Association in mixed solvents / by George Barger.

  • Barger, George, 1878-1939.
Date:
[1905?]
Catalogue details

Licence: Public Domain Mark

Credit: Association in mixed solvents / by George Barger. Source: Wellcome Collection.

    4/14 (page 1043)
    phenol and acetic acid, the question arises : what is their molecular weight in mixtures of, say, alcohol and benzene ? Two cases, those of benzoic acid in benzene and ethyl alcohol mixtures and of cinnamic acid in chloroform and methyl alcohol mixtures, were described in the previous paper (p. 325). Further examples have been studied in order that general conclusions might be deduced. Of each pair of solvents, one was associative, the other non-associa- tive; their boiling points, and probably, therefore, also their vapour pressures at the ordinary temperature, were not very different. The solutes may be grouped as follows : 1. Acids. 2. Alcohols and phenols. 3. Oximes and other hydroxyl- ated nitrogen compounds. Organic acids are strongly associated in dilute solution, and with increase of concentration the association soon reaches a maximum. They were mostly examined in a concentration of 0T—0'2 mole (compare footnote, Trans., 1904, 86, 295). With phenols and alcohols, the association increases slowly but steadily with the concentration, and some members of this class were studied in more concentrated solutions. Azobenzene was employed as the standard substance in nearly all cases. By the cryoscopic method, it had been shown to have a normal molecular weight over a large range of concentration in benzene solution (Auwers, Zeit, physikal. Chem., 1893, 12, 713). Moreover, its use as a standard was shown to be well justified in the previous com¬ munication on this method. For a few mixtures containing a large percentage of alcohol, in which azobenzene was not sufficiently soluble, a-naphthol was used as a standard after it had been found that a-naphthol was normal in mixtures containing less alcohol. In the following tables, the concentration is expressed in gram- molecules per litre of solution and the composition of the mixtures in percentages by weight of the non-associative solvent. For each determination, the two limiting concentrations of the standard are given in gram-molecules per litre. The molecular weights have been calculated for a solution midway between these limits; the association factor (A) or ratio of the observed molecular weight to the “ normal ” one has also been tabulated. Curves were plotted with the association factor as ordinate and the percentage composition of the solvent as abscissa ; these curves are dis¬ tinguished by Roman numerals.