Influence of Light on Uranium Compounds

It has already been mentioned that uranyl salts in solution exhibit a greenish fluorescence. If the solutions are exposed to light in the presence of readily oxidisable organic substances, such as alcohol, reduction takes place, uranous salts being formed. At the same time the fluorescence disappears, the uranous salts not exhibiting this property. This reduction under the influence of light is known as photolysis, and has been the subject of much investigation. The action is accelerated by the presence of substances which destroy fluorescence, and the two phenomena appear to be connected. It has been suggested by Baur that when light is absorbed by the uranyl ion, the uranium assumes a more active condition. If any substance is present with which the active uranium can react, it does so, and suffers reduction; but if not, the reverse reaction takes place, with emission of fluorescence. Baur further suggests that during absorption of light the hexavalent uranium is resolved into octavalent and trivalent uranium thus:

5U^VI + light → 3U^VIII + 2U^III,

and that, as the absorbed energy is given off in the form of fluorescence, the reverse reaction occurs, the hexavalent uranium being again produced. This reverse reaction, however, is prevented from taking place by the presence of substances which destroy fluorescence; these are generally either reducing agents, such as oxalic acid, potassium iodide, or uranous salts, which might be expected to destroy any salts of octavalent uranium; or oxidising agents, such as ferric salts, which would react with salts of trivalent uranium. In the decomposition of oxalic acid by means of uranyl sulphate under the influence of light, carbon monoxide and carbon dioxide are evolved, and the trivalent uranium compounds react with unchanged hexavalent uranium, forming products in an intermediate stage of oxidation. A small quantity of formic acid is found in the solution, but this is unable to accumulate since it is used up in reducing the uranyl-ion to uranous-ion. The final product of the reduction of uranyl oxalate is a violet hydrated form of urano-uranic oxide.

Many oxidation and reduction reactions, especially in organic chemistry, are in consequence considerably influenced by the presence of uranyl salts under the action of light. The reaction may also be used as a means of preparing uranous salts, by exposing to light a solution of the uranyl salt containing the requisite quantity of the acid entering into the constitution of the salt, and a readily oxidisable substance such as alcohol. In the case of the chloride the reaction is as follows:

UO₂Cl₂ + 2HCl + C₂H₅.OH = UCl₄ + CH₃.CHO + 2H₂O.

If insufficient acid is added, basic salts result; while if the reaction is allowed to proceed further, a black precipitate of uranous hydroxide is obtained.

Photoelectric Cells

A noteworthy effect which appears to be in some way connected with photolysis was discovered by Baur, who observed that the potential difference at a platinum electrode immersed in a solution containing both uranyl and uranous salts was considerably less positive when exposed to sunlight than when kept in the dark. If two platinum electrodes are immersed in a solution of the mixed salts, and it is arranged so that one electrode is exposed to a bright light while the other is kept in the dark, the cell can be made to furnish a current. The displacement of potential appears to be due to changes which occur in the uranyl salt under the influence of light, possibly with formation of pentavalent and octavalent uranium. The effect is weakened by the presence of those substances which inhibit the fluorescence of uranyl compounds.

Many uranium salts, like other fluorescent and phosphorescent substances, are triboluminescent - that is, they emit light when crushed.