LXI.—Contributions from the Chemical Laboratory of the University of Edinburgh. No. VII. The dissociation constants of organic acids

James Walker
1892 Journal of the Chemical Society Transactions  
THE physical properties of chemical compounds have been classified in three groups-additive, colligative, and constitutive. The only purely additive property is mass : each element in a compound contributes its own mass t o the total, so that the mass of the compound is the sum of the masses of its constituents. Other properties which are essentially additive are the specific heat, specific volume, and specific refractive power ; but in all these cases a disturbing influence is apparent-similar
more » ... is apparent-similar atoms do not always contribute the same amount to the total, but the quantities vary according to the function exercised by the atoms in the compound. Thus, carbonyl oxygen has a value for the atomic volume different from the value attributed to hydroxyl oxygen ; and so it is with other elements. Here the additive character of the property is modified by constitution. This influence is still more marked in properties such as the boiling point, melting point, colour, and crystalline foriii, SO that these propet4iies have been termed constitutice. The third group of properties consists of those which at-e independent of the nature, number, and arrangement of the atoms in the compound, and which assume the same value for the molecules of all compounds, no matter how these may be constituted. Examples of such colligative properties are the volume occupied by gases under fixed conditions of temperature and of pressure, and the depression of the freezing point of solutions. Substances that are prepared in the course of a chemical investigation can only be identified by means of their own physical properties o r by those of other substances into which they may be transformed. The first step is to ascertain the composition of the compound by some method of analysis ; here advantage is taken of the additive character of mass. Next, the molecular weight is determined from some colligative property, for example, from the vapour density, the depression of the freezing point or elevation of the boiii n g point of some liquid in which the substance is dissolved. The only problem which remains is the determination of the arrangement of the atoms within the molecule-that is, the constitution. As a rule, chemical methods are resorted t o for the strict solution of this problem, the substance being transformed into some other of known
doi:10.1039/ct8926100696 fatcat:rxthf246hncz5isyjldsh6oeoe