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Thus Bredig's constant (K) for a base, bears to the true ionisation-constant (K,), the ratio where K is the ratio

K2
1+K2

2

of non-ionised electrolyte to non-electrolyte in the solution. K2, as well as K1, will vary with the nature of the compound, since the facility with which the nitrogen goes from the trivalent to the pentavalent form will depend on the nature of the groups already attached to the nitrogen atom. Therefore, a comparison of the constants K, of different compounds can give no accurate information as to the variation of ionisation with constitution. Since, however, it has not yet been found possible to determine K, this comparison is the only one possible.

The numbers quoted from Bredig's paper are affected by another error, for recent work has shown that the value for the mobility of hydroxyl which was used by Bredig in the calculation of Ac for the bases is too low. Since the order of the ionisation-constants would not be affected by this, and the recalculated numbers would not be the true ionisation-constants, Bredig's numbers are given as they stand.

The following conclusions were drawn by Bredig :—

The quaternary bases from aliphatic amines are, in general, so strongly ionised that no constant can be obtained. The phosphonium, arsonium, stibonium, sulphinium, and tellurinium bases are also very strongly ionised, but the tin and mercury bases are only weakly ionised.

The following numbers are the values of the molecular conductivity of the above-mentioned bases at the dilutions given (T 25°):

=

Tetramethylammonium hydroxide (CH3),NOH 219
Tetramethylphosphonium hydroxide (CH ̧)POH
Tetramethylarsonium hydroxide (CH3),ASOH 211 216

Tetramethylstibonium hydroxide

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226

(228)

214

221

223 218

(CH3),SBOH

178

181

183

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These are the true values of the molecular conductivity since all the base is present either as ions, or as non-ionised molecules corresponding to these ions.

Secondary bases are more strongly ionised than primary or tertiary bases.

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It is, however, quite possible that this is due to the effect of the alkyl group in changing the power of the nitrogen to pass from the trivalent to the pentavalent form by taking up

water.

Among metamers, constitutive influences have sometimes a very great effect.

Propylamine.

Trimethylamine

K3 X 10

0'047

0'0074

We should expect this rule to hold, for, leaving out any specific effect of the alkyl group on the ionisation, the power of the nitrogen atom of passing from the trivalent to the pentavalent condition is probably very different in two such compounds as those given. With compounds in which we should expect this influence to be less, we find the differences in the constants are much less, e.g.

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Organic Di-acid Bases.-No very striking relations have been observed among compounds of this class. Bredig1 has found that those di-acid bases which ionise in stages show the same relation between the first and second ionisation as the dibasic acids; i.e. the greater the extent of the first ionisation, the less the extent of the second.

Neutral Salts. Since salts are made up of the anions and

1 Loc. cit.

cations of the acids and bases, we naturally look for relations between the degree of ionisation of a salt and the degree of ionisation of the acid and base from which it is formed. Such relations have not yet been found, and it seems that certain metals have so strong a tendency to go into the ionic form that it matters little whether they are combined with a complex which readily becomes an ion, or not.

The salts, like other very strongly ionised compounds, do not follow the dilution law of Ostwald.

The results may be summed up in one general rule.

Salts which have an analogous constitution are ionised to the same extent in dilute solutions of equal concentration.

The numbers given are calculated from the tables in the Leitvermögen der Elektrolyte, and correspond to a temperature of 18°, and a dilution of ten litres.

Salt. KC NaCl LiCl NH4Cl KI KNO, AgNO3 KCIO3 KC2H3O2

Degree of 0.85 0.84 0'82 0·85 0·86 0·83 0.81 0.82 0.85

Salt.

Degree of ionisation

Salt.

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BaCl2 SrCl2 CaCl2 Ba(C2H3O2)2 Ca(C2H3O2)2 Ba(NO3)2 ZnCl2

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It is worthy of remark that although acetic and hydrochloric acids, at a temperature of 18° and a dilution of IO litres, are ionised to 1°3 per cent. and 91 per cent. respectively, potassium acetate and potassium chloride are both ionised to 85 per cent. at the same temperature and dilution.

Salts of the two last classes (e.g. BaCl2, MgSO) are among the compounds which give complex ions. At the dilution to which the numbers refer, the concentration of the complex ions would be small. But the cadmium salts, which show a great tendency to give complex ions, are ionised to a much smaller extent than corresponding salts of other metals, and therefore form an exception to the general rule. The mercury salts are also very weakly ionised.

The Valence Rule.-An empirical rule has been found by Ostwald1 and his pupils which shows a connection between the variation of conductivity with dilution, and the valencies of the anion and cation from which the salt is formed. It may be expressed in the form—

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where A, and A, have their usual meanings, n, and no are the valencies of the anion and cation, and C, is a number depending only on the dilution and temperature.

The rule applies only to those salts which ionise "normally," i.e. which do not undergo hydrolysis or give complex ions; and it approximates most nearly to the fact when v is such that A, A, is comparatively small.

In practice the rule is generally used in the form—

A1024 A32 = n1n2C

The value of C is found to be approximately 10 at 25°, e.g.

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This rule has been often used to determine the basicity of acids, and is especially useful as a test for hydrolysis in cases -e.g. sodium salts-where abnormality of ionisation can be only due to hydrolysis.

It has been found by this method that KH,PO, KHASO behave as salts of monobasic acids, and that K2HPO, K2HAsO behave as salts of dibasic acids, except in dilute solutions, when hydrolysis occurs."

1 Zeitschr. phys. Chem., 1. 109, 529 (1887); 2.7901 (1888).
2 Walden, Zeitschr. phys. Chem., 2. 49 (1888),

3 Walden, loc. cit.

§ 4. DOUBLE AND COMPLEX SALTS.

Hittorf,1 from his work on the transport numbers for salts, divided double salts into two classes: (1) those which ionise simply, i.e. behave as ordinary salts, and (2) those which split up into the ions of the salts of which they are composed, i.e. behave in solution as mixtures of the salts from which they are formed. After Ostwald, the salts of the first class are called complex salts, and the name "double salts" is reserved for those of the second class.

It is easy to distinguish the two classes by means of ordinary chemical reactions. Thus, a solution of potassium ferrous sulphate gives the reactions of the ions K, Fe", and SO", but a solution of potassium ferrocyanide gives no reaction for Fe" or CN', but only for K and a complex ion (FeC.N)”.

Examples of complex salts are

KзFeCN, KAg(CN)2, (NH,);Fe(C2O), K2PtCl
Examples of double salts are-

K2SO,MgSO6HO, K2SO„Al(SO4)32 4H2O

Later work has, however, shown that the distinction between complex and double salts is one of degree and not of kind.

The electrical conductivity of a mixture of solutions of two salts can be calculated from the conductivities of the separate solutions, if no reaction occurs between the salts or their ions. On mixing solutions of two salts which together form a complex salt, a reaction takes place the formation of complex from simple ions and the conductivity becomes less than that calculated. But a mixture of solutions of two salts which together form a double salt should give the calculated conductivity, since, according to the definitions of double salts, no reaction should take place. Now, it has been found 2 for

1 Pogg. Ann., 106. 513 (1859).

2 Jones and Mackay, Amer. Chem. Jour., 19. p. 83.

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