PHOTOCHEMISTRY 



297 



than 0.01 of a millimole was oxidized in 20 min. Metadinitrobenzene, 

 paranitrotoluene, and various other substances were found to be inhibitors. 

 Phenol urea was found to be an accelerator. These various substances 

 act as inhibitors not only for the photochemical reaction, but also for the 

 thermal reactions. The close parallelism shows that the thermal reaction 

 as well as the photochemical reaction is a chain reaction. 



Oxalic Acid Sensitized hy Uranyl Ion. — The investigation of this 

 reaction by Leighton and Forbes (34) gives perhaps the most precise 

 photochemical measurements described in the literature. Earlier work 

 on this reaction was unsatisfactory, and wide discrepancies in the results 

 of different investigators were apparent. In this investigation mono- 

 chromatic light from a mercury-vapor lamp and a monochromator was 

 passed through a quartz cell and the absolute energies were measured 

 with a thermopile and galvanometer standardized against a standard 

 lamp of the Bureau of Standards. Several corrections are described in 

 detail. 



Oxalic acid is decomposed by very short ultra-violet light, but it is 

 not decomposed by the longer ultra-violet light or by visible light. 

 However, in the presence of the uranyl ion, this light does cause oxalic 

 acid to decompose into carbon dioxide, carbon monoxide, and water. 

 The oxalic acid is titrated with dilute potassium permanganate. The 

 uranyl salts are strongly fluorescent in ultra-violet light. It is likely that 

 a complex is formed between the oxalic acid and the uranyl ion, which 

 absorbs the light and causes disruption of the oxaUc acid molecule. The 

 uranyl ions are not affected by the reaction and can be used over again 

 in the decomposition of an indefinite amount of oxalic acid. 



Using a solution 0.01 molal in uranyl sulfate and 0.05 molal in oxaHc 

 acid at 25° C, the following quantum yields were obtained at different 

 wave-lengths : 



The temperature effect for a 10° rise is 1.03. 



For the shorter ultra-violet Hght, it has been found (17) that a ten- 

 fold dilution of this solution gives better results, and the quantum yield 

 is not changed by the dilution. For practical use it is found that the 

 uranyl oxalate is easier to purify than the uranyl salt and the oxalic acid. 



This reaction is probably the best for general use in actinometry. 

 The quantum yield is accurately known to a very few hundredths of a unit 

 and it is independent of concentration over wide range ; it is only slightly 

 dependent on wave-length and on temperature. It is reproducible and is 

 free from the erratic variations that frequently accompany chain reac- 

 tions. Investigators in different laboratories have obtained excellent 



