Passage of Current Through Solid Crystalline Compounds

C. Tubandt. Über Elektrizitätsleitung in festen krystallinischen Verbindungen II Mitt. Überführung und Wanderung der Ionen in einheitlichen festen Elektrolyten. Zeitschr. anorg. u. allg. Ch. 115, 105—126, (1921).

When current passes through an electrolyte solution, electricity is transported partly by anions and partly by cations; by determining the changes in the concentration of the solution at the anode and cathode, one can find quantitatively what fraction of the total current is carried by ions of each kind, i.e., determine the corresponding transport numbers. This method is not applicable to solid electrolytes, and in this connection our information about the mechanism of the transport of electricity in solid electrolytes is very limited. Only in the case of solid glass or quartz has the question been fully clarified. As Warburg and Tegetmeier showed, in the electrolysis of these bodies only the sodium (or lithium) ion from the sodium silicate contained in the solid solution moves, whereas the calcium ions and the acid remain immobile. This result was, without sufficient grounds, extended to other electrolytes, as a consequence of which the assertion often encountered in the literature arose that in solid electrolytes only cations possess mobility, while anions are immobile.

The method used by the author is based on the following considerations. Just as, in the case of solutions, the ratio of the mobilities of the ions can be found by measuring changes in concentration, so, in the case of solid electrolytes, the same ratio can be determined from the changes in weight undergone by two cylinders of the substance under study, pressed against one another with polished surfaces, when ions are transported across the boundary between these cylinders. The practical execution of this method is connected with two conditions: 1) the transport of substance must not cause the neighboring cylinders to grow together, so that at the end of the experiment they could be separated from one another without difficulty; 2) the products of electrolysis must be separated in such a form as to make their quantitative determination possible. Experience has shown that the first condition is always fulfilled in practice, provided only that the substances are impeccably pure. Far greater difficulty is presented by the fulfillment of the second condition, since in the electrolysis of most salts the metal is deposited at the cathode in the form of fine filaments, which very rapidly grow through the entire mass of the electrolyte and alter the whole me-

mechanism of the transport of electricity. To avoid this, a small cylinder of silver iodide was inserted between the cathode and the electrolyte under investigation, since from this compound the metal is deposited in the form of a sufficiently compact mass, without a tendency to form dendrites. In some cases it was necessary to introduce also a third intermediate salt. By this method the following substances were investigated:

Silver iodide (regular). The following system was subjected to electrolysis: silver anode, three cylinders of silver iodide, platinum cathode. We shall give the results of one of the experiments \((t = 150^\circ)\): in the coulometer 0.8337 g of silver was deposited; on the cathode 0.8339 g of silver was deposited; the changes in weight of the first, second, and third cylinders were respectively: \(-0.0003\) g, \(+0.0002\) g, \(-0.0001\) g; the loss in weight of the anode was 0.8333 g. Thus, the weight of the separate \(AgJ\) cylinders remained entirely constant, while from the anode there passed into the anodic cylinder, and from the cathodic cylinder there was deposited, an amount of silver equivalent to the quantity of electricity passed. It follows from this that through every cross-section of this electrolyte, when the current passes, silver ions pass in an amount determined by Faraday’s law, while the iodine ions remain immobile; for if the latter took part in the transport of electricity, the anodic cylinder would have had to increase, and the cathodic one to decrease, in weight. The absolute mobility of the silver ion in silver iodide is equal to \(0.55 \cdot 10^{-3}\) cm/sec at \(145^\circ\) (assuming complete dissociation of \(AgJ\)). Analogous results were given by the investigation of silver chloride and bromide; as in the case of silver iodide, the transport of electricity in these salts is effected only by silver ions, while the anions remain immobile. The mobility of the \(Ag\) ion in \(AgCl\) near the melting point of this salt is equal to \(0.030 \cdot 10^{-3}\) cm/sec.

Lead chloride. The following system was subjected to electrolysis: cathode, three cylinders of lead chloride, silver anode. At the end of the experiment, the lead deposited on the cathode was weighed together with the cathodic cylinder of lead chloride, and the \(AgCl\) formed on the anode together with the anodic cylinder. We shall give the results of one of the experiments \((t = 250^\circ)\): weight of the silver deposited in the coulometer, 0.1215 g; change in the weight of the cathodic cylinder \(-0.0401\) g, of the intermediate cylinder \(+0.0005\) g, of the anodic cylinder \(+0.1617\) g, of the silver anode \(-0.1214\) g. It follows from this that the amount of chlorine which left the cathodic cylinder is equal to 0.0401, the amount of chlorine which penetrated into the anodic cylinder 0.400, whereas according to Faraday’s law, on the assumption that the entire transport of electricity is effected by \(Cl\) ions, this amount should have been 0.099. Thus in \(PbCl_2\) the cations prove to be immobile and the transport of current is effected by the anions. A similar result was obtained also for \(PbF_2\).

Silver sulfide. The investigation of this substance, concerning which there is in the literature a whole series of the most contradictory reports, is extremely hampered by the tendency of the metal deposited on the cathode to grow through the electrolyte. By introducing an intermediate layer of \(AgJ\), however, it was possible to prove the following: the \(\alpha\)-modification of \(Ag_2S\), stable above \(179^\circ\), conducts the current like the silver halide salts (the mobility of the silver ion in this case is unusually high, near the transformation point it is equal to 0.11 cm/sec). The \(\beta\)-modification \(Ag_2S\) (below \(179^\circ\)) proved to be a mixed conductor: about 80% of the current is transported, as in the case of the \(\alpha\)-modification, by silver ions; the passage of the remaining 20% is in general not connected with the transport of matter. In other words, \(\beta\)-\(Ag_2S\) is an 80% conductor of the second kind and a 20% conductor of the first kind.

In the case of cuprous sulfide, the modification stable above \(91^\circ\) behaves analogously to \(\alpha\)-\(Ag_2S\); the character of the electrical conductivity of the modification stable below \(91^\circ\) has so far not been established.

A. Frumkin.