Optimum Composition of Uranyl Oxalate Solutions for Actinometry

Abstract

Continued experience with the uranyl oxalate actinometer has led us to vary the concentration of its components, the better to adapt them to the frequency and intensity of light employed. We have measured the quantum yields, 4, of such solutions in terms of d,s for a "standard" solution, 0.01 M in uranyl sulfate and 0.05 M in oxalic acid, carefully investigated. 1*2 Commercial uranyl sulfate is often of dubious quality, and then requires laborious purification.2 Heidt and Daniels2 obtained uranyl oxalate in sufficient purity merely by combining commercial uranyl nitrate with oxalic acid, washing the precipitate and drying at 110'. They photolyzed a solution 0.01 111 in uranyl oxalate and 0.05 114 in oxalic acid at 1313 mp, and found I$ = 0.57, identical with I#I~ found by them and by Leighton and Forbes1 a t the same wave length using uranyl sulfate.There are at least three good reasons for introducing less than 0.05 mole of oxalic acid per liter.(1) The number of quanta absorbed by the actinometer must be calculated from a relatively small difference between two titrations of total oxalate. If this total is decreased, a smaller difference can be determined with equal percentage accuracy, and the time required for actinometry in the course of a photochemical experiment can be shortened.( 2 ) Excess of oxalate exerts a strong inner filter effect if X < 254 mp. Thus at A208 mp the extinction coefficient K H~c~o , = 1300 as compared with KUO,C~O~ = 4500. (3) at A208 mp unsensitized photolysis of oxalic acid occurs (d, = 0.02).3 For reasons (2) and (3) confusion results if concentrations vary.If less than five molecules of the weak oxalic acid are added to one of uranyl sulfate, liberation of hydrogen ion brings the reaction to equilibrium short of "complete" conversion to uranyl oxalate. Consequently the inner filter effect of uncombined uranyl ion is not at a minimum, nor the concentration of uranyl oxalate (presumably the photolyte) at a maximum. Then d, falls below d,, and becomes :t function of ~oxicentration.~ But if one starts (I) W.G.Leightonand G.S Forbes,Tars JOURNAL,^^,^^^^ (1930) (2) Heidt and Daniels, ibid ,, 54, 2384 (1932).(3) Brackett and Forbes, ibid., 56, 4459 (1933) (4) Bucbi, 2 physik Chcm , 111, 269 (1924) with 0.005 M uranyl oxalate, and maintains even half a molecule of oxalic acid in excess throughout the photolysis, $I can be assumed constant at d,, as shown below. The advantages of restricting excess of oxalate can thus be realized without encountering diminished and variable quantum yields.No great decrease in oxalate concentration is feasible through cutting down uranyl concentration when 436 mp > X > 366 mp because transmissions become inconveniently large unless reaction layers are made unduly thick. But when X = 205 mp, "standard" solution absorbs 7770 of incident light 0.01 mm. from the front window. Vigorous stirring is then necessary to avoid depletion of oxalate, liberation of uranyl ion and consequent decrease of d, in this thin layer. Using 0.001 ,I1 uranyl sulfate or oxa...

Photochlorination in a tubular reactor

“…Initial runs were made without light in the quartz reactor (10). With very precise analytical conditions, a sm.all amount of dark reaction was detected (by the formation of small quantities of hydrogen chloride).…”

Practical Chemical Actinometry—A Review

“…At the above typical concentration,c Φð254Þ ¼ 0:60 and Φð435Þ ¼ 0:58 with a minimum between, Φð366Þ ¼ 0:49 has been reported at 25°C (94). Subsequent works confirm these results (94,95,103,104).…”

“…Description of the actinometer. Pioneering studies in the 1930s (94)(95)(96) formed the basis for the wide application of uranyl oxalate as an accurate chemical actinometer. The careful work of Leighton and Forbes (94) was the first to determine reliable quantum yields for the photolytic destruction of oxalic acid in the uranyl oxalate aqueous system.…”

Photochemical reaction pathways of ruthenium(II) complexes. Photoredox and photosubstitution reactions of the acetonitrile complex pentaammineacetonitrileruthenium(2+) and of the dinitrogen complex pentaamminedinitrogenruthenium(2+)

“…The principal reaction product is Ru(NH3)sCH3CN3+ and the CTTS process can be represented as Ru(NH3)5CH3CN2* + hv-Ru(NH3)5CH3CN3* + eV (12) The back-reaction, reduction of Ru(III) by e_aq, would be exceedingly fast (k = 7.4 X 1010 M~l sec-1 for Ru(NH3)63+)29 so observation of net photooxidation requires that some other solution component act as a sink for the reducing equivalents generated under the reaction conditions (pH 3). The reactivity of the hydronium ion (k = 2.2 X 1010 M-1 sec-1)29 makes H+aq the probable sink for the hydrated electron (eq 13) since for the initial stages of * + e\, --• (13) conversion H+ is orders of magnitude more concentrated than the more reactive Ru(III) species.…”