In the Laboratory
Coulometric Titrations in Wine Samples: Studies on the Determination of S(IV) and the Formation of Adducts
W
Denise Lowinsohn and Mauro Bertotti* Instituto de Química, Universidade de São Paulo, São Paulo, SP 05508-900, Brazil; *
[email protected] The determination of SO32᎑ by using iodine (or triiodide) as a titrant according to the equation I3᎑ + SO32᎑ + H2O
3I᎑ + SO42᎑ + 2H+
(1)
has the major drawback that low results are obtained as a result of air oxidation of S(IV) during the titration (1, 2). Furthermore, a report has addressed the formation of elemental sulfur from the reduction of S(IV) by iodide in acidic medium during a direct titration, which could occur in relatively concentrated SO2 solutions (> 0.04%) (3). Hence, recommended procedures for quantifying sulfite in foods and beverages to which the substance is added as a preservative are based on different approaches. In the classic Monier–Williams method, SO2 is purged with an inert gas from acidified sample, collected in electrolyte-trapping solution containing excess of hydrogen peroxide, and then determined as sulfuric acid by titration with standard NaOH solution (4 ). Another Association of Official Analytical Chemists (AOAC) procedure is based on the determination of SO2 in a flow injection analysis (FIA) configuration, where the gas diffuses across a semipermeable membrane into a carrier stream of malachite green solution. From spectrophotometric measurements it is possible to relate the decrease in the color of the solution to the concentration of SO2 (4). The Ripper method, based on the direct oxidation of sulfite with iodine, has been largely used in the wine industry but its precision is not satisfactory (5). We describe an experiment related to the determination of sulfite in wines by coulometry. The experiment fits into an Instrumental Analysis course for chemistry and pharmacy students, where the fundamentals of this electroanalytical technique are to be introduced. Students are often motivated when they work with “real” samples; we chose white wine because sulfite may be added in relatively large amounts to prevent oxidation processes. The analyte is determined by generating the titrant at the anode. There are some interesting discussions on the kinetics of the reaction of iodine with S(IV) present in the sample as an additive and on the release of S(IV) from this adduct. Experimental Procedure
Reagents Water used to prepare the solutions was distilled and then processed through a water purification system (Nanopure Infinity, Barnstead, resistivity = 18.1 MΩ). Chemicals were of analytical grade and were used with no further purification. Apparatus Coulometric experiments were performed using a Metrohm E 211A coulometer, the current being maintained at 10.0 mA. The electrolyte consisted of a 1 M HAc–0.1 M Ac᎑ buffer solution (volume = 30 mL, pH ~3.7) to which potassium iodide was added (final concentration = 0.1 M).
Platinum electrodes were used as anode (a gauze cylinder) and cathode (a wire spiral) and a 100-mL beaker was used as an electrochemical cell. To restrain the contact between the electrolyte solution and OH᎑ ions generated at the cathode (since iodine is transformed into hypoiodite in alkaline medium), the auxiliary electrode was placed in a tube electrolytically connected to the solution through a diaphragm of sintered glass. The tube contained a 0.2 M sodium sulfate solution and a few drops of phenolphthalein, added to demonstrate that the pH increases in the solution where the cathode is immersed as current flows in the circuitry. Care was taken in the transfer of wine samples and thiosulfate solutions to the electrochemical cell; hence piston burets (Metrohm AG Herisau E274, 5 mL capacity) were used to achieve the desired accuracy. Starch was used as endpoint indicator in the coulometric titrations.
Procedure The basic procedure involves first the generation of a known excess of iodine at the anode as follows: 2I᎑
I2 + 2e᎑
(2)
the concurrent reaction taking place at the cathode being 2H2O + 2e᎑
H2 + 2OH᎑
(3)
After the generation of the excess of iodine in the working solution, 1.000 mL of white wine (Forestier, Brazil) is added, and since iodine is in excess all sulfite is oxidized according to eq 1. In the next step the remaining iodine is consumed by adding a known amount of standard thiosulfate solution, its excess being subsequently consumed by iodine coulometrically generated at the anode as follows: I3᎑ + 2S2O32᎑
3I᎑ + S4O62᎑
(4)
Wine samples were introduced to the electrochemical cell with no previous treatment, except when the determination of total sulfite was required. In this case, solid NaOH was added to wine samples to give a 2 M solution, which is enough to release all S(IV) as discussed below. Appropriate aliquots of these alkaline solutions were then transferred to the acetate buffer solution that contained the electrogenerated triiodide, and the determination of total S(IV) follows the procedure described above. The pH of the electrolyte solution does not change appreciably after the addition of a small volume of the alkaline sample . Data related to the S(IV) contained in wine were corrected by performing experiments in samples to which formaldehyde had been added (final concentration = 1 M) to bind all S(IV) in the form of a more stable compound (6 ) that is not oxidized by iodine. Therefore, the blank titration yields the concentration of other substances in the wine susceptible to iodine oxidation, which would constitute an interference in the proposed method.
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
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In the Laboratory
The proposed coulometric method was validated by using the AOAC distillation–titration procedure (4). Wine samples were treated with HCl to generate SO2, which was then purged by bubbling argon through the solution. SO2 was collected in a 3% H 2O2 solution and the sulfuric acid formed was titrated with standard NaOH solution, methyl red being used as endpoint indicator.
1.5
C / mM
1.0
0.5
Hazards 0.0
Concentrated solutions of NaOH, HCl, HAc, and H2O2 may cause burns; contact with skin and eyes should be avoided. There are no special disposal methods for chemicals used in this experiment. Discussion Sulfites are widely used as preservatives in the food industry to inhibit bacterial growth and to prevent oxidation and discoloration in foods (7–9). In wines, S(IV) is found in two main forms: bound to unsaturated compounds and phenolic constituents, and free as HSO3᎑, the prevalent species (10) at the mildly acidic conditions found in most wines (pH around 3.5) (11). Equation 5 represents the formation of an adduct between HSO3᎑ and a carbonyl compound (12): H
H C R
O
+
HSO3
−
R
C
SO3−
(5)
OH
S(IV) adducts are only stable at acidity values where the monoprotonated species is prevalent (e.g. 3 < pH < 8, according to pK1 and pK2 values for H2SO3). At conditions of high acidity or basicity, respectively, SO2 or SO32᎑ is formed and the adduct is no longer stable with respect to the pH. The determination of S(IV) in wines is of concern because of its harmful effects for some individuals at concentrations above the limits established by law. The determination of the free fraction is important because it refers to all species that may rapidly and quantitatively be converted to SO2, these species controlling the functional activities of S(IV) during the preparation and storage of the wine. Since S(IV) may be released from the adducts at the low pH of the stomach, the total amount of S(IV) (bound plus free) is also a relevant parameter. Our coulometric method involves a study on the determination of S(IV) in wine samples by the procedure described above. Since the samples are added to an iodine solution and the adduct formed between S(IV) and acetaldehyde (the prevalent carbonyl compound in wines [13]) is not sufficiently chemically stable, it is expected that some bound S(IV) may break down leaving more S(IV) to react with iodine. Hence, the work was extended to evaluate S(IV) concentration as a function of the residence time of the sample in the electrochemical cell prior to the addition of thiosulfate and the subsequent coulometric titration. Wine samples were added to the iodine solution and allowed to react with the oxidizing species for various periods of time before the addition of thiosulfate. Figure 1 shows the results of coulometric titrations; the amount of S(IV) obtained depends on the contact time between wine and triiodide. Hence, it is possible to conclude
104
0
10
20
30
40
50
Time / min Figure 1. Concentration of S(IV) determined from coulometric experiments. Portions of white wine (1.000 mL) were added to the working solution containing a known excess amount of triiodide. The plot shows the dependence of S(IV) concentration on the residence time of the sample in the electrolytic cell before the titration is performed. Each point represents the average of 3 determinations.
that adducts in wines are not quite stable toward oxidation reactions with triiodide, which makes evaluation of the free S(IV) concentration by this procedure impossible. Classical procedures to determine the total S(IV) content in wines recommend the treatment of the sample with NaOH solution (14); this releases bound S(IV) as a consequence of the formation of SO32᎑. H R
C OH
SO3−
+
OH
−
H
R C
+
SO32−
+
H2O
(6)
O
Therefore, an extra step could involve determination of the total S(IV). This was done by increasing the pH of the wine sample by adding NaOH. We have observed that a 2 M NaOH solution (final concentration) will release all S(IV) from the adducts. The calculated value for total S(IV) concentration in the wine sample was (3.7 ± 0.2) mM; the lower values obtained in the direct titration at pH = 3.7 show that the release of S(IV) from the adducts is not complete even if the sample is allowed to stand in the electrochemical cell containing iodine for extended periods of time. The proposed method was validated by comparing the results for total S(IV) obtained after the addition of NaOH to the wine sample with the ones found by using the official distillation–titration method (4). In this case, the distillation was performed by gentle heating of the acid solution with simultaneous bubbling of argon, which allows all S(IV) to be released from the adducts as SO2 is formed. The average of three determinations was (3.6 ± 0.2) mM, confirming the reliability of the coulometric procedure. This experiment illustrates a number of principles related to analytical and organic chemistry. First, from standard potential values found in literature for both redox couples (I3᎑/I᎑ and SO42᎑/SO32᎑) it is possible to calculate the equilibrium constant for reaction 1 at a given pH by taking into account that at equilibrium the potential of both half-cells is equal. Even though one of the products (iodide) is in large excess,
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
In the Laboratory
the reaction would go to completion in direct titrations of sulfite with iodine (or triiodide) because the calculated Keq is sufficiently large. However, accurate results are difficult to obtain in direct titrations owing to air oxidation of sulfite during the experiment; this justifies the addition of the sample to a relatively large amount of triiodide, where the oxidation to sulfate is immediate. Second, the determination of the concentration of sulfite according to the proposed method requires the knowledge of the reactions at the anode and in solution. Students have to write down all chemical equations to work out their calculations on the basis of the principles of Faraday’s law and the stoichiometry of the reactions. The advantages of generating a titrant by constant-current coulometry are discussed, as is the need to maintain an electrode process with 100% current efficiency. Third, the determination of bound, free, and total sulfite is not easy when wine samples are to be analyzed, owing to the relatively labile adduct formed in that matrix, and we think that this distinction should not be the main focus of the experiment. A more important question concerns the difference between S(IV) adducts formed with formaldehyde and acetaldehyde. The adduct formed between acetaldehyde and hydrogen sulfite is a less stable compound because of the inductive effect of the –CH3 group, which is more efficient than –H as an electroreleasing group (12). The discussion on the breakdown of the adduct by adding sodium hydroxide and the possibility of determining all S(IV) is also very meaningful. At this point students may debate whether their results actually stand for the total S(IV), and some discussion about other methodologies for determining S(IV) is likely to appear (for instance, by removing SO2 from the sample in an inert atmosphere and collecting the analyte in a solution containing triiodide). Fourth, the endpoint in our method was detected by using starch because we were working with a white wine sample. For red wines a visual endpoint indicator may not be suitable, and this difficulty could be addressed with the students. Good possibilities would involve endpoint detection by instrumental methods; amperometry and biamperometry are well suited. Discussions of the difference between electrodes used to generate the titrant (and side products at the cathode) and those used to monitor the electroactive species (I3᎑) or the reversible couple (I3᎑/I᎑) are always useful to teach relevant aspects of electroanalytical chemistry (15). The possibility of titrating organic acids in wine samples by using the cathode as a source for producing OH᎑ ions quantitatively would also be discussed. Finally, teamwork and the exchange of results among students play a key role in engaging them in cooperative learning (16–19). We suggest that students work in small groups to obtain the data, each group being responsible for only one titration of the wine sample (and a repetition to ascertain the repeatability of the determination). To validate the results obtained by the coulometric procedure, one group should perform the experiment by the AOAC method. The experiment is complemented with a further discussion based on the overall results and questions like those presented above.
We think that this experiment, intended to be carried out in a learning environment, has many elements to challenge and empower students. Conclusions Faraday’s laws of electrolysis are well illustrated in a coulometric experiment. We believe that this experiment is well suited to explore this subject in a different way, where some very important concepts of analytical chemistry are discussed in the context of a real sample that has to be manipulated adequately. Moreover, the experiment opens new possibilities for discussion of matters such as the completeness of chemical reactions, distribution of species as a function of pH, endpoint detection, and nucleophilic addition. Acknowledgment This work was supported by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo). W
Supplemental Material
Notes for the instructor and material for students are available in this issue of JCE Online. Literature Cited 1. Kolthoff, I. M; Sandell, E. B. Textbook of Quantitative Inorganic Analysis, 3rd ed.; Macmillan: New York, 1952. 2. Laitinen, H. A. Chemical Analysis; McGraw-Hill: New York, 1960. 3. Furman, N. H. Standard Methods of Chemical Analysis, 5th ed.; Van Nostrand: New York, 1939. 4. AOAC, Official Methods of Analysis of AOAC International, Vol. 2: Food Composition, 16th ed.; Cunniff, P., Ed.; AOAC International: Arlington, VA, 1995. 5. Vahl, J. M.; Converse, J. E. J. Assoc. Off. Anal. Chem. 1980, 63, 194. 6. Lindgren, M.; Cedergren, A.; Lindberg, J. Anal. Chim. Acta 1982, 141, 279. 7. Walker, R. Food Addit. Contam. 1985, 2, 5. 8. Cooker, L. E. J. Assoc. Off. Anal. Chem. 1986, 69, 8. 9. Wedzicha, B. L. Food Addit. Contam. 1992, 9, 449. 10. Wedzicha, B. L Chemistry of Sulphur Dioxide in Foods; Elsevier: Amsterdam, 1984. 11. Decnop-Weever, L. G. Anal. Chim. Acta 1997, 337, 125. 12. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 3rd ed.; Allyn & Bacon: New York, 1973. 13. Amerine, M. A.; Berg, H. W.; Kunkee, R. E.; Ough, C. S.; Singleton, V. L.; Webb, A. D. Technology of Wine Making, 4th ed.; AVI: Westport, CT, 1982. 14. Pizzoferrato, L.; Di Lullo, G.; Quattrucci, E. Food Chem. 1998, 63, 275. 15. Lötz, A. J. Chem. Educ. 1998, 75, 775. 16. Wenzel, T. J. Anal. Chem. 1995, 67, 470A. 17. Wright, J. C. J. Chem. Educ. 1996, 73, 827. 18. Dougherty, R. C. J. Chem. Educ. 1997, 74, 722. 19. Wenzel, T. J. Anal. Chem. 1998, 70, 790A.
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