In the Laboratory
Inhibition of Sulfite Oxidation by Phenols: Screening Antioxidant Behavior with a Clark Oxygen Sensor László Sipos Erdey-Grúz Tibor Technical School of Chemistry, Debrecen, Hungary H-4024
In this article I describe a simple laboratory exercise to introduce the measurement of dissolved oxygen with a Clark oxygen sensor for a sophomore-level course. The dissolved oxygen of natural waters is very important for living creatures in lakes, streams, and oceans, so its measurement in laboratory courses is relevant to environmental studies (1, 2). Oxygen content is also important from another point of view. In a lot of man-made products (processed foods, plastics, fuels, and lubricants) oxygen content is minimized because it deteriorates these materials by oxidizing them, decreasing their quality and causing unpleasant smell (5, 6 ). To prevent the undesired effects of oxygen, antioxidants are used. This term refers to organic compounds that are added to oxidizable materials to retard autooxidation and prolong the useful life of the substrates (4). The chemical classes that are effective as antioxidants are relatively few. They include hindered phenols, secondary aromatic amines, certain sulfide esters, trivalent phosphorus compounds, hindered amines, metal dithiocarbamates, and metal dithiophosphates. The concept of antioxidants is attributed to S. L. Bigelow for sodium sulfite solution and to A. Lumiere, L. Lumiere, and A. Seyewetz for the protection of photographic developers. The latter authors used the term antioxidant first. Phenols (mainly hydroquinone) are often used with sodium sulfite in photographic developers (7). There are two main analytical techniques to measure dissolved oxygen in aqueous environments: the Winkler method and the Clark oxygen sensor. The Winkler method involves nice chemistry and gives the opportunity to discuss basic chemical concepts like redox reactions and reduction potentials. However, it is slow and time-consuming and therefore has little practical value. In practice, the Clark oxygen sensor is used for the measurement of dissolved oxygen because it allows quick observation of changes in dissolved oxygen content and makes computer-assisted data acquisition possible. This is very important not only to environmental analysis but to fermentation industries as well. The Clark oxygen sensor is a complete voltammetric cell, which consists of a platinum cathode as working electrode and a silver anode. These electrodes are mounted inside a cylinder that contains a buffered solution of potassium chloride. The cylinder is closed with a thin (~20 µm), replaceable, oxygen-permeable membrane of Teflon. The oxygen is reduced on the cathode to water, and under constant polarizing potential the resulting current is directly proportional to the oxygen concentration of the analyte solution (3). The reaction of sulfite with oxygen (eq 1) is slowed or inhibited completely in the presence of certain phenols.
2SO32᎑ + O 2 → 2SO42᎑
(1)
This can be followed easily by the decrease in dissolved oxygen concentration. The experiment demonstrates the effect of antioxidants and differentiates phenols according to their effectiveness. Extension of the technique to structurally similar compounds is also possible. Sulfite oxidation with molecular oxygen, like many other oxidation processes, is a radical chain reaction (8). The chain starts with the formation of SO3ⴢ ᎑ radical, which occurs by means of light or transition metal ion present (eqs 2, 3). hν
SO32᎑ → SO3ⴢ᎑ + e ᎑ Me3+ + SO3 → Me2+ + 2᎑
(2) SO3ⴢ᎑
(3)
The sulfite radical reacts with oxygen to give monoperoxysulfate radical (eq 4), which gives monoperoxysulfate anion and sulfite radical with hydrogen sulfite ion (eq 5). The monoperoxysulfate anion oxidizes sulfite to sulfate (eq 6) and the sulfite radical formed can react with oxygen again, starting a new cycle. Phenols act as radical trapping materials and break the chain by reacting with the radicals (eq 7) (9). SO3ⴢ᎑ + O2 → SO5ⴢ᎑ SO5ⴢ᎑
+ HSO 3 → HSO5 + ᎑
᎑
HSO5 +
SO 32᎑
᎑
(4) SO 3ⴢ᎑
(5)
→ SO4 + HSO4
(6)
2᎑
᎑
PhOH + Rⴢ → PhOⴢ + RH
(7)
Experiment
Materials and Measurement All substances were of analytical grade and used as received. The hand-held oxygen measuring device (Leybold Didactic) was connected to one Clark oxygen sensor (Schott-Geräte, made in Germany) and to a computer supplied with Cassy universal data acquisition software. The data were exported to MS Excel 5.0 and processed there. Procedure Deionized water (50 mL) was added to a 100-mL beaker placed on a magnetic stirrer and equipped with a Clark oxygen sensor at room temperature. Phenol (0.01 mmole) was added and stirred until completely dissolved (~30 s). To this homogenous and well-stirred solution, 1 g of dehydrated Na2SO3 or K2S2O5 was added and the oxygen content was recorded at 5-s intervals. Using the same stock supply of deionized water, the starting values are practically equal. The data acquisition does not need necessarily computer assistance.
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In the Laboratory
Results and Discussion
Inhibition in Basic Solution Upon addition of solid Na2SO3 to water, the dissolved oxygen concentration is reduced to zero within 10 s. In the presence of phenols, the oxygen content decreases more slowly (Fig. 1). The slope of the curves reveals that ortho- and parasubstituted phenols are more effective antioxidants than metasubstituted ones under these conditions. An interesting comparison can be made with L-ascorbic acid. The dienol moiety in the molecule is similar to pyrocatechol, and the inhibitory effect of the two substances is the same. The continuous decrease of oxygen content is attributed to the oxidation of phenols, because in 0.16 M Na 2SO3 solution (pH = 9.3) the oxidation of phenols is considerable. This makes the comparison difficult and uncertain, especially with trihydric phenols, which are highly oxidizable (Fig. 2).
Inhibition in Acidic Solution At neutral and acidic pH phenols do not oxidize. This can be proved at the beginning of the experiment by measuring the oxygen content in the 0.2 mM phenol solution.
Then the sensor shows constant value. To provide an acidic medium, K2S2O5 was used (pH = 4.6 in 0.09 M solution). In this case the curves decrease slightly or level off after ~3 minutes of salt addition, which indicates very slow or no sulfite oxidation in the solution (Figs. 3 and 4). Under these conditions, the concentration of dissolved oxygen at a given time can be correlated with the effectiveness of the different kinds of phenols. The finding that ortho- and para-substituted phenols inhibit better than meta-substituted ones is consistent with the stability of aryloxy radicals in eq 7. Substituents of +M type, like the hydroxyl group, enhance the stability of aryloxy radicals more in ortho and para positions than in meta (10). Conclusion The change of dissolved oxygen content in sulfite solutions in the presence of different kinds of phenols is slow enough to follow with the Clark oxygen sensor, but fast enough to complete the measurement within 10 minutes. This makes the experiment ideal for a laboratory course and allows comparison of phenols according to their antioxidant effectiveness.
Figure 1. Dissolved oxygen content versus time in 0.16 M sodium sulfite solution in the presence of dihydric phenols and L-ascorbic acid.
Figure 3. Dissolved oxygen content versus time in 0.09 M potassium pyrosulfite solution in the presence of dihydric phenols and L-ascorbic acid.
Figure 2. Dissolved oxygen content versus time in 0.16 M sodium sulfite solution in the presence of trihydric phenols.
Figure 4. Dissolved oxygen content versus time in 0.09 M potassium pyrosulfite solution in the presence of trihydric phenols.
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In the Laboratory
Acknowledgment I would like to thank Miklos Zsuga for his help in preparing this article. Literature Cited 1. Stagg, W. R. J. Chem. Educ. 1972, 49, 427–429. 2. Crosson, M.; Gibb, R. J. J. Chem. Educ. 1992, 69, 830–832 3. Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th ed.; Saunders: Philadelphia, 1992; p 547. 4. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Ed.; VCH: Weinheim, 1985; Vol. A3, pp 91–111. 5. Donnelly, T. H. J. Chem. Educ. 1996, 73, 158–161. 6. Chan, W. H.; Lam, K. S.; Yu, W. K. J. Chem. Educ. 1989, 66, 172–173. 7. Byrd, I. E.; Perona, M. J. J. Chem. Educ. 1982, 59, 335–336. 8. Bäckström, H. L. J. J. Am. Chem. Soc. 1927, 49, 1460–1472. 9. Altwicker, E. R. Chem. Rev. 1967, 67, 475–531. 10. Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca, NY, 1969; pp 1241–1245.
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