way that it is entirely surrounded by the liquid with the exception of the top.
The water of the bath is kept in brisk ebullition for thirty minutes, any evaporation from the flask being replaced by the addition of boiling distilled water. The stopper should be kept for a few minutes before use in hot distilled water and is then inserted, the flask removed, wiped dry, and, after it is nearly cooled to room temperature, placed in the balance and weighed when balance temperature is reached. A convenient size of holder will enable the analyst to use eight or ten flasks at once. The temperature at which water boils in each locality may also be determined; but unless at very high altitudes, or on days of unusual barometric disturbance the variations will not be great, and will not appreciably affect the results.
51. Alternate Method of Estimating the Weight of Water in Flasks.—Formulas for calculating the volume V, in cubic centimeters, of a glass vessel from the weight P of water at the temperature t contained therein, and the volume Vʹ at any other temperature t’ are given by Landolt and Börnstein.[28] They are as follows:
| V = P | p |
| d |
| Vʹ = P | p | [1 + γ (tʹ- t)]; |
| d |
in which p = weight (in brass weights) of one cubic centimeter H₂O in vacuo. This is so nearly one gram that it will not affect the result in the fifth place of decimals and may therefore be disregarded. Hence the formula stands:
| Vʹ = P | 1 | [1 + γ (tʹ- t)] |
| d |