An Electrodialvzer for Starch ., R . M. HIXON AND VERA DAWSON MARTIN Iowa Agricultural Experiment Station, Ames, Iowa
T
HE apparatus described in this article was developed for
The center cell was constructed of a glass cylinder of about 1500-cc. capacity with parchment membranes stretched over the ends. The whole was clamped between the two end plates 20 em. (8 inches) square, by means of 10-cm. (4-inch) bolts, with rubber gaskets 3 mm. (0.125 inch) thick serving as spacers between the parchment and the glass plates. The capacity of the electrode compartments thus formed was almost 75 cc. The electrodes were held in place against the glass plates as shown in Figure 1 by rectangles of glass tubing and rubber bands. The glass rectangles also served the purpose of supporting the parchment membranes. The positive electrode was an oval (14.38 X 16.25 X 0.0025 om., 5.75 X 6.5 X 0.001 inch) of platinum foil and the negative electrode was an oval of copper foil as recommended by Humfield and Alben (1). Electrical connections were secured by short wires leading through holes in the top of the glass plates and held in place beneath the foil by the rubber band arrangement. The outlets to the electrode chambers were short glass tubes through stoppers in holes in the plates. Rubber tubes carrying small funnels were connected to these outlets in order to facilitate draining and refilling the electrode chambers. The apparatus was cooled by allowing cold water to flow over the sides when the whole cell was supported over a pneumatic trough.
the electrodialysis of starch and starch products. The process was used to remove all ions and at the same time t o cause a coagulation of portions of the carbohydrate material which would facilitate its recovery. Various forms of apparatus have been used for electrodialysis. The apparatus of Taylor and Kerecztesy (4) was unsatisfactory because the electrodes were too small and too far apart. T h e Loddesol ( 2 ) modification of Pauli's (3) apparatus mas better suited for this purpose because i t was a three-chambered apparatus used in a horizontal position, with the electrodes fairly close together. Other dialyzers described in the literature either handle very small volumes or, if of large capacity, use electrodes other than platinum in hard-rubber containers to reduce costs of construction.
Any source of direct current electricity can be used with this type of apparatus. In the electrodialysis of carbohydrates i t was convenient to use a vacuum-tube rectifier with either resistance or transformer steps in order to vary the voltage applied. Variation in the current source from 0 to 500 volts with a capacity of 100 milliamperes is necessary. The progress of the electrodialysis was followed by noting the voltage and current going through the cell, and also by titrating the liquid from the electrode chambers with 0.1 N acid or base. With starch products the cations were removed most rapidly, as noted by Watson ( 5 ) .
TABLE I. RATEOF DIALYSISOF 1 PERCENTCORNSTARCH PASTE Time Hours 0 0.5 1.0 2.0
Amperage iMilliamps.a 160
105
85 65 55 40 30 28 28b 2EG 28d
Rate of Electrolyte Removal. 0.1 N Acid or Base Anode Cathode Cc./hour 5.2 4.2 3.2 2.3 1.9 1.6 1.3
3.0 4.0 5.0 6.0 1.1 7.0 0.6 11.0 0.3 15.0 a To get comparative readings, current source was adjusted t o before each reading. b Free electrolyte practically all removed. 0 Distinct, separation of gel phase is apparent. d Separatlon of gel phase 18 complete.
FIGURE 1. ELECTRODIALYZER
An electrodialyzer for quantitative work on starch should meet the following requirements: 1. Since the concentration of starch used cannot be much greater than 1 Der cent, the dialyzate compartment should have high capacity. 2. Since the conductance of starch Dastes is very low.' the electrode area must be large to give high c6rrent capacity. 3. To avoid excessive costs of platinum, the anode must be designed to use thin foil. Graphite or base metals cannot be used for quantitative work. 4. The volume of the electrode compartments should be small for convenient recovery of the small amounts of electrolytes removed from the starch". o. The dialyzer must be easily dismantled for cleaning and removal of the coagulated starch paste.
a
9.4 1.5 0.9
0.7 0.6 0.6 0.5 0.4 0.3
0.1 200 volts
The data presented in Table I are characteristic of the operation of the electrodialyzer on a 1 per cent cornstarch paste. The applied voltage was increased from 200 at the beginning of dialysis to 500 volts in the course of 4 or 5 hours, taking care that the current was never more than about 100 milliamperes.
Literature Cited
The apparatus described below is a modification of the Loddesol cell to meet these requirements. Figure 1 is a photograph of the completely assembled apparatus; i t also shows an extra center chamber, the construcas a Of One end plate, and the vacuum-tube rectifier source of current.
(1) Humfield and Alben, J. Am. SOC.Agron., 20,36 (1928). (2) Loddesol, Ibid., 24, 74 (1932). (3) Pauli, Biochem.Z.,152, 355 (1924). (4) Taylor and Kerecztesy, IND.E m . CHEM.,28, 502 (1936). (5) Watson, Ibid., 26,640 (1934). JOURNAL Paper No. 5-593 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 426.
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