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are not formed by different modes of ionisation of the same group, as was the case with the above compounds. The presence of both a positive and a negative group in the molecule is accompanied by a weakening of both the acid and basic properties, so that in general, amido acids are weak acids and weak bases. But if one group is much more strongly positive than the other is negative, or vice versa, the weakening effect can proceed so far that either the acid or the basic property becomes immeasurably small. Thus, orthoamidobenzene-sulphonic acid has no basic property (i.e. cannot form salts with acids) owing to the great influence of the strongly negative SOH group; and betaine gives no sodium salt, since the acid properties of the compound have disappeared, owing to the effect of the strongly positive - N(CH) OH. Since the -NH,- group is only weakly basic, all simple amido acids are more strongly acid than basic.

The ionisation-constants of these compounds cannot be determined from the conductivity, unless they have very much stronger acid properties than basic, as in the amido-sulphonic acids, or very much stronger basic properties than acid, as with betaine. For in other cases the mode of ionisation is complicated, and, moreover, the conductivity is very small. The ionisation - constants given below were calculated by Winkelblech from the results of his experiments on the hydrolysis of the salts of amido acids with acids and bases.

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A comparison of the numbers for the amido-benzoic acids with those for aniline (I'I × 10-1) and benzoic acid (60 × 10-8) shows that the decrease in ionising power of the basic group is much greater than that of the acid group. It is also seen that the NH2 group and the COOH group have

the greatest effect on one another when they are nearest together, that is, when they are ortho to one another.

From the above numbers it appears that the stronger base is also the stronger acid, with the exception of asparagine. In the case of the amido-benzoic acids, this is easily explained by the weakening effect of the groups on one another, which is less the greater the distance between the groups. But no explanation has been found of the fact that in the series glycocoll, sarcosine, betaine, which in this order contain groups of increasing basic properties, the basic properties decrease almost as rapidly as the acid properties. It must, however, be borne in mind that the values given for the ionisation-constants for the "basic" ionisation are not the real ionisation-constants (see p. 128).

In the ionisation of such a compound as glycocoll CH2.NH OH

COOH

(assuming the nitrogen to have gone over into

the pentavalent condition by taking up water) the ions CH..NH, CH.NHOH

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CH,NH

, OH', H, and
C

coo

are possible. COOH COO' Since the amphoteric electrolytes have only small ionisationconstants, the first two of these ions will be present to a very small extent. The last of the above ions carries equal positive and negative charges, and is therefore neutral. (Such ions are called zwitter-ions, or, according to the old nomenclature, intramolecular salts.) This ion can exist to only a small extent in aqueous solution, since its reaction with water is quite analogous to the hydrolysis of the salt of a very weak acid with a very weak base. And the concentration of OH' and H will also be small. Thus, on the whole, a solution of an amphoteric electrolyte, the acid property of which is not very much greater than its basic property, contains mostly non-ionised molecules, and possesses only a small electric conductivity.1

1 For a full theoretical discussion of amphoteric electrolytes, see Walker, Proc. Roy. Soc., 73. 155 (1904).

T. P. C.

L

CHAPTER III

THEORY OF CHEMI-ELECTROMOTIVE FORCE

§ I. VOLTAIC AND ELECTROLYTIC CELLS FARADAY'S laws express in the most general form the part that quantity of electricity plays in chemical reaction; that, however, is only one aspect of the relations between chemistry and electricity. On the chemical side it is necessary to consider the intensity with which a reaction tends to occur, or what may be described as the affinity producing it; on the electrical side, not merely the quantity of electricity moved, but the work done in its motion. This last is expressed by electromotive force (or its synonyms voltage or difference of potential), and electromotive force and affinity answer to one another in the same way as quantity of electricity and amount of reaction do. To elucidate this abstract statement by examples and precise rules is the chief business of modern theoretical electrochemistry.

An arrangement for performing an electro-chemical reaction is known either as a voltaic or an electrolytic cell (to the latter class belong the voltameters already considered). There is no difference in essential construction between them, however; the former term is applied to a cell in which the reaction is capable of yielding work, and therefore current; the latter when current has to be supplied at the expense of some external source of energy. So closely are the two related that the same appliance may act as both. The familiar lead accumulator is a voltaic cell whilst discharging, an electrolytic cell when being charged.

Each consists essentially of a pair of electrodes, with

their corresponding electrolytes, the electrolytes serving as connection between the electrodes. Thus the Daniell cell consists of (a) a zinc electrode in a solution of zinc sulphate (b), a copper electrode in a solution of copper sulphate; and the two half cells are combined by putting the solutions in conducting contact. The complete scheme is therefore

Zn: ZnSO1aq.1 : CuSOaq. : Cu

But there need not be two distinct electrolytes; thus, when dilute sulphuric acid is electrolysed between platinum plates, the scheme is

Pt: H2SO1aq.: Pt

The first distinction to be drawn with a view to classifying cells is that the process occurring at a single electrode may or may not be reversible. The two cells just mentioned yield examples of this. When current is led through a Daniell cell from the zinc to the copper-the direction in which it naturally flows, more ions of zinc are formed from the solid electrode; if by means of another battery current be sent the reverse way through the cell, it will carry some of the zinc ions with it, and deposit them in the metallic state on the electrode. The two processes are the precise converse of one another, and no new substances are formed in them. This, then, is a case of a reversible electrode, and in particular it is reversible with respect to a cation, viz. Zn".

When current is led through the water voltameter, from left to right in the above scheme, the consequence at the lefthand electrode is the production of oxygen, which after a while appears in bubbles there. If, however, the current be led through in the opposite sense, it cannot produce the reverse effect of taking oxygen out of the electrode into the solution, for there is no oxygen in the electrode; what happens is liberation of hydrogen there; such an electrode is therefore irreversible. We shall at present consider only reversible electrodes, and make the further classification into those reversible with respect to a cation (first kind) or an anion (second kind).

Aq. (aqua) means an indefinite quantity of water.

A reversible cell may therefore be made up of any combination of those two kinds.

The first kind of electrode consists usually of a metal, immersed in a solution of one of its own salts. We have already had one instance of this in the zinc of a Daniell cell; the copper of the same cell is another. It was pointed out on p. 23, however, that current may be led into a solution, not by formation of a new ion, but by raising the charge of one already existing. Hence arises another way of making an electrode of the first kind: if a platinum plate be immersed in a mixture of ferrous and ferric salt, when current is led in by it some Fe (ferrous) ions will be converted into Fe (ferric); when current is led out by the same electrode, ferric ions will be reduced to ferrous; i.e. we have a reversible electrode.

The second kind of electrode is not so simply made, as negative substances, oxygen, chlorine, and so on, are not themselves conductors. They may, however, be dissolved in platinum and similar metals. Thus, if a sheet of platinum be immersed half in a chloride solution and half in gaseous chlorine, the platinum is no longer to be regarded as in itself constituting the electrode'; the chlorine is the active material. When the arrangement is used as anode, the current is led in by discharge of negative ions, i.e. formation of gaseous chlorine from the chlorions of the liquid when as cathode, the reverse process occurs. But as the chlorine which the platinum can hold in solution is very limited in amount, it would be easy by passing too much current out to use it all up and reduce the electrode to the irreversible condition described above in the case of oxygen. Such an electrode is, however, strictly reversible if not spoilt by too much current, for if the chlorine contained by the platinum is only slowly used up, it will be continuously replaced by diffusion from the gaseous atmosphere above. For the actual construction of gas electrodes, whether for use with anionic gases such as chlorine and oxygen, or with the cationic hydrogen, see p. 239.

A more usual method of forming an electrode of the

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