2428
Anal. Chem. 1983, 55,2428-2429
Errors in Ferrioxalate Actinometry Sir: The measurement of light fluxes (intensities),necessary in a number of chemical contexts such as photochemistry, transient absorption, or emission spectrometry, etc., may be approached either by use of calibrated optical detectors (absolute) or by means of chemical actinometers (relative). In view of the difficult and time-consuming nature of absolute measurements together with their requirement for specialized equipment, the majority of chemical investigators have relied on chemical actinometers. Of these, many workers have turned to the ferrioxalate photodecomposition, extensively studied by several groups (1-9). This actinometer offers a number of advantages; it is sensitive, convenient, simple, and inexpensive. Nevertheless it does have some defects; specifically, the quantum yield of production of iron(I1) varies with wavelength, concentration of ferrioxalate, and, to a small extent, temperature. From the combined studies of Parker, (3,5) Lee and Seliger (6),Demas (8),and Hamai (9)and their co-workers, however, the quantum yield is now known at several specific wavelengths in the range 250 nm to 510 nm over which the system can be utilized. The closest rival to the system, potassium “reinckate” (trans-[Cr(NH,),(NCS)~]-),is somewhat less sensitive having only 25% as large a quantum yield of photodecomposition. Also it is less convenient owing to its moderately rapid thermal decomposition. It does, however, offer extension of the operating wavelength range to about 670 nm. For wavelengths longer than this there is currently no convenient actinometric system of equivalent precision.
That such inhibition indeed occurs is shown by the results summarized in Table I. For synthetic solutions equivalent to those developed and diluted as recommended in the literature (5),the absorbance of light at 510 nm increases to its final value at a rate that depends markedly on the ratio [phen]/[Fe2+];see rows 3 to 8 of Table I. We emphasize that with ferrioxalate either absent or present at 0.006 M, these effects are not seen, rows 1 and 2. The data show further that at a [phen]/[Fe2+]ratio of -8:1, a ratio that would pertain for 3 mL of 0.006 M ferrioxalate about 8% decomposed, developed, and diluted to 25 mL (and therefore lying well within the range recommended (5)),color development to the correct final absorbance is complete within 5 min. But if 0.15 M ferrioxalate is used in an otherwise identical set of conditions, 17 h is required for equilibrium to be achieved and the final absorbance reached is only 70% of the correct value, row 3. Note that this would correspond to less than 1%photodecomposition of this more concentrated solution. This effect, we believe, arises from competitive complexing of the phenanthroline by Fe3+in equilibrium with the ferrioxalate.
RELIABILITY
PREVENTION
Since ferrioxalate decomposition occupies such a preeminent position in the current practice of chemical actinometry, with a very large number of quantum yield determinations referenced to it, it is important to establish its complete reliability and reproducibility under all conditions of use. It is therefore worrying that unexplained irreproducibility has been informally reported in its use at 365 nm. More specifically Bowman and Demas (7) reported an important potential source of error arising from slow color development owing to photooxidation of phenanthroline solutions, which had been exposed to ultraviolet light, and cautioned against the use of such aged solutions. It is possible that such effects may have played a role in the irreproducibility referred to earlier, but we have recently observed a second potential error source which we report here. Over more than a decade in our laboratory we have used 0.006 M ferrioxalate solutions in 0.05 M HzS04with complete reliability. In our procedure the exposed ferrioxalate -3 mL is mixed with 6 mL of a 0.05% phenanthroline/0.75 M sodium acetate/0.2 M H2S04buffer solution followed by final dilution to 25 mL. The premixed phen/buffer solution (developer) was always stored in the dark and no slow color development was ever observed, even with actinometer and developer solutions that had been stored for long times, even years. This supports the earlier conclusion (7) that phenanthroline photolysis products were responsible for inhibition of the development of the F e ( ~ h e n ) color. ~~+
Errors arising from this cause might in principle be avoided in any of these ways: (1) Addition of Excess Phenanthroline. Rows 5-8 show that if the [phen]/[Fe2+]ratio exceeds 201, only 20 min is required to reach full absorbance while 501 requires
