Photochemical determination of the solubility of oxygen in various media

Tdma,

Vol. 37, No. 9, pp. 905-909,

1990

0039-9140/90

Printed in GreatBritain

$3.00 + 0.00

Pergamon Pressplc

PHOTOCHEMICAL DETERMINATION OF THE SOLUBILITY OF OXYGEN IN VARIOUS MEDIA CHRIS FRANCO and JOHNOLMSTEDIII*

Department of Chemistry and Biochemistry, California State University, Fullerton, CA 92634, U.S.A. (Received

11 August 1989. Revised

16 February

1990. Accepted 22 February 1990)

Summary-A

photochemical method for determining the oxygen concentration in air-saturated nonaqueous solvents has been developed. Solutions containing a sensitizer (Rose Bengal or Methylene Blue) and 1,3-diphenylisobenxofuran (DPIBF) as an oxygen acceptor are irradiated at 546 or 633 nm and the absorbance at 404 nm is monitored. The dissolved oxygen content is found from the change in absorbance and the known 1: 1 stoichiometry of addition of singlet oxygen to DPIBF. The solubilities found, accurate to +6%, for oxygen in air-equilibrated solvents, are (mM): acetone, 2.37; acetonitrile, 2.42; dimethylsulfoxide, 0.33; ethanol, 1.94; N-methylformamide, 1.31. Measurements on mixed acetone-N-methylformamide solvents showed that the solubility of oxygen does not vary with solvent composition in a predictable manner.

The solubility of oxygen in non-aqueous solvent systems is of considerable interest both in photochemistry, where oxygen-quenching of excited states is one of the predominant bimolecular processes, and in electrochemistry, where cathodic production of superoxide anions from dissolved oxygen is an important reduction process. The values cited in compilations of oxygen solubility data’** and in a recent report of determinations based on a gas chromatographic method3 show significant variation. For dimethylsulfoxide (DMSO), for example, some values cited are nearly twice as large as others.’ Since we required accurate values for oxygen concentrations in solution for a study of oxygen quenching of luminescence,4 we have developed a rapid and straightforward photochemical method for measuring them. It relies on the clean photo-oxygenation reaction of a wellknown singlet oxygen acceptor, 1,3-diphenylisobenzofuran (DPIBF).5 Though Demas has exploited luminescence quenching reactions for determination of oxygen concentration6 and photo-oxygenation reactions for chemical actinometry,’ we believe that the use of photo-oxygenation to determine oxygen concentrations has not previously been reported.

In the presence of a suitable sensitizer (S; in these determinations, both Rose Bengal and Methylene Blue were used), illumination of an oxygen-containing solution with light in the visible region results in generation of singlet oxygen: S+hv +s*d’s 3S+O,+S+‘O* When DPIBF is also present in the solution, the singlet oxygen photo-oxygenates the DPIBF (D), leading eventually to oxidation products: D + IO2--+DO2 + products Since DPIBF is the only component of the solution that absorbs radiation of wavelength 404 nm, the progress of this reaction sequence can be readily monitored by measuring the decrease in absorbance at 404 nm. As long as oxygen is present in the system, illumination will result in disappearance of DPIBF. When all the oxygen has been consumed, the reaction sequence stops. The net decrease in absorbance is quantitatively related to the loss of DPIBF and, since the reacting ratio of oxygen and DPIBF is 1: 1, the amount of oxygen originally present in the solution can be determined.

*Author for correspondence. 905

CHRISFRANCOand JOHNOLMSTED III

906 EXPERIMENTAL

Reagents DPIBF (Aldrich), Rose Bengal (Eastman, 82% certified dye content), and Methylene Blue (Aldrich, certified) were used as received. Spectral grade ~-methylfo~amide, dimethylsulfoxide and acetonitrile and HPLC-grade acetone were obtained from Aldrich and used without further purification. Absolute ethanol was obtained from Spectrum. Apparatus All absorbance measurements were made with a Beckman DU-7 spectrophotometer. When Rose Bengal was the sensitizer, an Illumination Industries Model LH371Q mediumpressure mercury arc lamp was used as the light-source, with its output filtered through water, a Corning 3-73 yellow glass filter, and an Oriel 546~nm interference filter. For sensitization with Methylene Blue, the output from an LGK 7628 He-Ne laser operated at 6 mW was passed through a defocusing lens so that the light beam filled nearly the full face of the irradiation cell. Determination DPI%F

of the molar absorptivity

of

Enough DPIBF to yield a 1.00 x 10V2&f solution was weighed into a IO-ml standard flask, solvent was added up to the mark, a small magnetic stirrer bar was added and the solution was stirred, while protected from light, until all the solute had dissolved. At regular intervals during the stirring, IO-$ aliquots were removed, quantitatively diluted to 3.00 ml, and the absorbances at 404 nm measured. This was continued until complete dissolution of the solid was indicated by no further change in the absorbance, which was then used to compute the molar absorptivity of DPIBF in the solvent used.

syringe, then the cuvette was filled by syringe with the solvent of interest to a point well above the stopcock. The sealed cuvette was placed in a cell holder and the contents were mixed by magnetic stirring until the DPIBF was completely dissolved, as indicated by constancy of the absorbance at 404 nm for a 10.0~~1sample, withdrawn and mixed with 3.00 ml of solvent. The sealed cuvette was then repeatedly irradiated with 546~nm light for constant periods of time (t~ically 20 set), the absorbance at 404 nm being measured after each irradiation, To determine the oxygen concentration of the solution, absorbance vs. time plots were prepared (e.g., Fig. l), from which the total change of absorbance due to i~a~ation, A.4, could be found. By use of the molar absorptivity, AA was converted into amount of DPIBF consumed, which corresponded to the amount of oxygen present in the solution before the irradiation was started. RRSULTS AND DISCUSSION

The results for five different solvents and a set of mixed ~-methylfo~amid~~tone solutions are presented in Table 1. Each value is the mean of 3-5 determinations. The absolute average deviations (AAD) indicate the reprod~ibility of the dete~inations; the relative average deviations ranged from 2.5% (DMSO) to 8% (acetone) and were typically 6%. We can judge the accuracy of our method by the results for acetone, for which there is

Procedure All measurements were made at room temperature (25 f 2”). All solvent samples were pre-eq~~brated with air by passage of an air stream through them for at least 20 min. A glass cuvette of known volume, fitted with a stopcock, was charged with a weighed amount of DPfBF and a small magnetic stirrer bar (also of known volume). Sufficient stock solution in the test solvent sensitizer to give a final absorbance at 546 nm greater than 1.0 was added by

limo (se@ Fig. 1. Plot showing the change of absorbance of diphenylisobenzofuran (at 404 nm) with time of irradiation, for air-saturated acetone containing Rose Bengal as sensitizer. For this irradiation, AA = 0.132.

901

Solubility of oxygen Table 1. Solubilities of oxygen in air-saturated solutions This work

Solvent

Solubility, IO-3lU

Absolute average deviation, lo-‘A4

Acetone

2.31

0.19

Acetonitrile Dimethylsulfoxide

2.42 0.33

0.14 0.008

Ethanol N-methylformamamide 3 : 1 NMF : acetone* 1: 1 NMF : acetone* 1: 3 NMF : acetone*

1.94 1.31 0.79 0.82 1.76

0.09 0.04 0.05 0.05 0.11

Literature values, lo-‘M 1.89-2.42 2.40 2.3 1.70 0.32-0.58 0.44 2.0 -

References

1 2 3 3 I 3 1, 8, 9

*Mole ratio.

agreement among four independent literature values that the solubility of oxygen in acetone saturated with air is 2.35 f 0.05 x 10e3M at 25”. Our value, 2.37 + 0.19 x 10d3M, is in excellent agreement with this consensus value. We obtained the same value with either Methylene Blue or Rose Bengal as sensitizer. Ethanol provides a second benchmark for judging our method. The consensus value as evaluated by Battino’ has been verified by Tokunaga’ and most recently by Ltihring and Schumpe,’ all of whom find a concentration of 2.0 + 0.05 x 10m3M in an air-saturated solution. Our measurements with Methylene Blue as sensitizer gave 1.94 + 0.09 x 10m3M. Measurements with Rose Bengal gave a value 15% lower, but the stoichiometry of consumption of DPBIF by oxygen is not 1:l for this combination of sensitizer and solvent. It has been shown that Rose Bengal sensitizes production of superoxide anions as well as singlet oxygen,” and in a protic solvent such as ethanol, the superoxide is destroyed by proton abstraction from the solvent.” For dimethylsulfoxide, the reported values cover a relatively wide range (we have converted values for equilibrium with pure oxygen at 1 atm pressure into values for equilibrium with air, for purposes of comparison): 3.23 x 10w4M,124.4 x 10-4M,3*‘34.64 x 10T4M,14 and 5.80 x 10-4M.‘s Our result, 3.28 f 0.08 x 10e4M, is in excellent agreement with the lowest of these values but is not compatible with the others. For this solvent, Methylene Blue is not a suitable sensitizer; irradiation causes decomposition of the sensitizer, accompanied by generation of a gaseous product, indicating complex photochemistry.

In the case of acetonitrile, only one previous determination seems to have been made, Achord and Hussey3 reported a value of 1.7 x lo-‘M, significantly lower than our value of 2.42 k 0.14 x 10m3M. We also obtained a lower value than theirs for dimethylsulfoxide, indicating that the discrepancy, which falls well outside the reported uncertainties in the experiments, is not due to a systematic error. As was the case for acetone, our solubility determinations for acetonitrile gave the same results with either sensitizer. Errors in our determinations could arise from incomplete saturation with air, incorrect molar absorptivities, oxygen desorbed from the cell walls, photodegradation of DPIBF by competing reactions, or competing photoreactions with oxygen. Since the solvent samples were likely to be air-saturated as received, and air was bubbled through them for 20 min, lack of saturation is unlikely. The molar absorptivities of DPIBF in each solvent were independently determined. Though they differed slightly from solvent to solvent, all values were about 2.2 x 10“1.mole-’ . cm-‘, which is of the same order as that reported for DPIBF in dimethylformamide (2.8 x 10“ at I,,, = 415 nm).16 The initial absorbances of the solutions prepared for irradiation were also found to be consistent with the independently determined molar absorptivities. We have found evidence for either desorption of oxygen or a competing photoreaction in dimethylsulfoxide and N-methylformamide, where the plots of absorbance US. irradiation time showed a small but distinct downward slope after the consumption of oxygen was apparently complete. Irradiation of argonsaturated solutions under identical conditions

908

CHRISFRANCOand JOHNOLMSTED III

also yielded slowly decreasing absorbances, with the same slopes. Since this slow consumption of DPIBF occurs with only two of the four solvents we believe that it is due to a solventdependent competing photodegradation reaction rather than to desorption of oxygen from the cell walls, which should be the same for all the solvents tested. In any case, we have corrected our results for this slow process by extrapolating the final slope to zero time and measuring AA from the initial absorbance to the intercept. For two of the solvent-sensitizer combinations, our observations indicate that competitive reactions involving oxygen are involved: Methylene Blueedimethylsulfoxide and Rose Bengal-ethanol. Comparison measurements with different sensitizers provide a sensitive way of identifying the presence of such complications. Mixed solvent systems are frequently attractive for studies of photophysical processes, as they afford an easy way to adjust solvent properties (viscosity, dielectric constant) as well as substrate solubility. There are virtually no data available on the solubility of oxygen in mixed solvents other than water-alcohol mixtures. We have chosen to look at the N-methylformamide-acetone system, which yields an extremely wide range of dielectric constants. A consideration of the thermodynamics of ternary mixtures of a gas and two liquids shows that the Henry’s law constants (H) may be expected to follow a logarithmic equation:” lnH,i,=x,lnH,+x,lnH,-AG,/RT

(1)

where AG, is the molar excess Gibbs free energy of the solvent mixture containing the two solvents 1 and 3 in mole fractions x, and x3, and Hi is the Henry’s law constant for the ith solvent system, relating mole fraction of dissolved gas, x2, to gas pressure p2: ~z=-G-fi Figure 2 shows the logarithms of the Henry’s law constants obtained from our measurements, vs. mole fraction of solvent. Also shown is the solubility curve calculated from the logarithmic equation by using excess Gibbs energies from the literature.18 It is clear that the oxygen solubility relationship is more complicated than the

1 4 I

E

, 4.5 0.0

. '. 0.2 0.4

. 0.6

-. 0.8

10

MoleFraction-NMF

Fig. 2. Variation of the Henry’s law constant H with solvent composition for oxygen in acetone-N-methylformamide mixed solvent. Open squares are experimental data points (error bars indicate average absolute deviations), the solid line is computed from the thermodynamics [the solid squares were calculated by using equation (l)].

logarithmic equation predicts. The shape of the observed variations is similar to that computed from the excess free energies of mixing, but the magnitude of variation at high acetone concentrations is much larger than predicted. While this behavior may appear anomalous, similar observations of solubilities that exhibit maxima or minima at intermediate solvent compositions have been reported, e.g., acetylene dissolved in acetone-hexane mixtures.” Our observation of such deviations from simple expectation emphasizes the necessity of determining experimental solubilities for work associated with the oxygen contents of mixed solvent systems. The solubility of oxygen in an acetone-rich mixed solvent is sufficiently lower than the oxygen content of the pure solvents for bubble formation to be observed on mixing of the two components, if equilibration is rapid enough. For a 1: 1 mole ratio solution, 0.026 ml of gas should be liberated per ml of solvent. We have looked for evidence of bubble formation is freshly-mixed acetone-l\r-methylformamide solutions, both stirred and unstirred, with and without boiling chips present to provide nucleation centers. No bubbles were observed. As our solubility determinations are consistently reproducible, we attribute the lack of bubble formation to slow equilibration between the gas and liquid phases after mixing of these solvents.

Solubility of oxygen for this reasearch was provided by the Department Associations Council of the Associated Students, California State University, Fullerton. Most of the experiments were done by Mr. Franc0 in an undergraduate research project. Acknowledgemems-Support

REFERENCES 1. R. Battino (ed.), Solubility Data Series, Vol. 7, Oxygen and Ozone, Pergamon Press, Oxford, 1981. 2. S. L. Murov, Handbook of Photochemistry, p. 89. Dekker, New York, 1973. 3. C. L. Hussey and C. L. Achord, Anal. Chem., 1980,52, 601. 4. C. J. Timpson, C. C. Carter and J. Olmsted III, J. Phys. Chem., 1989, 93, 4116. 5. J. Olmsted III and T. Akashah, J. Am. Chem. Sot., 1973, 95, 6211. 6. J. R. Bacon and J. N. Demas, Anal. Chem., 1987, 59, 2780.

909

7. J. N. Demas, R. P. McBride and E. W. Harris, J. Phys. Chem., 1976, SO, 2248. 8. J. Tokunaga, J. Chem. Eng. Data, 1975, 20, 41. 9. P. Liihring and A. Schumpe, ibid., 1989, 34, 250. 10. V. S. Srinivasan, D. Podolski, N. J. Westrick and D. C. Neckers, J. Am. Chem. SOL, 1978, 100, 6513. 11. D. T. Sawyer and M. J. Gibian, Tetrahedron, 1979, 35, 1471. 12. J. H. Dymond, J. Phys. Chem., 1967, 71, 1829.

13. E. L. Johnson, K. H. Pool and R. E. Hamm, Anal. Chem., 1966, 38, 183. 14. W. R. Baird and R. T. Foley, J. Chem. Eng. Data, 1972, 17, 355. 15. N. V. Chaendo, G. I. Sukhova, N. K. Naumendo and I. A. Kedrinskii, Russ. J. Phys. Chem., 1979, 53, 1133.

16. A. Zweig, G. Metxler, A. Maurer and B. G. Roberts, J. Am. Chem. Sot., 1967, B9, 4091. 17. J. H. Hildebrand, J. M. Prausnitz and R. L. Scott, Regular and Related Solutions, p. 131. Van Nostrand Reinhold, New York, 1970. 18. U. Seyffert, K. Francke and K. Quitsch, Z. Phys. Chem. (Leipzig),

1974, 255, 969.