ion Exchange Membranes in Coulometry STEPHEN W. FELDBERG and CLARK E. BRICKER Department of Chemistry, Princeton University, Princeton,
b Cation exchange membranes have been used to separate the anode and cathode compartments during the coulometric determination of bases in both aqueous and nonaqueous media. Even though these membranes are not 100% permselective, they are sufficiently selective in preventing the mixing of ions produced at the electrodes during the generation of the titrant to provide a distinct advantage.
S
cation exchange resins made in the form of membranes exhibit permselective properties and may be used as a replacement for salt bridges and sintered-glass disks in coulometric procedures. Separation of the anodic and cathodic oxidation reduction products is particularly difficult when working with highly mobile hydronium and hydroxyl ions in the coulometric titration of bases. The successful application of cation exchange membranes to this phase of coulometry is indicative of wide application to all other phases. YNTHETIC
PRINCIPLE
When a membrane is used in the titration of bases, a cation exchange membrane separates the anode and cathode half cells. The base to be titrated is placed in the anode cell. As these exchange membranes are permselective, the cation exchange membrane is impermeable to the hydroxyl ion. The only cations which can migrate to the cathode cell'are sodium ions (if sodium hydroxide is being titrated). It is, however, essential to have a large concentration of supporting electrolyte to: Maintain constant current. The internal resistance drop across the generating electrodes must not be dependent upon the amount of base present. Thus, the concentration of the supporting electrolyte should be a t least 100 times greater than the concentration of the base being titrated. Increase the fraction of the current carried by the supporting electrolyte. Because ion exchange membranes are not 100yo permselective, it is necessary that the largest possible fraction of the current be carried by inert electrolyte. An ion can cross a membrane by diffusion or electromigration. When a cation exchange membrane is used to 1852
ANALYTICAL CHEMISTRY
N. 1.
separate the generating electrodes, hydroxyl ions in the anode compartment will crosa the membrane only by d 8 u sion because these ions migrate toward the anode. However, hydroxyl ions generated in the cathode cell will tend t~ migrate and diffuse into the anode cell. APPARATUS
A cation exchange membrane, 0.024inch thick, CR-61A (Ionics, Inc., 152 Sixth St., Cambridge, Mass.) was used. The ion exchange resin in this membrane contains a sulfonic acid functional group attached to a polystyrene matrix. Circular disks, 13/ls inch in diameter, of the ionic membrane were mounted in a bottomless glass vial (Figure 1). A platinum generating electrode and the calomel reference electrode for the pH measurements were mounted inside this membrane cell. A constant current supply described by Reilley, Adams, and Furman (3)was used. In addition, a constant voltage transformer was placed in the alternating current line ahead of the constant current supply to minimize the fluctuations in the voltage. The generatin currents used were 20 ma. or less an! deviated less than =tO.Ol% during a titration. These currents were determined by measuring the internal resistance drop across a standard resistance. The circuit is shown schematically (Figure 2). The titration cell consisted of the bottom of an &ounce polyethylene
bottle fitted with a rubber stopper. The stopper was bored so that a glass electrode, the generating electrode, the membrane cell, and a nit,rogen input tube would form a nearly air-tight seal. An additional hole, cut in the rubber st,opper and fitted with a glass plug, allowed the addition of the substance to be determined without disassemblingthe cell. The detection circuit consisted of a glass electrode, a saturated calomel reference electrode, and a Leeds & Northrup pH meter, Model No. 7664. Because basic solutions were usually being titrated, it was essential t o carry out the titration in a carbon dioxide-frce atmosphere to obtain the most abrupt change in pH a t the equivalence point. All solutions were deaerated with nitrogen prior to P. titration and a cover of nitrogen was always maintained n w r the solution during a titration. Efficient stirring during n titlation is essential as any local increase of hydronium ion might allow this ion to migrate across the cation membrane. The mognetic stirrer used did not affect the pH readings. PROPERTIES OF I O N EXCHANGE MEMBRANE
The amount of d8usion of hydroxyl ions acrosa the cation exchange membrane was investigated by allowing various concentrations of sodium hydroxide in the membrane cell to stand for 11-hour periods in contact with a 0 V.
A
M B Figure 2.
C D
Figure 1. Membrane cell
cv. cc. RI. P.
SI.
A. Glass vial
R2.
8. Cap from Leibig condenser C. 0 rings D. Ion membrane
SZ. C.
Y. PH.
X.
Circuit diagram
Constant voltage transformer Constant current supply Standard resistor Potentiometer DPDT reversing switch Dummy resistor DPDT switch Electric clack Magnetic stirrer p H meter Titration cell, including membrane (dotted line), generating, indicating, and reference electrodes
neutral electrolyte in the titration cell. A negligible amount of hydroxyl ion diffused across the membrane if the concentration of hydroxyl ion in the membrane cell was less than 0.01N. The permselectivity of this membrclhe was estimated by placing 1N sodium chloride in the cathode chamber and 1.V sodium nitrate in the anode chamber and then passing a known number of coulombs through the system. The amount of chloride ion which crossed the membrane was determined with standard silver nitrate. The transference number of the chloride ion in the cation membrane was calculated to be about 0.03. The absence of 100% permselectivity only slightly diminishes the efficiency of an ion elchange membrane as used in this work. By placing sufficient supporting electrolyte in the membrane cell, and by replacing the electrolyte in this cell after each titration, the migration of hydroxyl ion into the titration cell can be minimized. At the beginning of a titration, the solution in the membrane cell is neutral. If Y . nieq. of hydroxyl ion are to be determined, then X/2 meq. is the average amount of hydroxyl ion in the membrane cell during a titration. Let Ti equal the volume of liquid and the niilliequivalents of supporting electrolyte in the membrane cell. The mobilities of the hydroxyl ion and the anion of the supporting electrolyte can be designated hoH and A, respectively. Assuming that the ratio of the mobilities of the anions is approximately the same in the membrane as it is in solution, the following equation applies : Fraction of current carried by
--
R
hydroxyl ion
=
AOH 2v 0.03 . m
Az
The ratio of the mobility of hydroxyl ion to the mobility of z (Clod-, NOZ-, SO;') is apDroximately 3. Assuming 0.2 meq. of hydroxyl ion to be titrated, rn equal to 15 meq. in a V of 15 ml., the fraction of current carried by the hydroxyl ion across the cation membrane is 0.0006 or 0.06%. This hydroxyl ion which migrates across the membrane results in a positive error due to thc introduction of extra hydroxyl ions into the titration cell. Although the per cent error increases with sample sizc, the lack of 100% permselectivity of the membrane will cause undetectable errors in the titration so long as samples of less than 0.5 meq. are being titrated. This calculation assumes that the inert electrolyte in the membrane cell is neutral a t the beginning of each titration and that its concentration is maintained a t 1N. In practice, the
concentration of the electrolyte in the membrane cell was varied between 0.25 and 1.5.11 and no detectable difference in tne accuracy of the titrations was noted. In view of the lack of 100% permselectivity of the ion exchange membrane, the choice of the electrolyte in the anode and cathode compartments becomes important. I t is advantageous to use a salt which is suitable for generating hydroxyl ions as well as hydronium ions to permit accurate backtitrations. A chloride medium is unsatisfactory because chlorine as well as hydronium ion and oxygen would be produced a t the anode and 'thereby give an unknown efficiency for the generation of hydronium ions. A sodium or potassium nitrate solution is probably satisfactory as the electrolyte in an anodic cell, but if there is ever a need to reverse the polarity for back-titrations, this medium is not suitable because nitrate ions can be reduced. The best electrolytes for use in both the anode and cathode compartments ale potassium sulfate, sodium sulfate, and sodium perchlorate. Because the last salt is more expensive than the sulfates, potassium sulfate was used for nearly all the study in aqueous solution. However, in nonaqueous medium, use of the more soluble sodium perchlorate is necessary to have sufficient electrolyte present. A solution of copper sulfate was tried in the cathode compartment to prevent the generation of hydroxyl ions which migrate through the membrane during a titration. Because copper ions which diffused through the cation exchange membrane were precipitated by the hydroxide ions from the titration cell, an increasing error in the hydroxyl determination was observed when a succession of determinations was run using the same cation exchange membrane. Consequently, the use of this electrolyte was discontinued. In contrast to coulometric cells employing a sintered-glass disk to separate the working electrodes, the effect of hydrostatic pressure and consequent water flow through the membrane was negligible. The water flow through a membrane due to hydrostatic pressure was less than 0.01 ml. per hour with a 24-inch head. PROCEDURE FOR PRECISION TITRATIONS O F HYDROXYL IONS
Add approximately 100 ml. of 0.25M potassium sulfate to the polyethylene beaker (anode compartment) and 15 ml. of the same solution to the cell contrtining the cation exchange membrane. Pass a stream of nitrogen through the stirred solution in the anode compart-
ment for 2 to 3 minutes, and then add a few drops of 0.1N sodium hydroxide. Generate hydrogen ions in the anode compartment until the pH of the solution is within 0.1 pH of the desired value, which is the pH a t the inflection point in the titration curve. This pH is 7.6 for sodium hydroxide. Add the sample to be analyzed and again generate hydrogen ions until the pH reaches the preset value. From the number of coulombs required, calculate the milliequivalents of hydroxyl ion in the sample. For best results, fresh potassium sulfate solution should be used for each determination to avoid building up carbonate in the electrolyte. RESULTS AND DISCUSSION
Titration of 14 samples of 0.1N sodium hydroxide by the above procedure with sample size varying between 0.1 and 0.6 ml. showed an average deviation of 3=0.16%. There was no bias toward high or low results. Interferences with determinations of hydroxyl ions are ions that are oxidized more easily than water, ions which precipitate in basic solution and, obviously, other basic ions such as carbonate, acetate, etc. The interference of some ions such as chloride that are oxidized in competition with the oxidation of water can be eliminated by the addition of hydroquinone to the anodic medium ( 1 ) . These interferences are not peculiar to this determination and are not the result of any inefficiency of the ion exchange membrane. The cation exchange membrane lasted indefinitely in aqueous medium. In fact, the same disk was used continuously until some experiment either clogged the membrane or dissolved the matrix. Titration of solutions of sodium carbonate in the recommended procedure gave an inflection in the titration curve for the bicarbonate end point but no apparent indication of the carbonic acid end poi:.t. This observation, in contrast to conventional acid-base titrations of carbonate, is caused by the rapid migration of hydrcgen ions through the membrane as the pH of the solution approaches the carbonic acid end point (pH 4). This limitation of cation exchange membranes precludes their use where the pH a t the end point of the titration is less than about 6. Only those bases with a Kb greater than lo-' can be titrated accurately by the recommended procedure. However, by using nonaqueous media, this limitation can be essentially eliminated. With the same apparatus and procedure as described, but using 0.5M sodium perchlorate in methanol for the anode compartment and saturated aqueous potassium nitrate for the cathode compartment, 12 aliquota of a V O L 31, NO. 1 1 , NOVEMBER 1959
1853
sodium carbonate solution varying between 0.006414 and 0.02566 mmole were analyzed with an average deviation of 7 parts per thousand. The molarity of the sodium carbonate solution, prepared by weight, was 0.03207 and the average value from these determinations was 0.03198. Similarly, titrations of sodium acetate were carried out in G-H solvent (2) (50% ethylene glycol and 50% isopropyl alcohol) containing 0.5M sodium perchlorate. The average error was 0.87,. In all titrations it is important to run a preliminary sample, about the same size as the samples to be titrated, to plot the titration curve and determine the pH of the end point and thus the pH to which the titration medium must be preset.
The cation exchange membranes appeared very stable in methanol but in G-H solvent they began to discolor after several days. An analogous attempt to titrate acids using anion exchange membranes (ARX-44 from Ionics, Inc.) was' not successful. This was due t o the presence of tertiary amine groups in what was supposedly a quaternary ammonium-type exchange resin. The tertiary amine groups were protonated by the acid and therefore results were always low. However, if a strong anion exchange resin membrane is used, there should be no difficulty in carrying out acid titrations by this technique. The success of these acid-base titrations using ion exchange membranes as an ion barrier between the half cells indicates that these membranes
should be very adaptable to other coulometric procedures. In contrast to the uee of sintered disks and salt bridges for separating half cells the problems of hydrostatic flow and nonpermselectivity are eliminated. Furthermore, the low electrical resistance of these membranes (about 70 ohms) should simplify maintaining constant current during a titration with less sophisticated current supplies. LITERATURE CITED
( 1 ) Hanselman, R. B.,Streuli, C. A., ANAL. CHEM.28, 916 (1956). (2) Palit, S. R., IND.ENQ.CHEY.,ANAL. ED. 18, 246 (1946). (3) Reilley, C. N., Adam, R. N., Furman. N. H., ANAL.C ~ M 24, . 1044 (1952). RECEIVED for review June 10, 1959. Accepted August 21, 1959.
Determination of Organic Nitro Compounds by Controlled- Pote ntia I Cou Io met ry JURGEN M. KRUSE Eastern Laboratory, E. 1. du Ponf de Nernours and Co., Gibbstown, N. 1.
b The application of controlled-potential coulometry to the determination of nitro compounds was studied. The electroreduction of the nitro compounds in a variety of organic and semiaqueous solvents indicated that a background current posed the chief obstacle to the determination of traces of nitro groups. Methods were developed to reduce and compensate for this background current, so that satisfac!ory measurement of as little as 20 p.p.m. of CI nitro body in an organic sample could be accomplished.
T
HE quantitative determination of small amounts of nitro impurities in organic compounds, and particularly of negatively substituted aromatic compounds. is often difficult, as most of the cv&ting methods for the quantitative analysis of organic nitro bodies require either the prior identification or the isolation of the suspected impurity. hmong the methods which have been iised are quantitative reduction ( I , 1. 10).diazotization after reduction ( 6 , l e ) , liolarography either in aqueous (a. 9 , 12 ) o r Iionaqutwus sol~cnts ( I O ) , and various c,olorimetricmet hods. Invcstigation of chemical reduction procedures for the drtermination of ric.gativel~-substituted aromatic nitro rompounds indicatccl that compounds of this typr are not rrduccd quantitatively
1854
ANALYTICAL CHEMISTRY
even by such reducing agents as zinc amalgam in acid, stannous chloride, tin-acid couples, or titanous chloride. Reduction methods therefore did not appear attractive for the measurement of an unknown impurity. Both polarographic and colorimetric methods require the isolation or identification of the unknown impurity and had to be ruled out. The use of coulometry, especially in a nonaqueous or semiaqueous system, circumvents the requirement of ing the specific nitro impurity and obviates the need for a standard. s u c h a method of coulometric reduction of nitro compounds was described recently ( I O ) and demonstrated the usefulness of coulometric reduction in nonaqueous systems. In the present work, which was carried out prior to the appearance
Figure 1 . 1. 2.
of this paper, the coulometric reduction of nitro compounds in some nonaqueous and semiaqueous systems was studied. Solubility considerations indicated that a nonaqueous system was necessary. The high electrical resistance and ditrerent nature of this type of solvent presented a number of problems which had to be studied in detail. APPARATUS AND REAGENTS
The cell design and coulometric apparatus used were essentially those described by Lingane (7). Line Voltage was controlled with a variable transformer and then rectified. The circuit design is depicted schematically in Figure1. suggested by ~ i a single~ compartment cell with a silver-silv(,r chloride reference electrode was used; this electrode was positioned as closely
Circuit for controlled-potential coulometry
Variac Rectifler 3. 100 ohm potentiometer, 10 watts 4. 500-ohm potentiometer A,. 0-1 0 n o . ammeter A?. 0-1 00 ma. ammeter
AB.
0-500 ma. ammeter
C. EC. V.
Coulometer Electrolysis cell 0-1.5 volt precision voltmeter Galvanometer Reference electrode
G. RE.
~
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