(6) Magee, R. J., Scott, I. A. P., Wilson, C. L., Talanta 2, 376 (1959).
LITERATURE CITED
( 1 ) Boyd, G. E., J. Chem. Educ. 36, 3 (1959). (2) Farrar, L. G., Thomason, P. F., Kelle,y, M. T., ANAL.CHEM.30, 1511 (1958). (3) Howard, 0. H., Weber, C. W., Zbid., 34,530 (1962). (4) Kelley, M. T., Jones, H. C., Fisher, D. J., Ibid., 31,488,956 (1959).
( 5 ) Kelley, M. T., Miller, H. H., Ibid., 24, 1895 (1952).
(7) Miller, F. J., Thomason, P. F., ANAL. CHEM.33,404 (1961). (8) Zbid., 32, 1429 (1960). (9) Miller, H. H., Kelley, M.T., Thomason, P. F., in I. S. Longmuir, “Ad-
vances in Polarography,” Vol. 1, pp. 716-26, Pergamon Press, New York,
1960. (10) Parker, G. W., Reed, James, Ruch, J. W., U. S. Atomic Energy Commission Rept. AECD-2043, Jan. 9, 1948.
(11) Shults, W. D., Thomason, P. F.; ANAL.CHEM.31,492 (1959). (12) Thomason, P. F., U. S. Atomic Energy Commission Rept. ORNL-2453, Dec. 31, 1957.
RECEIVED for review September 20, 1962. Accepted November 21, 1962. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. The Oak Ridge National Laboratory is operated by the Union Carbide Corp. for the U. S. Atomic Energy Commission.
Some Applications of Ion Exchange Membranes in Automatic Coulometric Aqueous Acid-Base Titrations PAULINE P. L. HO and MAX M. MARSH Analytical Research Department, Eli Lilly and
b Anion and cation exchange membranes, combined in a single compartment, have been applied to the separation of the anode and cathode compartments in coulometric acid-base titration. In the range of 0.200 to 1 .OOO meq. of acid or base, the standard deviations were found to be less than f0.004. An automatic coulometric titration setup i s described.
I
N THE usual coulometric procedure.
particularly for internal electrolytic generation of reagent, a salt bridge or a sintered-glass disk is commonly employed to separate the cathode and anode compartments. However, in many cases-e.g., the electrolysis of water in acid-base titrations-a clearcut separation is difficult because of the high mobilities of the hydronium and hydroxyl ions. Feldberg and Bricker ( I ) found that a synthetic cation exchange membrane \$ as satisfactory as a cathode isolator in coulonietric titration of bases. Although these membranes were not 100% permselective, they were aufficiently selective to prevent the mixing of anodic and cathodic oyidation reduction products. Hydrostatic pressure exerts almost negligible effect on these membranes; this, combined with inertness and ion selective properties, provides a distinct advantage over the sintered-glass disk or other barrier for electrode isolation. T o facilitate the coulometric titration and overcome the tedious procedure in ordinary potentiometric titrations, in which a preliminary sample must be run in order to preset the pH change at the end point, an automatic titration apparatus has been set up. It consists of a coulometer (3) for current supply, a titration compartment, and a simple
618
ANALYTICAL CHEMISTRY
Co., Indianapolis, Ind.
end point-detecting circuit. The end point of a titration can be detected either spectrophotometrically or potentiometrically with the XIalmstadt second derivative end point termination method (4, 5 ) . The acid-base titration procedure is further simplified by mounting cation and anion exchange membranes together inside the electrode isolation cell, so t h a t the same cell can serve for both purposes. Only the total resistance across the membranes increases. The perniselective capacity of each type of membrane is not altered. EXPERIMENTAL
Apparatus. The basic components of t h e automatic spectrophotometric titrator are illustrated in Figure 1. An integrating motor type of coulometer (5’) was used for the electrolytic generation of H f and OH- ions. The quantity of electricity passed through the titration cell is directly proportional to the voltage applied to the terminals of a special low inertia motor on which a mechanical counter is geared to the motor shaft; thus, the integrated current is directly registered as number of counts instead of involving exact measurement of time for current flox at constant current. The coulometer has been calibrated in terms of counts per milliequivalent, using the silver deposit method. The titration compartment lvas made by modifying a n obsolete spectrophotometer. The light source consists of a coiled filament incandescent lamp, connected to the line through a voltageregulating transformer. Wavelength, in the region of 380 to 700 mp, can be easily selected by turning the grating of the monochromator via a wavelength dial on top of the box. The sample holder used for this experimental work was a 100-ml. borosilicate glass
beaker, but beakers ranging from 50 to 150 ml. can be used. For the maximum efficiency of mixing, a motordriven glass stirrer was mounted on the cover of the titration compartment; the speed could be adjusted from 0 to 2000 r.p.m. by turning the knob of a potentiometer mounted on the box. The titration cell assembly is shown in Figure 2,.4. Both the coulometer and the stirring motor were connected to the relay of the control system (4). They can be started simultaneously when the automatic button is pushed, and stopped when the inflection point of the titration is reached. The light detector is an inexpensive CdS photoconductive cell. The output signal is converted to voltage through a simple electric circuit and fed into the input of the control unit, similar to the one in the commercially available Sargent-hlalmstadt Spectro-Electro titrator (6). The setup is suitable for both spectrophotometric and potentiometric titrations. For potentiometry, the voltage signal between the two electrodes is fed into the control unit directly. Adaptors for the mounting of electrodes have been made on the cover of the titration compartment. A positive voltage swing is required for proper operation of the commercial control unit at the end point of the titration. At a potentiometric end point, electrode reactions employing a change in electromotive force in either a positive or negative direction (with respect to a reference electrode) were expected to be encountered; a t the spectrophotometric end point a n increase or a decrease in absorption mas anticipated. To make these reactions or absorption changes compatible with the control unit, a reversible switch was connected to the output terminals of the light detector and the electrodes, so that proper signal could always be applied to the input of that unit. The cation exchange membrane, Nep-
GENERATMG CATHODE
-
COULOMETER
l 3 - J
6ENERATlNG ANODE
PPRAFFIN -TITRATION CELL
- I O N EXCHANGE MEMBRANES
A
Basic components of automatic coulometric
Table 1.
Samsle XaOH NaOH SaOH Sa3P04
4 4 4
HC1 HC1 KHCsH4Oa
3
H3PO4 and
3
EXCHANGE MEMBRANE
Titration cell assembly
membrane. The reverse is true for acid titration, in which the diffusion of H + ion is prevented by the anion exchange membrane. To maintain the current a t a constant value, a 1M solution of sodium sulfate was used for the preliminary experimental work. However, since our later interests have been directed toward nonaqueous solvents, a more soluble salt, sodium perchlorate solution, was preferred. I n the titration of base, the sample beaker is the anode compartment, and the isolated platinum spiral is made cathode; the condition is reversed when acid is being titrated. The number in the counter is read after every determination; the net count is obtained by subtracting the count before the electrolysis from that after electrolysis. The number of milliequivalents in the sample can be obtained by dividing the number of counts by a factor, expressed in counts per milliequivalent, lvhich is the calibrated value set by the silver deposition method or other calibration procedure. RESULTS AND DISCUSSION
The results of acid-base titrations are summarized in Table I. In the range of 0.2 to 1.0 meq. of acid or base the standard deviations n-ere k0.004
Coulometric Acid-Base Titrations Using ion Exchange Membranes for Separation of Cathode and Anode Compartments
s o . of
detns.
of H20 will flow through the isolating tube in 48 hours. A platinum spiral was used as the isolated electrode which serves as a cathode in the titration of bases and an anode in the titration of acids. It is only necessary to reverse the polarity of the leads which connect the electrodes to the coulometer, to convert from acid to base titrations. The electrode in contact with the sample consists of a small piece of platinum gauze, shielded with glass tubing which guides the gas bubbles formed as reaction product to the surface of the solution instead of diffusing throughout the sample solution. The shielded electrode is placed close to the shaft of the glass stirrer to ensure maximum efficiency of mixing. Procedure. The procedure for each titration is simple, since it does not require the preparation of any standard reagent. When base is being titrated, the cation exchange membrane prevents OH- ion, the product of cathode reaction, from diffusing into the anode compartment. The current is largely carried by the inert electrolyte-e.g., sodium ion-migrating through the
L-ANION
B
Figure 2.
SOURCE
ton CR-61, and the anion exchange membrane, Nepton AR-IllA, were used to titrate base and acid, respectively (Ionics, Inc., 152 Sixth St., Cambridge, Mass.). "he cation membrane is a sulfonated polystyrene type; the anion membrane is a mixed strongly and weakly basic amine type. Both are reinforced in a Dyriel fabric matrix. Circular disks, 17 m n . in diameter, were cut out with a cork borer and mounted with a Teflon screw cap on a borosilicate glass tube as sh0n.n in Figure 2,B. The edge of the cap ITas sealed with paraffin wax to prevent leakage. A hole was cut a t the center of the screw cap to expose an area of about 0.5 sq. em., which is the only channel of contact betlieen the cathode and anode compartrents. The resistances of the cation and anion exchange membranes, when mounted separately, were 35 and 19 ohms, respectively. However, when the two disks were mounted together in the manner described above, the total resistance was measured as about 86 ohms, wet with distilled water. With a column head of 9 em., approrimately 0.8 ml.
WAX CAP
CATION EXCHANGE MEMBRANE
w::::T1 i.I-'"] Figure 1. titrator
TEFLON
H2POiH8PO4--! and POa-.
Indicator A, Current, Taken, Found, Std. mp ma. meq. meq. dev. used A. Titration of bases with electrolytically generated H + ion Phenol red 550 50 0.196 0.193 *o. 001 Phenol red 550 50 0.493 0.489 1 0 ,001 Phenol red 550 50 0.988 0.988 *o.ooo Mixeda 550 50 0.209 0.210 10,003 B. Titration of acids with electrolytically generated OH- ion Phenol red 550 20 0.200 0.206 *0.001 Phenol red 550 20 0.500 0.528 *0.003 Thymol 557 20 0,200 0.215 k o . 004 blue C. Successive titration of polyacids with electrolytically generated OH- ion Methyl red 550 20 0.209 0.219 10.001 Xlixeda
550
20
0.209
0.215
0.00
Rel. error, ?,: -1.5 -0.8 fO.0 +0.5
+3 +5 +7
+5 +2.8
Equal mixture of thyrnolphthalein and phenolphthalein (0.1yosolution).
VOL. 35, NO. 6, MAY 1963
619
Table 11.
Sample
NO 1
2 3 4 5
Coulometric Titration of Acid with Electrolytically Generated in 0.1 M Sodium Naphthalene-2-sulfonate
HC1
Indicator used Phenolred
Methylred
1 2 3
HCI
1 2 3
I
