lodometric Method for the Determination of Dithionite, Bisulfite, and Thiosulfate in the Presence of Each Other and Its Use in Following the Decomposition of Aqueous Solutions of Sodium Dithionite James P. Danehy and Charles William Zubritsky III Department of Chemistry, University of Notre Dame, Notre Dame, Ind. 46556
Dithionite, bisulfite, and thiosulfate can be determined quantitatively in the presence of each other with satisfactory precision by a method involving three operationally distinct iodometric titrations. Data are presented graphically for the decomposition of aqueous solutions of sodi.um dithionite as a function of pH at initial 0.1M dithionite; initial concentrations of dithionite at pH 5.5; and initial concentrations of bisulfite, thiosulfate, and hydrosulfide at pH 5.5 and initial 0.1 M dithionite. The cumulative ratio of bisulfite formed to dithionite decomposed does not vary significantly from unity over the span of 16 to 94% decomposition in the range of pH 4.00 to 6.75, but the ratio of bisulfite formed to thiosulfate formed is consistently higher than 3: 1 through much of the course of reaction, though it usually falls to a cumulative value of a little more than 2:1 terminally.
Cuprimetric (1, 3-5) and argentometric (1, 6, 7) methods give reliable results, even in the presence of large. amounts of bisulfite-sulfite, but ferrimetric (3, 8) methods can be used only if bisulfite-sulfite is not present. Titration with Methylene Blue (9, 10), with which neither bisulfite-sulfite nor thiosulfate interferes, works well. The use of iodometry poses an immediate, but by no means insuperable, problem in that all three of the species of -interest (dithionite, bisulfite, thiosulfate) reduce iodine quantitatively. Merriman (11) devised a method in which the initial addition of excess formaldehyde to the dithionite is followed by titration with standard iodine solution. The formaldehyde not only cleaves. the dithionite quantitatively (Equation 2), but also binds any bisulfite present so that only the hydroxymethanesulfinate formed reduces iodine. Na2S204
Sodium dithionite (Na 2 S 2 04, known commercially as sodium hydrosulfite or sodium hyposulfite) is a relatively inexpensive compound which has long been used extensively in the textile and paper industries as a bleaching agent and as an assistant in the vat dyeing of cellulosic fibers. It is a more powerful reducing agent than sodium bisulfite, to which it is very rapidly oxidized in aqueous solution by aerial oxygen. In the absence of air, however, aqueous solutions of dithionite are not intrinsically stable but undergo a disproportionation, whose stoichiometry was established by the Jellineks (1), at rates which depend on the temperature, concentration of bisulfite-sulfite present, and, most importan~ly, on the pH value of the solution. 2Na2S204
+
H20
---+
2NaHSO;l
+
Na 2SZ0 3
(1)
The anhydrous solid is rather stable at room temperature, but Patel and Rao (2) have shown quantiatively that the crystalline dihydrate is unstable, as might have been expected from the behavior of aqueous solutions of dithionite. In view of the properties of sodium dithionite, a simple and reliable method for determining its content, as well as those of the two principal contaminants to be expected (bisulfite and thiosulfate), would be useful. Surprisingly, no such method has yet been established. While a wide variety of analytical methods for the determination of dithionite have been reported, they either do not allow for the interference of other substances which may be present or, if noninterference has been established, they have not provided for the concurrent quantitative determination of these other substances so that a realistic specification of the product can be made. (1) K. Jellinek and E. Jellinek, Z. Phys. Chern., 93, 325 (1919). (2) C. C. Patel and M. R. A. Rao, Proc. Nat!. Inst. Sci. India, 19, 239 (1953).
+
2CH 20
+
H20 ----. HOCH:zS02Na
+
HOCH zS03 Na (2)
This method was subsequently recommended as the preferred one by the American Association of Textile Chemists and Colorists except "... where the hydrosulfite is known or suspected to contain significant amounts of thiosulfate, sulfide, or other oxidizable impurity not inactivated by formaldehyde." (4). Wollak (12) described an attractively simple procedure for determining the dithionite, bisulfite, and thiosulfate contents of a product by carrying out three operationally distinct iodometric titrations. However, the few data presented by him are quite insufficient to support the method, and the "A, B, C" values given do not represent operationally the rather complex stoichiometric relations between the three titers. Zocher and Saechtling (13) concluded that two of Wollak's titrations give consistent results, but they rejected as unsatisfactory the one which gives a value for thiosulfate. Like Wollak, they did not give an explicit scheme for handling the data. They did, however, make the analytlcally.remarkable statement: "For practical purposes it is best to disregard the thiosulfate content of dithionite preparations. Usually very little (3) C. C. Patel and M. R. A. Rao, Proc. Natl. Inst. Sci. India. 15, 115 (1949). (4) Committee on Analytical Methods Report, Amer. Dyest. Rep., 46, P443 (1957). (5) M. S. Spencer, Trans. Faraday Soc.. 63,2510 (1967). (6) A. Seyewetz and Sioch [sic], Bull. Soc. Chirn. Fr. [3], 35, 293 (1906). (7) J. H. Smifh, J. Amer. Chern. Soc., 43,1307 (1921). (8) F. L. Hahn, Anal. Chirn. Acta, 3,62 (1949). (9) R. G. Rinker, T. P. Gordon, D. M. Mason, R. R. Sakaida, and W. H. Corcoran, J. Phys. Chern., 64, 573 (1960). (10) R. G. Rinker, S. Lynn, D. M. Mason, and W. H. Corcoran, Ind. Eng. Chern., Fundarn., 4,282 (1965). (11) R. W. Merriman, Chern. Ind., 42,290 (1923). (12) R. Wollak, Fresenius' Z. Anal. Chern., 80,1 (1930). (13) H. Zocher and H. Saechtling, Fresenius' Z. Anal. Chern., 117,392 (1939).
Table I. Determination of the Dithionite, Sulfite, and Thiosulfate Contents of Anhydrous Sodium Dithionite with and without the Addition of Known Amounts of Sulfite and Thiosulfate a 8203 2 - added, mmol
A
B
0.'280 0.280 0.560 0.560
3.94 3.90 4.00 4.10 4.22 4.18 4.02 4.06 3.99 4.04
0.02 0.02 0.07 0.07 0.12 0.12 0.02 0.02 0.02 0.02
0.109 0.109 0.218 0.218
a
C
6.05 6.02 6.22 6.22 6.27 6.28 6.81 6.79 7.27 7.27
Present
Found
0.970 0.970 0.970 0.970 0.970 0.970 0.970 0.970
0.975 0.965 0.965 0.990 0.995 0.985 0.995 1.000 0.988 1.000
8203 2 -, mmol
803 2 -, mmol
820. 2 -, mmol
Titers, mequiv
803 2 -;- added, mmol
Present
Found
0.082 0.082 0.082 0.082 0.362 0.362 0.642 0.642
0.080 0.085 0.145 0.070 0.030 0.065 0.400 0.360 0.660 0.615
Present
Found
0.149 0.149 0.258 0.258 0.040 0.040 0.040 0.040
0.040 0.040 0.14 0.14 0.240 0.240 0.040 0.040 0.040 0.040
Detailed procedure and specifications of samples and reagents given in Experimental.
RESULTS AND DISCUSSION
is present." Quite recently Nair and Nair (14) have reported briefly a modified form of the Wollak procedure, which includes a titration with aqueous chloramine-T solution as well. We have now completed a critical evaluation of the procedure suggested by Wollak and find it to be unexceptionable. It is the purpose of the present report to present data which validate the method, to derive the explicit relations between the titers by which concentrations of the components can be calculated, and to present data which supplement recently published studies on the anaerobic decomposition of aqueous solutions of sodium dithionite.
EXPERIMENTAL Reagents. The samples of anhydrous sodium dithionite used in this study were reagent grade, purchased from J. T. Baker Chemical Co., Fisher Scientific Co., and Mallinckrodt Chemical Works. That the product (Fisher) with which most of the work was done was indeed anhydrous was shown by the fact that a sample (0.2000 gram) kept 48 hr in a vacuum desiccator over P205 lost no weight (±0.1 mg). The iodine, potassium iodide, sodium thiosulfate pentahydrate, and auxiliary chemicals were all of ACS reagent grade. Solutions were prepared and standardized by accepted methods. Procedure. Titration A. Add 35 ml of formalin (~370/0 aqueous formaldehyde) to 50 ml of boiled and cooled distilled water and add 0.3 gram Na 2CO a·I0 H 20, which brings the solution to ~pH 9.· To the solution so prepared, add a weighed solid sample, or an aliquot of an aqueous solution, of sodium dithionite, swirl to dissolve, add 25 ml of water and 5 ml of 200/0 aqueous AcOH, and titrate with "-'O.IN iodine. The mequiv of iodine consumed is titer
Quantitative Relations between the Three Titers. Determination of Thiosulfate. Upon addition of a sample to an excess of iodine solution, any thiosulfate present is oxidized to tetrathionate (Equation 3), (3)
while both dithionite and bisulfite are oxidized to sulfate. Addition of sulfite then reduces the excess iodine to iodide, and excess sulfite quantitatively cleaves the tetrathionate to trithionate and thiosulfate (Equation 4). 8 40 62 - + 80/-
(14) V. R. Nair and C. G. R. Nair, Res. Ind., 16,47 (1971).
8 30 62- + 8:20/-
(4)
Addition of formalin binds the remaining excess sulfite so that the only species present which can consume iodine is thiosulfate. The titration of this derived thiosulfate with iodine gives titer B. From the stoichiometry of Equations 3 and 4, it is clear that the thiosulfate corresponding to titer B is one-half of that present in the original sample. Therefore, mmol 8 2 0 3 2 - = 2B. Determination of Dithionite. Upon addition of a sample to an excess of formalin, reactions represented by Equations 2 and 5 take place. NaH80 3
+
CH 20
~
HOCH 280 3 Na
(5)
Thiosulfate does not react with formaldehyde. Both the thiosulfate and the hydroxymethanesulfinate are oxidized by iodine in accord with Equations 3 and 6. HOCH~802Na
A. Titration B. Add dithionite to an excess of "-'O.IN iodine (75 ml is sufficient for ~0.2 gram Na2S204) in which ~1 gram AcONa· 3H 20 has been dissolved. Slowly add aqueous N a2S0a solution (25 grams anhydrous Na 2SOa per liter of water) until the iodine solution is decolorized, and then add an additional 30 ml of Na2SOa solution. Add two drops of phenolphthalein solution, neutralize with "-' IN NaOH, and hold for 5 minutes. Add 10 ml formalin, then 10 ml 200/
