tribasic acids. Of isomeric acids, those with two carboxyl groups attached to one carbon atom are stronger than those which have the three carboxyl groups attached each to a different carbon atom. Substitution of alkyl groups for hydrogen increases the constant of a tribasic acid. An unsaturated acid has a greater ionisation-constant than the saturated acid to which it corresponds. Phenols. There are classes of compounds, of which the most important are those derived from phenol, which, although they do not contain one of the characteristically acid groups such as COOH or SO、H, are ionised to an appreciable extent in solution. A discussion of the results obtained would lead to no new conclusion, but the ionisation-constant of phenol is given in order to show the small tendency to ionise among compounds of this class. 2 Summary of the Results for Acids.-Wegscheider has compiled, from his own and other measurements, a table of factors representing the effect of the substitution of various atoms and groups in monocarboxylic acids, on their ionisationconstants. The numbers—which do not apply to unsaturated acids or those exhibiting stereo-isomerism—are the factors with which the ionisation-constant of a monobasic acid must be multiplied to obtain the ionisation-constant of the compound in which the atoms or groups to which the factors refer have taken the place of hydrogen. In compiling this table, it has not been possible to take into account small differences of constitution, which, however, have an appreciable influence on the constant. So that these numbers, as will be seen from the calculations based on them, are only approximate. 1 Walden, Zeitschr. phys. Chem., 10. 563 (1892). From this table the ionisation-constant of any substituted monocarboxylic acid corresponding to the substituent radicals given, can be calculated from that of the acid itself. And, since the ionisation-constant of a polybasic acid is the sum of the ionisation-constants corresponding to the carboxyl groups it contains, we can calculate the constants of an extremely large number of substituted and unsubstituted polybasic acids, from the ionisation-constants of the simple monobasic acids. If the table were complete, we could in this way calculate the ionisation-constants of all other acids from those of the simple monobasic acids. As was mentioned above, the numbers thus obtained are not accurate. The differences between the experimental and calculated numbers are greatest (a) in acids which have two carboxyl groups and a strongly negative group attached to one carbon atom; (b) in acids which are derived from succinic acid by substitution of two alkyl groups, one in each CH2 group; and (c) in di-substituted aromatic acids in which the groups are in positions 1:2:6 or 1:2:3 (COOH = 1). The following examples show the method of calculation and the degree of accuracy obtained : CH2.COOH Succinic acid | may be regarded as propionic CH2.COOH acid, in which the carboxyl group is substituted in the B position. K1 X 10" for propionic acid = 0·0013 .. the constant for one carboxyl group = o'0013 X 2°41 .. K1X 105 for succinic acid (two carboxyls) = 2 X 0'0013 X 2°41 =0'00627 K, X 105 for succinic acid (Walden) o'00665 Hydroxyterephthalic acid— = K1 X 105 for benzoic acid = 0*0060 Factor for ortho substitution of hydroxyl = 17, ..K, X 105 for carboxyl group, ortho to OH=0'0060 X 17 X 2'62 =0*267 Factor for meta substitutes of hydroxyl = 1°45 K, X 10" for carboxyl group, meta to OH = o'o060 X 1'45 X 2'62 = 0'023 .. KX105 for hydroxyterephthalic acid=0'267 +0'023=0'290 K, X 101 for hydroxyterephthalic acid observed by Wegscheider OH COOCH HO COOH وو 0'0243 "" 0'0250 Acids containing electropositive substituent groups will be discussed under amphoteric electrolytes. Inorganic Bases. The inorganic bases are very strongly ionised, and do not obey Ostwald's law of dilution. Potassium hydrox ide KOH 197 206 213 222 225 228 230 (233) (234) It is seen that similarly constituted compounds are ionised to the same extent in solutions of the same dilution, e.g. KOH to the same extent as NaOH, and Ca(OH)2, Ba(OH)2, Sr(OH)2 also to the same extent. Ammonia is best considered in connection with the organic bases. Organic Bases.-Bredig1 has measured the conductivity of a large number of organic bases, and from his numbers he has calculated the ionisation-constants in the usual way. Now, it is probable that in solutions of ammonia and amines only a fraction of the compound is really present as base, the rest being simply dissolved, or forming a hydrate with water Zeitschr. phys. Chem., 13. 289 (1894). incapable of directly giving ions. Eg., in a solution of ammonia we have probably only a small amount of NH4OH and its ions, the rest of the base being either simply dissolved -whatever that term may mean-or forming a hydrate with water NH3,(H,O) which does not give into ions. The equilibrium in a solution of an ammonium base may be represented by the equations If K, be the reaction constant of the first reaction, and K, the real ionisation-constant of the base, Now, if a be the number of gram-equivalents used in making one liter of the solution of base NX, Bredig measured the electric conductivity of solutions of bases, and calculated the values of the expression vAx A,2 VA。 (A.-A.) -the ordinary formula for the ionisation-constant-and found that for many bases it was constant. |