QUANTITATIVE ANALYSIS 435 out of the pyknometer by blowing gently down the right-hand inlet tube by means of a piece of rubber tubing carrying a short calcium chloride tube. (4) Molecular weight determinations by depression of the freezing-point are more accurate and far less troublesome than those by the elevation of the boiling-point (p. 440), and the former method should always be preferentially employed if the solubility of the organic compound in the solvent is sufficiently high. (5) Certain solvents, particu-larly benzene, cause many organic compounds to form bi-molecular associates in solution, with the result that the experi-mental molecular weight is twice the true value. If association is suspected, the determination should be repeated in one or more other solvents, in order to ascertain whether consistent re-sults are obtained. The following practical points in the choice and use of the sol-vents mentioned in the above ^IG. °°-table should be noted: Benzene. Pure benzene (free in particular from toluene) must be used,
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