Current and Titration Efficiencies of Electrically Generated Manganic Ion Ferric-Manganous Sulfate Dual Intermediate System A. J. FENTON, Jr.,' and N.
H. FURMAN
Princeton Universify, Princeton, N. 1. The conditions for the anodic generation of fripositive manganese at platinum or gold anodes were studied as a function of acidity, current density, and concentration of manganese. Conditions were optimum in media containing both sulfuric and phosphoric acids at a platinum anode with 0.3 to 0.4M manganous sulfate, There is a limited feasible range of current densities. The ferric-manganous sulfate dual intermediate system gives good titration efficiencies regardless of whether ferrous or manganic ion is generated first.
(1 1 )
oxidized in sulfuric acid or mixed sulfuric-phosphoric acid media. Of particular interest were the effects of acidity, electrode material, concentration of manganous salt, and current density on the identity of the anode reactions and on the current efficiency with which these processes occur. Extensive use was made of current density us. electrode potential curves taken a t the working anode using the current scanning technique of Adams, Reilley, and Furman (1). Current efficiencies were estimated from these curves under various conditions of current density (18, 2O), electrode material, and concentrations of manganous sulfate and sulfuric acid. Some evidence for the nature of the electrode reaction was deduced from these data. The titration efficiencies for the electrode processes involved were obtained by electrically generating known increments of ferrous ion using conditions known to be 100% efficient for this purpose. The equivalents of manganous oxidation product(s) equal to the accurate known increment of ferrous ion were determined by coulometric titration. The equivalents of oxidant generated could then be compared directly with the equivalents of ferrous ion generated to obtain the titration efficiency under specific conditions. By reversing this procedure, oxidizing manganous ion first, the current efficiency was determined and checked against that estimated from current-electrode potential data.
(29) (21
APPARATUS AND REAGENTS
T
ANODIC oxidation of acidic manganous solutions may yield 111, various products-manganese(I1, IV, or VI1)depending upon the concentration of manganous salt, the concentration and kind of the acids that are present, the temperature, and the current density at the anode. The present paper is concerned primarily with the conditions under which manganese(II1) may be generated efficiently and utilized. The following is a brief tabular summary of pertinent literature: HE
Topic
Ref.
Stated generation of MnO; Proof of generation of Mn(ll1) under condition in 128) Conditions under which a partial yield of MnO; i s obtained Reduction potential of the Mdlll) Mn(ll) couple and acidity Manganic sulfate as a titrant Effect of phosphoric acid
(28) (72) (14)
(27)
(3)
(15) Manganic pyrophosphate Hydrolysis of manganic sulfate
125) (17, 3 0 ) 0 9) (6)
This investigation was undertaken to discern the operative electrode processes when manganous ion is electrolytically 1 Present address, Analytical Methods Research, The Procter & Gamble Co., Cincinnati 17, Ohio.
748
ANALYTICAL CHEMISTRY
Currents constant to =tl% from 20 pa. to 20 ma. were obtained from a constant voltage suppl! (23, 24) using a high resistance series decade box ( I ) . Higher currents TTere obtained from the high constant current tap of the coulometric supply (23, 24). The coulometric generating current a t any time was measured by taking the ZR drop across a suitable series resistance of a decade box (Otto Woulf, Berlin) with a vacuum tube voltmeter (Scientific Specialties, Boston). The voltage developed between the reference
and working electrodes was followed with a Leeds & Northrup pH meter (Catalog No. 7664). A low inertia integrating motor (Electro Methods, Ltd., Stevenage, Herts, Model 913) was used for current integration (22). The calibration or motor constant used was that obtained _ _ f(microequivain a previous study (IO); 1354 lents per count) = 0.0027. A lb
+
decade box (Otto Woulf, Berlin) was used as the series resistance for the motor. The electrolysis cell was a 4-ounce tall-form jar, fitted with a rubber stopper drilled with appropriate holes. The reference electrode consisted of saturated lead amalgam in contact with 6N sulfuric acid saturated with lead sulfate. An asbestos plug was used in the side arm to minimize diffusion f9). The half-cell potential was -0.272 volt (US. N.H.E.). - ~The isolated electrode was a platinum foil (1 x 1 em.) electrode placed in an asbestos plugged half cell that contained sulfuric acid of the same concentration as the auxiliary electrolyte. The geometrical areas in square centimeters of the generating electrodes were: A , 2.03; B, 3.18; C, 9.7; D , 13.6; and E, 2.05. The platinum electrodes A-D were constructed from bright foil with stout platinum wire contacts sealed into 6-mm. lead glass tubing (8). Electrode E was of pure gold foil to which a gold wire was fused that extended above the liquid level of the electrolysis cell. -4magnetic stirrer set a t a constant stirring rate was used with a Tefloncoated stirring bar. The stirring rate was so chosen that cavitation did not occur. The galvanometer used had a sensitivity of 0.033 ma, per mm. (G. M. Laboratories, No. 2564B) and was equipped with a Shallcross Ayrton shunt. Purification of the gas stream (carbon dioxide or nitrogen) was not critical, as the results were identical with and without purification. A solution of tetraphenylarsonium chloride (TPA - Cl), was prepared from the solid (Hach Chemical CO., Ames, Iowa) and converted t o the sulfate by addition of the stoichiometric amount of silver sulfate. The silver chloride was removed by filtration and washed with water, and the filtrate ~
diluted to give a 0.01-Y (TPA)Kh solution. h stock 0.5N manganous solution was prepared from manganous sulfate unihydrate. The 0.5-V ferric sulfate stock solution 1% as prepared by oxidizing ferrous sulfate with 307, hydrogen peroxide and de.;troying the excess peroxide by boiling. Absorption spectra in the visible wave length region were obtained with a Varren Spectracord (Warren Electronics, P,ound Brook, S.J.). PROCEDURES
Current-Electrode Potential Curves. The reyuisitp concentrations of sulfuric acid, water, and manganous sulfate \\ere diluted t o volume in a 50ml. volumetric flask. The contents were then transferred into the titration cell. The electrolysis leads were attached to the reference and working electrodes and stirring was begun. This three-electrode system eliminated the need for any I R drop corrections (1). A standardized scanning technique was employed. The electrolysis current was adJusted to the correct value R hen flowing through a dummy resistance that matched the internal resistance of the cell (350 ohms). The current was then switched to the electrolysis cell and the most positive voltage read (instantaneous voltage). The current was left on for an additional 1minute and the voltage read again a t that time (1minute reading). During this 1-minute interval the voltage fell slowly to a more negative value. The current was then turned off for 1 minute to permit breakdown of the diffusion and oxide layers a t the generating electrode, so that the electrode returned to the solution potential. During this waiting period the current Tvas readjusted and the same procedure followed for each point determined. Coulometric Titrations. The end point procedure of Cooke, Reilley, and Furman ( 4 ) was employed. The potential to be impressed across the indi-
eating and reference electrode was chosen by potentiometric titration using electrically generated reagents. A potential setting 50 mv. less than the end point gave the most stable galvanometer readings. Titration to galvanometer zero or plotting the galvanometer readings a t intervals near the end point yielded comparable results. In all cases a t current levels less than 20 ma. equilibrium was reached slowly so that 30 seconds were required for stable galvanometer readings. The generating media were mixtures of ferric sulfate, manganous sulfate, and sulfuric and phosphoric acids in various proportions. Fifty milliliters of the generating medium were put into the electrolysis cell and deaerated for 10 minutes. After deaeration the gas inlet tube was raised above the surface of the liquid to provide an inert atmosphere. The test solution was pretitrated to the end point potential as indicated by a zero galvanometer reading, the motor count noted, and generation of oxidant or reductant started. Thirty or more motor counts of reagent were thus generated, the motor count was noted, and leads were reversed. The solution was then back-titrated to zero galvanometer and the final motor count noted. Results were interpreted in terms of relative difference in motor counts for oxidant and reductant (8). RESULTS AND DISCUSSION
Current Density Electrode Potential Curves. The most distinctive feature of the current-electrode potential curves obtained was the transient nature of the potential when manganous ion was oxidized in solutions containing less than 12N sulfuric acid. When current was initially applied t o the electrodes, the voltage rose steadily in a manner similar to, but slightly slower
than that observed when cerous ion is oxidized. The potential, however, after reaching a voltage maximum drifted back to a more negative equilibrium value. To ascertain what electrode processes were responsible for this unusual behavior, current-electrode potential curves were obtained by measuring the most positive anode potentials attained as well as the lower equilibrium potentials. The equilibrium curve was constructed from those potential readings obtained after the current had been flowing 1 minute. These curves are shown in Figure l,A and B. The current-electrode potential curve obtained after rraiting an arbitrary 1 minute (Figure 1,A) is postulated as representing the conversion of manganous ion to hydrated manganese(1V) oxide. There are two reasons for this: A brown deposit of manganese dioxide was visible on the generating electrode a t sulfuric acid concentrations less than 4N. The general shape of the curves did not change appreciably as the acidity was raised although the deposit was not visible. Secondly, the curve shape was that expected when the electrolysis product is a solid with unit activity, Ellz being dependent only upon hydrogen ion and manganous ion concentrations (6). The curve obtained by measuring the most positive anode potentials as a function of applied current is the conversion of manganous to the trivalent manganic state. T.Vhen phosphoric acid is added to the supporting electrolyte curve C in Figure 1 is obtained. The similarity in the shapes of curves B and C is striking. Horn (14) was able to show that the conversion of manganous to manganic is the most likely process in phosphoric-sulfuric acid mixtures.
B t
r
4
I
c
::
4-
e 3 u
E,
1s N P
E
VOLT
Figure 1. Current voltage curves on platinum electrode 0.1 2M MnS04, 5 N H2S04 Equilibrium potentials read after 1-minute current flow E. Mart positive potential read C. 2% HaPo4 added D. Background, 5 N H2S01
A.
EA
Figure 2. electrode
VS
N.H.E
Current-electrode potential diagram for gold 0.06M MnSO4, 6 N HzSOd A. Voltage read after 1 minute
E. C. D.
E.
Most positive potential read 4% HsPO4 added 2% HaPo, added Background, 6N HsS04 V O L 32,
NO. 7, JUNE 1960
b
749
B
I
100'
I 500
I
400
h Figure 3. A.
A
I 600
(MP)
Transmittance curves of electrolyzed solutions
0.1 ml. KMn04 in 50 ml. HzO
B. 3N H2S04, 0.1M MnSO1, 2% H3PO4-solution electrolyzed a t current
density 0.5 ma./sq. cm. C. 6N HzSOa, 0.1M MnSO4-solution electrolyzed at current density 10 ma./sq. cm. a t gold electrode
From these observations it is postulated that a t sulfuric acid concentrations less than 8N, the manganic ion formed through primary oxidation a t the electrode rapidly hydrolyzes to form hydrated manganese dioxide and nianganous ion. Such a process would be indistinguishable from the direct conversion of manganous ion to hydrated manganese dioxide because the acid requirements and net electron transfers are identical. Figure 2 shows the current-electrode potential curves obtained a t a gold anode. The higher overvoltage for oxygen evolution on gold permits definition of a current density region higher than that possible on platinum. The identical electrode processes occur on gold as on platinum. The electrode reaction occurring at high current density was identified as the electrochemical conversion of manganous ion to the permanganate state. This was established by adding tetraphenylarsonium sulfate (31) to the electrolysis cell and electrolytically oxidizing manganous sulfate solutions
" I40
EA
1.50 VS.
N H.E
Figure 4. Current density-electrode platinum electrod e
ANALYTICAL CHEMISTRY
I
BO
potential curve for
4N H2SO4, 0.3M MnSOa, 2% H3PO4 B. 6N HzSO,, 0.4M MnSO4 c. Background 4N HzSO4, 2% H&'O4 D. Background 6N HzSOc
containing sulfuric acid a t relatively high current densities. A lavender deposit appeared on the generating electrode which was identical to that obtained when tetraphenylarsonium sulfate was added to permanganate dissolved in distilled water. A similar electrode product was obtained under comparable conditions on a platinum electrode. No deposit was observed under conditions of current density, acid or manganous ion concentration which favored the electrolytic formation of manganic ion or manganese dioxide. Experiments a t high current density with phosphoric-sulfuric acid mixtures (Figure 2, curves C, D) indicate that permanganate formation is a more difficult process than if sulfuric acid alone were present. This may be explained if we may assume that permanganate is formed through a persulfate mechanism-the persulfate be-
Current and Titration Efficiencies a t Low Current Density on Platinum" Current Current Efficiency Density, No. Ma./ of peq. peq. TheoTitn. Ma. Sq. Cm. Expts. Oxidant Ferrous retical Actual Efficiency 12.92 4.06 4 31.13 30 68 99.9 98.5 A 0 . 3 ....... 4 34.49 34.43 ........ 99.7f0.1 11.45 3.60 4 34.52 34.22 99.9 99.1 f 0 . 2 ....... 3 33.77 33.78 ........ 99.9 f O . l 99.9 99.3 A O . l ....... 9.92 3.12 4 23.11 22.95 99.8 3 ~ 0 . 2 3 39.11 38.92 ........ 4 44.18 44.11 99.9 99.8 h 0 . 2 ... .... 7.28 2.29 32.95 . . . . . . . . 100.1 AO.2 3 32.91 5.00 2.47 14.63 99.9 99.0 A O . l ....... 3 14.78 4.00 1.26 3 18.23 18.07 99.9 99.1 f O . l ....... 10.03 1.03 5 31.27 31.13 99.999.5f0.1 ....... 2.30 0.72 4 6.076 5.974 99.8 98.3 h O . l ....... 0.4M MnS04,0.1N Fes(S04)*,6N H2S04.
750
I 7 0
A.
ing formed by the electrolytic oxidation of sulfuric acid. The presence of phosphoric acid may alter the kinetics and energy requirements for the electrolytic conversion of sulfate to persulfate. Figure 3 shows transmittance spectra in the visible wave-length region obtained from solutions oxidized under various conditions. The similarity of these spectra indicates that the same final product is obtained. This product was identified as manganic ion by the wave length of maximum absorption and general band shape ( 7 , l a ) . Although manganese dioxide and permanganate as well as manganic ionvare formed initially by electrolytic oxidation, it is apparent that the former undergo secondary reaction with sulfuric acid and manganous ion to yield manganic ion. These secondary reactions may be written: hln +s
Table I. Generating Current,
I60
+ MnOl + 4H
and 4 ~ ~ +1 h$nO,2
+
+g
=
2Mn - 3
~ =+ 5Mnf3
+ 2H10
+ 4H20
Titration and Current Efficiencies. The current-yoltage curves discussed previously were used to estimate the theoretical efficiencies for manganic ion generated under various conditions. In obtaining these curves, voltage of the anode when generating oxidant a t the desired current density and the current density of the background a t the same voltage were measured. The theoretical current efficiency is calculated from the equation C , - (Ct,b/C0)X 100. C, is the current density when generating osidant and Cb is the background current density. This method of estimating current efficiencies has been
due to the nonstoichiometric reaction of manganese dioxide with manganous sulfate to form manganic ion. Under the experimental conditions used manganic ion or manganese dioxide was always in excess, so that the decomposition of either oxidant may be responsible. The data on titration efficiencies show that results may be quantitative if a reductant is present. I n these cases results are more favorable because the oxidant is consumed as rapidly as it is formed. I n addition, the partial interaction of current with the reductant is undoubtedly responsible to some extent for the improved results. Of all of the conditions studied in the investigation, the direct oxidation of manganous to manganic ion in mixed phosphoric-sulfuric acids had the highest current efficiency. Similar conclusions were reached by Horn (1.4) in a previous study. The process, however, is efficient only at low current densities. The addition of phosphoric acid to the solution apparently lowers the diffusion current of manganous ion (Figure 5, curves A and B ) by complexation. Theoretical current efficiencies of nearly 100% are obtained only at current densities less than 1.5 ma. per sq. em. This upper liiit is dependent, of course, on the concentrations of phosphoric and sulfuric acids and particularly on the concentration of the manganous salt. Either manganic ion or ferrous ion may
be generated first with little difference in precision or accuracy of the results under the optimum conditions. A dual intermediate system utilizing manganic and ferrous ions was found practical. Such a system would be useful for oxidations wherein a n excess of reagent is necessary. After this work was completed, Selim and Lingane published a paper on the generation of manganese(II1) in sulfuric acid medium (28).
REFERENCES
(1) Adams, R. N., Reilley, C. X., Furman, N. H., ANAL.CHEM.25, 1160 (1953). (2) Belcher, R., West, T. S., Anal. Chim. Acta 6,322 (1952). (3) Campbell, A. N., Trans. Faraday SOC. 22, 46 (1926). (4) Cooke, W. D., Reilley, C. N., Furman, N. H., ANAL.CHEM.23, 1662 (1951). (5) Delahay, P., “New Instrumental Methods of Electrochemistry,” p. 36, Interscience, New York, 1954. (6) Domange, L., Compt. rend. 208, 284 (1939). (7) Drummond, A. Y . , Waters, W. A,, J . Chem. SOC.1953, 435.
( 8 ) Fenton, A. J., Jr., Furman, N. H., ANAL.CHEM.29, 221 (1957). (9) Furman, N. H., Adams, R. N., Ibid.,
25, 1564 (1953). (10) .Furman, N. H., Fenton, A. J., Jr., Ibad., 29, 1213 (1957). (11) Grube, G., Huberich, K., 2. Elektrochem. 29, 8 (1923). (12) Hobart, E. W., private communication, 1956.
(13) Holluta, J., J . physik. Chem. 115, 145 (1925). (14) Horn, H., Ph.D. thesis, Korthwestern University, Evanston, Ill., November 1954.
(lij-Ikegami, Hiroshi, J . Chem. SOC. Japan 52,173 (1949). (16) Kolthoff, I. M., Lingane, J. J., “Polarography,” Vol. I, p. 217, Interscience, New York, 1952. (17) Kolthoff, I. M., Watters, J., IND. ENG.CHEM.,ANAL.ED. 15, 8 (1943). (18) Lingane, J. J., Anson, F. C., Anal. Chim.Acta 16, 165 (1957). (19) Lingane, J. J., Karplus, R., IND. ENQ.CHEN.,ANAL.ED.18, 191 (1946). (20) Lingane, J. J., Kennedy, J. H., Anal. Chzm. Acta 15, 465 (1956). (21) Morse, H. N., Hopkins, A. J., Walker, M. S., Am. Chem. J . 18. 401 (1896): (22) Parsons, J. S., Seaman, W., Amick, R. M., ANAL.CHEM.27, 1784 (1955). (23) Reilley, C. S., Adams, R. A., Furman, N. H., Ibid., 24,1044 (1952). (24) Reilley, C. N., Cooke, W. D., Furman, N. H., Ibid., 23,1030 (1951). (25) Saito, K., Saito, N., J . Chem. SOC. J a v a n 55. 59 (1952). (26) ’Selim, ‘R. &.) Lingane, J. J., And. Chim. Acta 21,536 (1959). (27) Sem, M., 2. Elcktrochem. 21, 426 (1915). (28) Tutundzic, P. S., Mladenovic, S., Anal. Chim. Acta 12, 382, 390 (1955). (291 Ubbelohde. A. R. J. P.. J . Chem. ‘ SOC.1935., -1605. - (30) Watters, S., Kolthoff, I. M., J . Am. Chem. Soc. 70,2455 (1948). (31) Willard, H. H., Smith, G. M., IND. ENG.CHEM.,ANAL. ED. 11, 305 (1939).
RECEIVED for review December 21, 1959. Accepted March 18, 1960. Taken from the P h D . dissertation of A. J. Fenton, Jr., Princeton University, 1958.
EIectroIytic Determination of Microgram Quantities of Water in Paper R. G. ARMSTRONG, K. W. GARDINER,’ and F. W. A D A M Central Research and Engineering Division, Confinental Can An instrumental analytical procedure has been developed for the rapid and accurate determination of water in the 1 1 - to 200-7 range. The method is designed for use with moisture-bearing samples as small as 0.6-mg. total weight without elaborate classical microanalytical procedures. It has been applied to determination of the moisture content of paperboard disks ranging in diameter from ‘/la to 6/32 inch, which contained from less than 1 to over 6% moisture. The procedure involves use of a micro oven, in which the water contained in the small paper sample is vaporized and carried into a commercially available electrolytic hygrometer by a dry nitrogen stream. The method of calibration is described
752
ANALYTICAL CHEMISTRY
Co.,Inc.,
Chicago 20, 111.
and data show the precision and accuracy attained during a typical practical application.
A
ISSTRUMEKTAL analytical technique for the determination of water in the range of 11 to 200 y in small paper samples-e.g., 0.6 mg.-has been developed to provide a rapid and easily applied means for analyzing selected specific areas of sheets and formed paper products. The electrolytic measurement of water, on which this procedure is based, has been reported by Keidel ( 2 ) , Taylor (S), and Cole et al. ( 1 ) . This principle has been adapted for the treatment of paper samples. The water in the sample is
N
vaporized in a micro oven by a controlled heating program. The released water vapor is carried hy a stream of dry nitrogen into the electrolysis cell, where the water is absorbed by a thin continuous film of suitably anhydrous phosphoric acid located between two platinum helical electrodes. The absorbed mater is electrolyzed by impressing a potential on the electrodes. The signal from the electrolysis instrument is proportional to the current passed during the electrolysis and is recorded on a strip chart, while the area under the signal-time curve is simultaneously integrated by a digital 1 Present address, Consolidated Electrodynamics Corp., Pasadena, Calif.
