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AN EXPERIMENT TO DETERMINE A PHOTOCHEMICAL QUANTUM YIELD ARTHUR A. VERNON and GEORGE S. FORBES Northeastern University, Boston, Massachusetts
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SATISFACTORY and relatively inexpensive experiment to determine the quantum yield of the photolysis of malonic acid can be performed by comparing the photolysis of this acid with that of oxalic acid. The quantum yield for the photolysis of oxalic acid sensitized by uranyl ion has been shown to be 0.6 over a wide range of concentration and wave length of absorbed light.' The oxalic acid-uranyl ion solution is used as an actinometer to determine the total quanta absorbed. The decomposition of oxalic acid is represented by the equation
HCIO,
+ hu
H*O
+ CO + COz
and malonic acid by &CsO,
U(kt' + hv , CH8COOH + COI
Into each of two 250-ml. beakers is put a mixture of 25 ml. of 0.01 M uranyl nitrate and 25 ml. of 0.01 M oxalic acid. Into each of two other beakers of the same size is put the same amount of uranyl nitrate and 25 ml. of 0.01 M malonic acid. The four beakers are placed in the reaction box with the two oxalic acid solutions in the middle and the two malonic acid solutions on the ends. The housing of the 8-watt germicidal mercury lamp is lowered onto the reaction box surface. After a 45-minute exposure, the positions of the solutions are exchanged and a second 45-minute exposure is made. This procedure averages the light intensity. After exposure the total oxalic acid of each of the two solutions is titrated hot with 0.04 N KMnO, by the standard procedure. The 10-mi. samples of exposed malonic acid-uranyl nitrate solutions are titrated similarly. Samples of each unirradiated original solution should be titrated to obtain the initial concentrations. 1 FORBES, G. S. AND W. LEIQHMN, J.Am. Chem. Soc., 52,3139 (1930).
350
It is important to stress the difference in the reactions of potassium permanganate with oxalic and malonic acid. They are represented by the equations: 2KMn04 GKMnO,
+ 5(COOHb + 3HgS04
ZMnSO, + &SO, + 10COs + 8Hn0 +GMnSO, 5CH9(COOH)2+ 9HB04 + 3KBOd + IOCOI + SHCOOH + 14&0
1.0 ml. of 0.04 N potassium permanganate is equivmoles of oxalic acid or 0.66 X 10" alent to 2.0 X moles of malonic acid. From the decrease in the number of molecules of oxalic acid during exposure and the known quantum yield, the total quanta absorbed by the solution can be calculated. Similarly, from the decrease in the number of molecules of malonic acid during exposure the quantum yield of malonic acid photolysis can be obtained assuming complete absorbance for each solution. A quantum yield of 0.27 has been reported for the photolysis of malonic acid in the presence of uranyl ions with only slight variation with wave length.% 'Pre~cn,W. C., A. LEVIMN, A N D W. A. NOTES, JR., J . Am. C h m . Soe., 51,80 (1929).
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