An Electrolytic Process for
...
Making Sodium Metaperiodate PERIODIC
ACID and its salts because of their selectivity of oxidation of cis-glycol configurations are becoming increasingly important in organic synthesis and in polysaccharide structural studies (7, 6 ) . Although several methods for the preparation of periodic acid and its salts by chemical and electrolytic oxidation of iodine have been reported (2, 5. 8), none is adequate for industrial use. Deficient in many respects for largescale application is the electrolytic procedure of Willard and Ralston ( 8 ) ,which utilizes a n anolyte of iodine suspended in hydrochloric acid and a catholyte of nitric acid. 'The corrosiveness of the electrolytes necessitates the use of costly platinum anodes and gold-plated copper cathodes. The procedure must also be conducted in two stages, production of iodic acid with subsequent electrolysis to periodic acid in a separate cell with different electrodes. This article describes the development of a practical method for the preparation of aqueous periodic acid solutions by electrolysis in a single cell using inexpensive electrodes. T h e process consists of oxidation of iodine in alkaline solution to sodium iodate, followed by electrolysis under acidic conditions to give periodic acid in a n over-all yield of 96y0 of theory. h-eutralization of the periodic acid solution produces a mixture of sodium periodate and sodium sulfate, from which sodium metaperiodate is isolated in 9370 yield in the first crystallization fraction. Residual periodate is readily recovered by precipitation as sodium paraperiodate. The effect of concentration of iodine, alkali, and sodium sulfate and of current density on the course of the oxidation was determined. Also reported is a phase equilibrium study ( 3 ) on the separation of sodium metaperiodate from sodium sulfate. the results of which were used to obtain efficient separation of sodium metaperiodate from sodium sulfate in neutralized periodic acid electrolysis solutions.
Electrolytic Oxidation
Diaphragm Cell.
The cell and direct current source used were the same as those employed in the preparation of periodate oxystarch (7). T h e cathodes, which were graphite rods 1 cm. in diam-
I
C.
L. MEHLTRETTER
and C. S. WISE
Northern Utilization Research and Development Division, U. Department of Agriculture, Peoria, 111.
S.
This chemical is becoming increasingly important in organic synthesis. Here is a practical method for making it at low cost. Equipment and techniques used might well be of interest in electrolytic production of other materials
1O O r
be
. 6 I 0 Ic3
0 -
I : LL
0 Z 0
1 I
tn Lu
>
P E R S Q . CM. 0.114 0.030
Z 0 V
0
10
20
TEMP. @c . 50
33 30
TIME, HOURS Figure 1. Oxidation of iodic acid to periodic acid is more efficient at lower current density VOL. 51, NO. 4
APRIL 1959
51 1
0
(99.770 purity) in a solution of 33 grams of sodium hydroxide and 10 grams of
I-
sodium sulfate in 500 ml. of distilled water was then introduced t o the cell and mechanically stirred. A current of 7.5 amperes was used during the electrolysis a t a potential of approximately 6 volts. T h e anolyte was maintained a t 50" C. by cooling the cell in a water bath. The catholyte, which was kept alkaline, was continuously or intermittently diluted with water from the reservoir to maintain its surface level above that of the anolyte and to reduce corrosion of the Alundum diaphragms. After approximately 14 hours, all of the iodine was converted to sodium iodate. The cathode compartments were then flushed with water to remove most of the alkali and connected with a reservoir containing 10% sulfuric acid. Acid solution was slowly passed through the cathode compartments to maintain slight acidity of the effluent during the second stage of the electrolytic oxidation. The anolyte was treated with 4 ml. of concentrated sulfuric acid to dissolve precipitated sodium iodate and the electrolysis was continued for approximately 12 hours a t the current density used in the first stage of oxidation to achieve essentially complete conversion of iodate to periodic acid. The diaphragms and anode were removed from the cell and the oxidation liquor was heated to 70' C. for about 30 minutes. It was then filtered to remove a small amount of lead dioxide that had shed from the anode. The clear colorless solution contained 145 grams of periodic acid and 4.3 grams of iodic acid. This analysis represents a conversion of 96y0 of the iodine to periodic acid and a total production of 99,47& of combined periodic and iodic acids. T h e production value includes the 0.470 of periodic acid absorbed by the diaphragms. Results a n d Discussion. Preliminary experiments showed that iodine in low concentrations in aqueous sodium hydroxide solution was readily oxidized to iodate by electrolysis. However, at higher concentrations of iodine an insoluble coating of trisodium paraperiodate formed on the lead dioxide anode, which lowered the efficiency of the cell. This effect was reduced by adding sodium sulfate to the anolyte in low concentration. Apparently scaling of the anode was prevented during the
I
I
20
40 60 80 TEMPERATURE, ' C .
100
Figure 2. Data on solubility of sodium metaperiodate and sodium sulfate in the presence of both solid phases make possible efficient separation eter with a submerged surface area of 24 sq. cm. each, were resistant to corrosion in both alkali and acid under reductive conditions. T h e lead dioxide-coated anode had a total effective area of 66 sq. cm. when immersed in the anolyte.
Table I. The Balance of Alkali and Iodine Is Critical in Avoiding Anode Scaling" Sodium Sodium Iodine, Hydroxide, Sulfate, Anode Scaling Grams Grams Grams None 25 20 0 50
loo*
15oc
30
0
20
0 20
33
10
50
20
60
20
0
+
+
Slight None None Slight
+ +
a Electrolysis of 400 ml. of solution at C.D. 0.114 ampere per sq. om. and 40° C . Additional alkali added in portions during reaction t o keep iodine in solution. * Anolyte volume approximately 500 ml. Addition of this amount of iodine t o alkaline solution produced white precipitate of sodium iodate in anolyte (500 ml.) before electrolysis.
512
Analytical Procedures. Periodic acid was determined in the oxidation liquors by the method of Fleury and Lange ( 7 , 4 ) . Combined periodic and iodic acids were determined as iodic acid (7) to follow the rate of conversion of iodic to periodic acid during the electrolysis. Final catholyte solutions and wash liquors from freshly used diaphragms were also analyzed to estimate loss of oxidant by migration and absorption, respectively. Preparation of Periodic Acid. The two cathode compartments of the cell were each charged with 30 ml. of 5% sodium hydroxide solution, and rubber stoppers holding the graphite cathodes and reservoir-connecting tubing were forced into place. Anolyte prepared by dissolving 100 grams of crude iodine
Table II.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Conversion of Iodic to Periodic Acid i s More Efficient at Lower Current Density
Anode Current Density, Amp./Sq. Cm.
Current, Amp.
Oxidation Time, Hr.
0.030
2.0
23
Periodic Acid Formed, % 100
0.114
7.5
12
97
Current Efficiency,% 90 45
SODIUM METAPERIODATE elecLrolyiis hy a continuous acid reaction at the surface of the anode through sulfate ion migration. Btcause continued electrolysis produced acidity in the anolyte. it \vas necessary to add periodically 3 to 5 ml. of 40y6 sodium hydroxide solution to prevent liberation of excessive amounts of free iodine and yet not coat the anode with trisodium paraperiodate. Because the balance of alkali and iodine is critical for avoiding anode scaling even in the presence of sodium sulfate, as shown in Table I , all of the additional alkali could not be introduced at the beginning of the electrolysis. A current efficiency of 357, was obtained in the oxidation of iodine to iodate at high current density (0.114 ampere per sq. cm.) and the effect of lower current densities was therefore not studied. Hoivever! the oxidation of iodic to periodic acid was more efficient at lolver current density, as previously shown (7.8). The results presented in Table I1 were obtained in the preparation of periodic acid from 100 grams of iodine under the conditions described and in a similar experiment conducted at the lower current density. The rate of oxidation of iodic to periodic acid under these conditions is shown in Figure 1. Oxidation at high current density was conducted at a constant temperature of 50" 1' C. to reduce cooling costs. Improved current efficiency is an added advantage gained at this higher temperature. TVhen 0.030 ampere per sq. cm. \vas used for the oxidation of iodic to periodic acid. cell temperature remained below 35' C. and no cooling of the anol);te \vas required.
*
Sodium Metaperiodate-Sodium Sulfate Phase Equilibrium Study
The efficient separation of sodium metaperiodate from sodium sulfate by fractional crystallization can best be accomplished by studying the phase diagrams Lvhich represent the equilibria existing in aqueous solutions of these salts ( 3 ) . However. data required for the preparation of such diagrams could not be found in the literature. The solubilities of pure sodium metaperiodate and sodium sulfate were therefore determined at various temperatures in aqueous solutions containing both salts in the solid phase. Temperature control within zk0.5' was considered precise enough for the development of a practical temperaturecycle process. Composition of the saturated solutions was determined on a portion removed in a wide-tipped pipet. previouslv heated to the temperature of the solution. and transferred to a tared
0
0 0 7
0" v, hl
0
Z
Na104, GRAMS P E R 100 G R A M S OF WATER Figure 3. Sodium metaperiodate i s separated from sodium sulfate b y fractional crystallization under optimum conditions
ground-glass stoppered weighing bottle. The weighed solution \vas evaporated and the residue dried at 100' C. to constant Lveight to ascertain total solids present. The residue was then dissolved in \cater and analyzed for sodium periodate by the method of Fleury and Lange ( 4 ) . Sodium sulfate \vas obtained by difference. The curves in Figure 2 illustrate the solubility at various temperatures of sodium metaperiodate and sodium sulfate in aqueous solutions containing the solid phases of both salts. Sodium metaperiodate has low solubility and sodium sulfate high solubility in water at 37' C., the temperature at which transition of decahydrate to anhydrous sodium sulfate occurs in the presence of solid phases of both the periodate and sulfate. The possibility of initial separation of a large proportion of the sodium metaperiodate in pure form from a mixture with sodium sulfate is thus indicated. The succeeding solid phase removed by crystallization will be sodium sulfate. Crystallization can be carried out by either of two methods. Cooling the sodium metaperiodate fil-
trate to the practical temperature of 26' C. will deposit decahydrate. as indicated in Figure 2. .4lternatively, the initial sodium metaperiodate filtrate may be evaporated to a definite weight and anhydrous sodium sulfate allowed to deposit at any temperature between 37' and 100' C. Only the isolation of the decahydrate is discussed here. Figure 3 illustrates the isothermal solubility curves for sodium metaperiodate and sodium sulfate at 26' and 37' C. Points a and a' represent the composition of solutions saturated with sodium sulfate, b and b' with sodium metaperiodate. and c and c ' with both salts a t 26' and 37' C.. respectively. Lines ac and u'c' represent the cornposition of solutions saturated with sodium sulfate in the presence of varying amounts of sodium metaperiodate. and lines bc and b I C ' the composition of solutions saturated with sodium metaperiodate which contain varying amounts of sodium sulfate. Actual compositions were not determined except a t the cardinal points of the curves and it was assumed that the solubility curves are straight lines between these points. VOL. 51, NO. 4
APRIL 1959
513
After removal of the first crop of sodium metaperiodate a t 37’ C., a solution of composition corresponding to c ’ is obtained. IVhen the solution is cooled to 26’ C., corresponding to point Y, deposition of sodium sulfate decahvdrate occurs until the solution attains the composition at c. Removal of \\Iter by evaporation a t 37’ C. produces deposition of sodium metaperiodate a t 1 , which continues along the line to c’. After separation of sodium metaperiodate, composition of the final solution is presented a t c I.
Separation of Sodium Metaperiodate and Sodium Sulfate Decahydrate. Sodium metaperiodate was prepared from 500 grams of trisodium paraperiodate as illustrated in the equation : SalHnIOs
+ H1S04
NaIO, f Na2S04 2H20 (1) --L
Table 111.
26
’C.
Table IV. Recovery of Sodium Metaperiodate and Sodium Sulfate Decahydrate from Trisodium Paraperiodate Was Good h-a?S01.10 H?O. Crops
+
Trisodium paraperiodate was used because it is readily obtained in quantitative yield by precipitation from alkaline solutions of periodic acid ( 2 ) . Addition of the theoretical quantity of sulfuric acid assured complete conversion of the alkaline paraperiodate to metaperiodate and sodium sulfate with little, if any. contamination by sodium bisulfate and mono- and disodium paraperiodates. T h e following separation is based on solubility data given in Table 111. At 37’ C. 3.4 grams of sodium metaperiodate and 46.0 grams of sodium sulfate can be dissolved in 100 grams of water to reach saturation for the salts. To keep all of the sodium sulfate from 500 grams of trisodium paraperiodate in solution at this temperature requires 525 grams of water. This quantity of water, however, dissolves only 18 grams of sodium metaperiodate a t saturation and the remaining 346 grams are insoluble (crop 1). The filtrate upon cooling to 26’ C. deposits 351 grams of sodium sulfate decahydrate (crop 2), Lvhich includes 196 grams of water of crystallization. After filtration of the dehydrate at 26’ C. the solution contains 329 grams of water. 87 grams of sodium sulfate, and 18 grams of sodium metaperiodate. Evaporation of the filtrate to leave 189 grams of water a t a temperature of 37’ C. deposits 12 grams of
Temp.,
sodium metaperiodate from soluticn (crop 3). The final filtrate thus contains 87 grams of sodium sulfate and 6 grams of sodium metaperiodate in solution. A comparison of the calculated and actual recoveries of sodium metaperiodate and sodium sulfate decahydrate from 500 grams of trisodium paraperiodate, according to the separation process described, is shown in Table IV.
95
94
3
2
..
..
..
..
58
64
..
..
The final filtrate contained 4 and 42%. respectively, of the amount of sodium metaperiodate and sodium sulfate originally present. If the filtrate is treated with alkali, the insoluble trisodium paraperiodate recovered can be returned to the process. In the actual performance of the separation crop 1 of sodium metaperiodate weighed 368.5 grams and \vas 92,5y0 pure (9470 recovery of total sodium metaperiodate). Two successive Luashings with 115 and 56 ml. of water. respectively. at 37’ C. produced 324 grams of product of 98yG purity. .4fter the filtrate had been cooled to 26’ C., a mixture of 326 grams of sodium sulfate decahydrate and 2.1 grams of sodium metaperiodate was isolated a? crop 2. Crop 3 was 6.6 grams of sodium metaperiodate (89% purity) isolated a t 37’ C. after appropriate evaporation. In commercial practice it might be preferable to eliminate the crystallization of sodium sulfate decahydrate and isolate the periodate in the filtrate of crop 1 as insoluble trisodium paraperiodate for return to the process. This effect was achieved essentially by combining crops 2 and 3 plus the filtrate of crop 3. with the t\vo washes of crop 1 and precipitating trisodium paraperiodate a t 90’ C. by adding excess sodium hydroxide
Solid Phase NaIO4 NatSOd. 10 HzO NaIO4 Na2SOa.10 HzO NaIOa Na9SOd Naps04 NaIOd
~~~~
INDUSTRIAL AND ENGINEERING CHEMISTRY
Separation of Sodium Metaperiodate from Electrolysis Solution. The electrolysis solution bvhich contained 140.3 grams of periodic acid (equivalent to 156.4 grams of sodium meraperiodate): 3 grams of sodium iodate, and 86.8 grams of sodium sulfate, was neutralized to p H 3 with 407‘ sodium hydroxide solution. The resulting mixture was evaporated to a weight of 435 grams to obtain the optimum conditions for deposition of sodium metaperiodate at 37’ C. as required by the solubility data in Table 111. The precipitated sodium metaperiodate was isolated by filtration at 37’ C.? Mashed once with 20 ml. of water a t this temperature. and dried at 100’ C. The product Lveighed 148 grams and \vas 98y0 pure sodium metaperiodate (937, recovery of total sodium metaperiodate). The presence of the small amount of sodium iodate did not appreciably affect the recovery of sodium metaperiodate. However, larger quantities might upset the phase equilibrium and contaminate the product. For isolation of dissolved periodate it would be preferable at this stage to precipitate quantitatively the 12 grams of sodium metaperiodate in the combined filtrate and washings as trisodium paraperiodate, as described earlier. -4cidification of the isolated paraperiodate salt according to Equation 1 would give a weight ratio of sodium metaperiodate to sodium sulfate in solution of 1 to 0.7 as compared to the 1 to 8 ratio in the filtrate of the electrolysis solution after removal of the first crop of periodate. The acidified solution can then be returned to the system. literature Cited (1) Bobbitt, J. M., Adibances in Carbohydrate Chem. 11, 1-41 (1956). (2) Booth, H. S . , lnorg. Syntheses 1, 168 11 939) -,,. j -
(3) Findlay, .‘The Phase Rule and I t s Applications,” Dover Publications, Kew York, 1945. 14) Fleury, P. P., Lange, J., J . pharm. chim. 17, 107 (1933). (5) Hickling, A , , Richards, S. H , J . Chem. SOC. 1940, p. 256.
(6) Jackson, E. L , “Organic Reactions,” vol. 11, p. 341, Wiley, New York, 1944. (7) llehltretter, C. L., Rankin, J. C.. Watson, P. R., IND.ENG.CHEM.49, 350 (1957). (8) Willard, H. H., Ralston, R. R., Trans. 62, 239 (1932). Electrochem. SOC.
Solubility Data
+
5 14
1
2 3
~~
+
37
KaIOl, % % Calcd. Isolated Calcd. Isolated
solution. The yield of trisodium paraperiodate was 60.3 grams, \Yhich is equivalent to 43.9 grams of sodium metaperiodate. Total recovery of periodate from the original 500 grams of trisodium paraperiodate was thus 99%.
15.5 5.4
30.4 26.4
RECEIVED for review September 11, 1958 ACCEPTED December 9, 1958
49.6 46.0
Division of Industrial and Engineering Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958.
28.6 3.4
