Polarographic Behavior in the Oxidation of Sod ium Forma Ide hy d e S uIfoxy late SIR: The stability and high reducing strength of sodium formaldehyde sulfoxylate (SFS) have made it particularly attractive for use in the dyeing industry, as a stabilizer for oxygen-sensitive compounds in solution, and for controlled reductions in numerous organic reactions. Therefore the mechanisms of its reactions have been of significant industrial and scientific interest. Goehring ( 5 ) reported that under neutral and alkaline conditions a t 25' C. the SFS anion, CH2(0H)SOz-, remained intact in solution. Oxidation of the ion under these conditions resulted in the formation of forinaldehyde and sulfite ion. Studies by Rinker, Gordon, and Corcoran (13) showed that under acidic conditions SFS undergoes decomposition rvith the initial reaction probably being
H+
+ CHz(0H)SOnHSOr
+
+ CHz(0H)
(1)
I n this case, the SFS becomes a source of HSO?., and so its chemical behavior is similar to that of S204-z ions. Decomposition studies performed by Kunini (11) were carried out in alkaline solution at elevated temperatures. For the temperatures between 70" and 120" C., he postulated the initial decomposition reaction to be CHn(0H)SOz-
+
HSOz-
+ HCHO
(2)
Reactions subsequent to the initial decomposition were strongly temperature dependent so that the type and distribution of products varied significantly within the temperature range studied. According to Furness ( 4 ) , SFS was oxidized irreversibly at a stationary platinum electrode in a system buffered with citrate a t a p H of 4. On the other hand, he found that under the same conditions of acidity, no anodic waves were obtained a t the dropping mercury electrode. The only quantitative study of the polarographic reactions of SFS was reported by Kolthoff and Tamberg (10). These authors obtained well-defined anodic waves in the p H range from 9 to 13 but no waves at or below p H 7 . .A plot of log (id - i)/i us. E yielded a straight line with a value of 0.9 for n, which indicated an irreversible process. The half-wave potential was dependent on p H but independent of SFS concentration. They also found that a direct linear relationship existed between the diffusion current and the concentration of SFS. Calculation of I , the averagr diffusion current con930
e
ANALYTICAL CHEMISTRY
stant, from the Ilkovic equation (3) yielded a value of 3.84 and likewise a value of 1.00 X cm.2 set.-' for the Fick diffusion coefficient of the SFS anion a t 25" C. The mechanism proposed by Kolthoff and Tamberg ( I O ) for the electrode reaction at a p H greater than 9 included the direct oxidation of SFS anion to the formaldehyde sulfite anion, CH2(OH)SO3-, followed by decomposition of the latter to formaldehyde and sulfite ion as shown in Equations 3 and 4:
+
CHz(0H)SOz2 0 H - -+ CHz(OH)S03- HzO
+ + 2e CHz(0H)SOs- + OH- * HCHO + S03-' + HzO
(3)
(4)
The purpose of the present study was to extend the work of Kolthoff and Tamberg over a larger p H range, namely from 2 to 12, in order to gain new insights on the chemical nature of SFS. EXPERIMENTAL
The electrolytic system consisted of an H-type cell mounted on a cushioned stand to minimize vibration. A saturated calomel electrode (SCE) was in the reference compartment. A cross member contained a glass frit and was filled with a 4% agar gel saturated with KCl. The other side was fitted with a dropping mercury electrode (DME) with drop times which could be varied from 2 to 6 seconds. T h a t compartment was also fitted with a stopcock to facilitate drainage and with appropriate side arms to permit deaeration of the test solution by bubbling nitrogen through it. By means of a two-way valve, the gas was diverted over the solution during electrolysis. The current flowing in the cell and the voltage between the DRIE and the SCE were automatically recorded on a voltage scanning Sargent Model XV Recording Polarograph a t a rate of 0.1 volt per minute. The cell was operated at 25' =t 1' C., and temperature changes were slow enough not to cause significant fluctuations in the experimental measurements. Xtrogen used for purging oxygen from the cell was Linde H P D grade (99.95Q/, purity) which was further purified by bubbling through chromous chloride solution and then humidified by scrubbing with oxygen-free distilled water. The SFS was a product of Rlatheson, Coleman, and Bell and was 99+% pure, with sodium sulfite and sodium thiosulfate being the major impurities. RIercury for the working electrode was doubly distilled. All other cheniicals such as buffer agents,
electrolytes, and formaldehyde were of reagent grade. Buffer solutions with p H values from 2 to 12 were prepared by mixing measured proportions of a solution which was 0.2m in S a O H with a stock solution which was 0.04F in acetic acid, phosphoric acid, and boric acid and O.lm in potassium chloride. Values of the p H were measured with a Beckman Model G p H meter. Before each run, the solution compartment of the cell was thoroughly rinsed with distilled water and allowed to drain dry. +4 volume of 25.00 nil. of the buffered electrolyte was added to the compartment, and then the system was purged with nitrogen for a t least 15 minutes. Changes in volume of the electrolyte as a result of the bubbling were negligible. Weighed amounts of the SFS were added and rapidly dissolved by brief, continued bubbling. Drop times were computed from the measured time of a t least 10 drops, with the D X E polarized a t the same potential a t which the diffusion current was measured. The rate of mercury flow was determined by weighing the mercury collected from the D M E over a measured time interval. RESULTS
Basic Media. .As shown in Figure 1, solutions of SFS which were 1 x 10-3 F gave single anodic waves in the p H range from 6.92 to 11.6. For the p H region from 8 to 12, the results of Kolthoff and Tamberg ( I O ) were essentially confirmed. Contrary to the findings of Kolthoff, however, a reasonably well-defined irreversible wave was obtained a t p H 6.92 with = 0.07 volt us, SCE. Also, a t p H values > 10, two small but distinct cathodic waves were obtained with half-wave potentials of - 1.32 volts and -1.63 volts us. SCE. Both waveb were irreversible. The nature of the first cathodic wave is still under consideration, and the second cathodic wave resulted from reduction of formaldehyde in accord with the reaction
+
HCHO
+ 2 H z 0 + 2e
-+
CH30H
20H-
(5)
For p H 2 6.92, values of the halfwave potentials were found to be independent of the SFS concentration between 0.3 X and 2.0 X F. The half-wave potentials were, however, dependent on the p H as may be noted in Figure 2, and became more cathodic with increasing hydroxyl concentration. This apparent decrease in free energy of oxidation with increasing basicity was indicative that OH- ions were con-
Figure 1 . Polarograms for sodium formaldehyde sulfoxylate in the p H range of 6.92 to 11.6 SFS concn. = 1 X 1 O - V
sumed in the electrochemical step of osidizing the SFS anion. On the other hand, as shown in Figure 2, the more rapid increase of Ell2 with decreasing concentration of hydrosyl ions in the pH range 8 to 7 indicated a change in oxidation mechanism was occurring because of thermodynamic limitations. T h e effect of added formaldehyde on the SFS electrode oxidation was studied. In agreement with Kolthoff and Tamberg (10) there was no detectable effect on the nature of the anodic waves. Thus, an initial reaction of the type
CH2 (OH)SOI-
Ft:
+
HS02HCHO (6) in which the intermediate HS02- is formed can be discarded since an excess of formaldehyde would certainly affect the characteristics of the waves. A plot of log [i,'(i
