Figure 1 . Cutaway of improved diaphragm electrolytic cell used in experimental work
T O WATTER RESERVOIR
a
cATHoDE% ,DIAPHRAGM
Expansion in wide potential usage of periodate o x y starch has been blocked by prohibitive cost of periodic acid needed for its preparation. An electrolytic method, already in pilot plant stage, is convenient, inexpensive, and may be applied to other polysaccharides and organics
I
PANOLY
TE
'STIRRER
C. L. MEHLTRETTER, J. C. RANKIN, and P. R. WATSON Northern Utilization Research Branch, U. S. Department of Agriculture, Peoria, Ill.
Electrolytic Preparation of Periodate Oxystarch
PERIODIC
A C I D and its salts selectively oxidize starch and form two aldehyde groups in each repeating unit of the starch structure (73; 7.1). The product, periodate oxystarch, can be considered a polycondensate of glyoxal and d-erythrose. Because of its unique functionality, it has large potentialities for use by itself or after chemical modification (78, 2-S. 26). But, methods of
preparation, although they produce quantitative yields, have been restricted largely to laboratory use in structural studies becausr they require equimolar quantities of costly periodic acid or its salts (7-5. 27). An electrolytic procedure \vhereby periodic acid is continuously regenerated would require only a fraction of the theoretical amount of oxidanr for pre-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
paring oxystarch and relatcd oxypolysaccharides and might have industrial significance. It could be applied as well to the oxidation of nurnt'rous othrr organic compounds of appropriate structure (72), It has been shown that oxystarch can be prepared electrolytically using 30(,'c8 of the theoretical amount of periodic acid (7). In the presrnt investigation.
quantitative oxidations were obtained utilizing amounts of oxidant as low as 7.5% of theory with high concentrations of starch. A divided electrolytic cell of improved design was developed, and the effect of more important variables on the course of oxystarch formation was studied. The variables chosen for the i study were current density, time of electrolysis, temperature, pH, diaphragm material, anode composition, kind and concentration of starch, and concentration of iodic acid. Optimum conditions of reaction found in this study are now the basis of a pilot plant investigation in which processing costs will be determined in detail.
C.D., Amp. per Sq. Cm. 0.036 X 0.030
A 0.024 I3 0.018
0 0.012 Experimental
Improved Diaphragm Cell. The electrolytic cell (Figure 1) consisted of a n 800-ml. borosilicate glass beaker containing two diaphragms for separating anolyte from catholyte. Best results were obtained with Alundum extraction thimbles because of the high stability of this material to acids and alkali. As obtained commercially, these were 2.54 cm. in outside diameter and 13.97 cm. high, with a wall thickness of 0.2 cm. Both medium and fine porosity grades were used with equal success. Each diaphragm contained 25 ml. of 5% sodium hydroxide solution as catholyte and a steel rod cathode 0.5 cm. in diameter and 15 cm. long extending to the bottom. T h e diaphragms were connected with a distilled water reservoir for dilution of the catholyte during electrolysis. Rubber stoppers, fitted snugly into the mouth of the diaphragms, held the cathodes and glass tubing in place. Alkali and hydrogen gas formed a t the cathode were removed through the tubing. Lead dioxide was used as the anode because of its efficient conversion of iodate to periodate and its low cost (70, 22, 23, 27). T h e anode was a sheet of lead 5.5 cm. wide, 15 cm. long, and 0.3 cm. thick, the lower half of which was coated with lead dioxide by electrolysis in I N sulfuric acid for several hours. I t was placed between the cathodes, 2.5 cm. from each, and submerged 7.6 cm. of its length in the anolyte to give a total effective area of 84 sq. cm. Immediately before use, it was electrolyzed for 30 minutes as before, using a current of 2 amperes. The beaker which was the anolyte chamber contained 300 ml. of a n aqueous solution of iodic acid formed by reaction of sodium iodate with the theoretical amount of sulfuric acid. Also dissolved in the anolyte were 52 grams of sodium sulfate to obtain good conduction of current. The various commercial starches investigated were added to the cell prior to electrolysis and the anolyte mixture was stirred mechanically throughout the oxidation.
Y 0
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TIME, HOURS Figure 2.
Oxidation rate of iodic acid to periodic aAd at
Temperature of the cell was regulated by means of a n outside water bath. The power source was a selenium oxide rectifier with a low-ripple direct current output of 0 to 15 amperes a t 0 to 30 volts. The ammeter and voltmeter furnished with the rectifier were used for measuring current and potential difference across the electrodes during the electrolysis. Reliability of the instruments was ascertained b y comparison with a n accurate 0- to 5-ampere ammeter and 0- to 10-volt voltmeter. Optimum Conditions for Oxystarch Preparation. Diaphragms of the cell were each charged with 25 ml. of 5% sodium hydroxide solution and rubber stoppers holding the cathodes and reservoir-connecting tubing put tightly in place. One mole of commercial cornstarch (182 grams with 10.80% moisture content) was then added to 300 ml. of aqueous solution containing 17.6 grams of iodic acid [0.10 mole per anhydroglucose unit (AGU) of starch] and 52 grams of sodium sulfate and the mixture transferred to the cell. The anolyte was vigorously stirred arid the electrodes were connected with the rectifier. A current of 2 amperes was passed through the cell during the entire electrolysis a t a potential of approximhtely 5 volts. A few drops of octyl alcohol were added to the anolyte to reduce foaming. After 48 hours, electrolytic oxidation was
.
25" C.
stopped and the anolyte mixture removed from the cell. If analysis of a small sample of product indicated a dialdehyde content below 95% of theory, the mixture was stirred overnight a t room temperature to effect more complete oxidation by periodic acid in solution. In either case, the final mixture was vacuum filtered through a sintered-glass funnel of medium porosity to isolate the insoluble product. The oxystarch was then washed by miving with 250 ml. of distilled water in a blender and filtered. After seven such treatments, approximately %ycof the iodate was removed from the product. Obtaining oxystarch completely free of iodate required considerably more washing with water. The white product, dried a t 40" C. for a t least 12 hours, was obtained in a yield of 98 to 100% of theory. Analytical Procedures Carbonyl Content of Oxystarch. Extent of oxidation of oxystarch samples removed during the course of the electrolysis was determined by a rapid method where 1 equivalent of alkali is consumed by each oxidized unit in the product ( 7 7 ) . Analyses for carbonyl groups in final oxystarch products were obtained by stoichiometric sodium borohydride reduction of duplicate samples (24), following the procedure of Lindberg VOL. 49, NO. 3
MARCH 1957
351
and Theander (LO) for periodate oxycellulose. The results were corroborated by the alkali method. Periodic a n d Iodic Acids. Periodic acid in anol>-tesolutions \vas determined by the method of' Fleury and Lange (8, 72). The combined filtrate and washings of the oxystarch samples removed periodically ivere used for determining the concentration of periodic acid present during oxidation. Determination of combined periodic and iodic acid in solution as iodic acid was carried out on suitable aliquots added to a solution of 1 rnl. of ethylene glycol in 15 ml. of distilled \vatu to reduce periodic to iodic acid. After standing for a t least 15 minutes. 3 ml. of 0.5.V sulfuric acid and 2 ml. of 20yc potassium iodide solution were introduced. Iodine liberated \vas titrated with 0.1,\.' sodium thiosulfate solution, using starch as the indicator. Such analyses were also made on final catholyte liquors and on the Ivashings of fresh]!- used diaphragms to determine loss of oxidant by migration and absorption. respectively. Effect of Variables
Current Density. Before oxidizing starch. the efTect olcurrent density on the formation of periodic acid in the anolyte solution Lvasdetermincd (Figure 2). The anolyte consisted of 0.0'5 and 0.36 mole of iodic acid and sodium sulfate, respectively. in 300 ml. of solution. As expected. rapid conversion of iodic to periodic acid occurred Lvith the higher current densities, and rate decreased markedly after 80 to 90% conversion with loss in current efficiency. Higher current efficiencies ivere gained by use of lower current densities. Too low current densities, however. Ivere impractical from the standpoint of increased rime of oxidation. In the presence of starch, periodic acid should be formed in excess early in the electrolysis to maintain a maximum rate of oxidation to oxystarch. '4 balance of current density, concentration of oxidant, and concentration of starch is thus required. Effect of various current densities on oxidation of a mole of cornstarch dispersed in 300 ml. of anolyte containing 17.6 grams of iodic acid (0.10 mole per anhydroglucose unit of starch) and 52 grams of sodium sulfare. is shown in Figure 3.. Periodic acid formed by electrolysis of this quantity of iodic acid represents 10% of that theoretically required to oxidize starch completely to oxystarch. LOTV oxidation rate of starch a t a current density of 0.012 ampere per sq. cm. indicated that insufficient periodic acid was formed during the electrolysis. Analysis showed the absence of periodic acid in the ano-
352
lyte. Current density of 0.036 ampere per sq. cm. produced a greater rate of oxidation early in the electrolysis because of the more rapid conversion of iodic to periodic acid. The rate decreased markedly after 24 hours \vith build-up of periodic acid in solution (Figure 5) probably because of increasing difficulty for periodic acid to prnerrate the starch granules. Time required for nearly complete conversion of starch ro oxystarch was rhus essentially that found when only half this current density was employed. Table I sho\vs an inverse relationship of current density and current efficiency. Since the most efficient current density, 0.018 ampere per sq. cm., was a t the borderline of a practical value (Figure 3 ) : it was decided to use the intermediate 0.014 ampere per sq. cm. in most of the experiments, Concentration of Iodic Acid. Electrolysis at the lotvest concentration of iodic acid consistent ivith efficient production of oxystarch is desirable because of its high cost. I n order to arrive a t this amount for 1 mole of starch at the current density chosen, effect of iodic acid concentration on rate of oxidation of starch to oxystarch was investigated. Figure 4 illustrates \.ariation in oxidation rate \\ith change in iodic acid concentration. More than 0.1 mole or less than 0.075 mole of iodic acid per anhydroglucose unit of starch is not advantageous. The anolyte consisted of iodic acid as indicated, and 0.36 mole of sodium sulfate in 300 ml. of solution to which 1 mole of cornstarch \vas added. Concentration of Starches. \Vhen the concentration of cornstarch was decreased to 0.5 mole ivhile maintaininq iodic acid concentration at 0.075 mole (0.15 mole per anhydroglucose unit of starch) in the usual volume of anolyte. the oxidation rate increased significantly after 12 hours of electrolysis. I n Figure 5 sho\ving the results, accumulation of periodic acid during each oxidation is plotted as a percentage of complete conversion of iodic acid initially present. Bctter agitation in the more dilute slurry and the presence of a considerable excess of periodic acid Lvould account for this result. The effect of intermediate concentration of starch on the oxidation rate was not appreciably different from that obtained with 1 mole of starch in which the iodic acid-anhydroglucose unit mole ratio was 0.075. O n e mole of cornstarch in 300 ml. of anolytr. containing from 0.075 to 0.10 mole of iodic acid per anhydroglucose unit of starch, thus appeared optimum in regard to over-all current efficiency of the electrolysis. Under these conditions, a considerable quantity of periodic acid
INDUSTRIAL A N D ENGINEERING CHEMISTRY
lOOr
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tf IO
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TIME, HOURS
Figure 3. Effect of current density on oxidation r a t e of cornstarch t o oxystarch a t 25" C. C.D., Amp. p e r Sq. Cm.
X 0.036
0 0.024 0 0.018
a 0.012 Table I. Relation of Current Density t o Efficiency for 9770 Conversion of Starch to Oxystarch h i o d e C'urrcnt ('imc n t Deiisity, C'iirwiit, I.ffii icnc y, A m p / F q . ('m, .itngeres 0.036 3.0 0.024 2.0 0.018 1.5
'rc
37 56 74
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Figure 4. Effect of iodic acid concentration on oxidation r a t e of cornstarch t o oxystarch a t 25' C.
vented adequate stirring of the mixture. As shown in Figure 6 , however, more dilute mixtures were readily converted to 95% oxystarch in 36 hours. T h e anolyte consisted of 0.075 and 0.36 mole of iodic acid and sodium sulfate, respectively, in 300 ml. of solution containing 1 and 0.5 mole of starch.
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Figure 5. Effect of cornstarch concentration on oxidation rate to oxystarch a t 25' C.
accumulated in the anolyte after 24 hours of reaction and could be used for completing oxidation outside the cell of oxystarches having upwards of 94% dialdehyde content. Under conditions optimum for cornstarch, wheat and potato starch did not oxidize well. The anolytes containing these polysaccharides had lower fluidity than those of cornstarch and this pre-
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Mole Wheat Starch, -Potato Starch\
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lor
20uI)1
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IO
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(0.075 Mole HIO,)
30
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TIME, HOURS
Figure 6. Effect of concentration of wheat and potato starch on oxidation rate to oxystarch a t 25' C.
p H Control. Most of the starch oxidations were maintained in the p H range 1 to 4 by adding solid sodium bicarbonate to the anolyte. T h e yield of oxystarch obtained averaged 98% of theory. Reaction of oxystarch with sodium bicarbonate was insignifitant because of low alkalinity of this salt and the fact that it was introduced in solid form and immediately attacked by the mineral acids present. Partial neutralization of acidity in the anolyte with aqueous sodium hydroxide solution lowered the yield of oxystarch approximately 10% because of sensitivity of oxystarch to alkali degradation (9, 7 7). Electrolytic oxidations with cornstarch in which acidity produced was not controlled, gave final p H values of about 0.4 with no detrimental effect on either oxidation rate or yield. This is in marked contrast , t o the periodate oxidation of xylan and of cellulose (75-77) a t room temperature where optimum oxidation was achieved a t a p H of 3.5 to 4, and a p H of 1 produced polysaccharide hydrolysis and a low yield of oxypolysaccharides. Temperature. Oxidations carried out in strongly acid solutions would probably be unsatisfactory a t higher temperatures because of hydrolysis of the initial polysaccharide and its oxidation product to reducing substances capable of reaction with periodic acid. I n fact, when electrolyses were performed a t 35 O C. for 48 hours, only 66% of the oxystarch containing 60% of dialdehyde units was recovered. Reduction of iodic acid to iodine was indicated by the blue starchiodine complex formed. Satisfactory results were obtained a t 25" C., and this temperature was employed in all experiments reported. Only slight cooling of the anolyte was required to maintain this temperature throughout the electrolysis. Anodes. Lead sheets were anodically polarized for several hours in 1N sulfuric acid at a current density of 0.035 ampere per sq. cm. to obtain a good coating of lead dioxide. They were then washed with water and air dried. Polarization for 30 minutes before use as suggested by Hickling and Richards (70) activated the anodes. Such pretreatment allowed reproducible starch oxidations to bk obtained with the lead dioxide-coated sheets of chemical lead and the two alloys d lead investigated as anode materials. Of the latter. one contained 1% silver and the
other (BAS, National Lead Co.) 4y0 antimony and small amounts of tin, tellurium, and silver. A lead dioxideimpregnated storage battery plate was used in several experiments, but was discarded because of excessive shedding of lead dioxide. The BAS alloy also was rejected for this reason. Most of the oxidations were performed with 1% silver-lead alloy. Small residues of lead dioxide observed in the anolytes after electrolysis could not be completely removed by ordinary gravity separation; thus the products generally contained about 300 p.p.m. of lead. A more detailed study of lead dioxide removal should produce oxystarches of considerably lower lead content for nonfood, industrial applications. Cathode and Catholyte. The effect of different cathode materials on the electrolysis was not studied, because a steel rod functioned satisfactorily in a n aqueous catholyte of 5y0 sodium hydroxide. Adding catholyte to the diaphragms prior to their introduction to the anolyte produced more efficient oxidations. Apparently, less iodic acid was absorbed by the porous cups by this procedure. The level of the catholyte was maintained several centimeters above that of the anolyte mixture by slowly flushing with distilled water. By this means, loss of iodic acid to the catholyte by diffusion was reduced and passage of hydrogen into the anolyte prevented. Moreover, such dilution of the catholyte minimized the corrosive action of alkali in the diaphragm. Diaphragm Composition. Unglazed porcelain diaphragms disintegrated after repeated use in the presence of alkaline catholyte solutions and were not satisfactory for prolonged studies. Alundum cups, on the other hand, gave excellent results and maintained their stability and electrical conductivity throughout the investigation. Fine and medium porosities were used interchangeably with little difference in their effect on the couise of oxidation of starch. Relatively high cost of Alundum cups led to investigation of less expensive porous materials for use in diaphragms. Although Vinyon bags have been used successfully in laboratory experiments in the electrowinning of metals (79), diaphragms prepared from both Vinyon and Orlon acrylic fiber were too porous for the present purpose and allowed hydrogen to diffuse into the anolyte and reduce iodic acid to iodine. Alkali also entered the anolyte and degraded the oxystarch present. Asbestos was not investigated because of the difficulty in constructing laboratory-size diaphragms from it. Cotton duck, of course, would react with the periodic acid formed in the anolyte. Table I1 VOL. 49, NO. 3
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MARCH 1957
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Table
II.
Effect of Diaphragm Composition on Oxidation of Cornstarch
Osidntioii I)iul~lirngiii
'rime,
Unglazed porcelain Alundum Vinyon Orlon (SO-1) Orion (SO-2)
48 48 24 48 48
-
Hr,
(hnipo~itii)n
. Oxystnrc,li, . ~ ~ %_ _ Oxidation" Tiel d
99 96 44 51 36
_
Oxidant Loss,
