Completely Automatic, Coulometric Titration Apparatus for Process Use Determination of Sulfur Dioxide in Gases with Concentrations Ranging from 0.1 to 100% by Volume EMBRECHT BARENDRECHT and WILHELMUS MARTENS Cenfral laboratory, Staafsmijnen in limburg, Geleen, Netherlands
b
Sulfur dioxide in concentrations ranging from 0.1 to 100% by volume can b e determined automatically at least every 5 minutes by a coulometric titration procedure. After absorption of the sulfur dioxide quantitatively in a dilute solution of sodium hydroxide and addition of this solution to an acidic solution of bromide, the sulfur dioxide is titrated with electrolytically generated bromine, the end point being indicated b y a modified potentiometric procedure. The quantity of electricity needed, which is a linear function of the sulfur dioxide content in the gas, is recorded automatically with a standard deviaIn general, the tion of less than 1%. automatic titration apparatus, described here in detail, needs no calibration. Moreover, the method can b e applied to other titration procedures (determination of hydrogen sulfide, mercaptans, and the like).
I
N THE manufacture of sulfuric acid,
it is necessary to record the amount of sulfur dioxide present in the process gas during sulfur combustion. Sulfur trioxide, mostly present as a mist, however, interferes with almost every, otherwise appropriate, method for the determination of sulfur dioxide. In this respect a coulometric method was considered attractive. The principle of the method is as follo!vs: A knonn volume of the gas-e.g., 5 m1.-is passed through a dilute solution of sodium hydroxide, in which the sulfur dioxide is absorbed quantitatively. The resulting basic solution of sulfite is led into the coulometric cell, which contains a n acidic solution of bromide. The sulfite is oxidized by the bromine reagent, which is generated by anodic oxidation (with a constant current) of the bromide a t a platinum electrode. The end point of this titration reaction is indicated by a modified potentiometric procedure. The electricity neededLe., the product of the constant oxidation current and the titration timebeing a measure of the bromine generated, and thus of the content of sulfur 138
ANALYTICAL CHEMISTRY
dioxide in the gas, is recorded as a linear function of the titration time. The successive operations are controlled automatically by a process timer. In addition to selectivity. the coulometric procedure possesses several advantages over other methods: Depending on the value of the constant anodic oxidation current, it is possible to determine concentrations of sulfur dioxide ranging from 0.1 to 100% by volume. In a wide range the composition of the solutions is not critical for maintaining an over-all efficiency of 100% for both the absorption and the anodic oxidation procesws. Consequently, standardization of these solutions and calibration of the titration apparatus are not necessary. The method can readily be made automatic. In another method. published recently ( I ) , the sulfur dioxide is titrated a t the rate at which it is formed. As this is inopportune in our case, the electrical part could be simplified if combined with a modified construction of the coulometric cell, as presented here. THEORY
Titration Reaction. This reaction can be disturbed by: Loss OF SCLFURDIOXIDE. Consequently, the solution must be as dilute as possible. Other conditions for eliminating or reducing this factor are: low acidity, a small interface between the solution and the air, and, finally, a low temperature. OXIDATIOKOF SULFUR DIOXIDEBY ATMOSPHERIC OXTGES. Therefore. the dilute solution of sodium hydroxide and the acidic solution of bromide are made free from oxygen by passing nitrogen through it. Moreover. the pH of the solution is kept below 7 . Electrolytic Generation of Bromine. I n a n acidic solution of bromide the folloiving electrode reaction takes place a t the anode:
3 Br-
-2e
--L
Br,
Other electrode reactions-for
(1)
in-
stance, the anodic oxidation of watermust be avoided. The circumstances under which only Reaction 1 takes place can be derived from currentpotential curves (3) (Figure 1). I n recording these curves, the current density a t a rotating platinum anode was varied and the potential of this anode was measured us. that of a reference-type electrode, a silver-silver bromide electrode, Re (current-scanning technique) (Figure 2 ) . The potentials are measured 20 seconds after closing of switch 8,. This switch is opened immediately thereafter, to prevent a n undesirable increase of the bromine concentration in the cell. The measurements are carried out successively at increasing current densities. Small concentrations of bromine can be measured roughly with the amperometric system, In (Figure 4, In). The folloning parameters were varied : concentration of bromide, concentration of sulfuric acid, and number of rotations per minute of the anode. By rotating the anode at approximately 600 r.p.m. a fast homogenization of the generated reagent is achieved. Aloreover, the upper limit of the 1007, current efficiency range can be shifted to higher values of j (compare curves 2b and 2c, Figure 1). Correlation of the relevant currentpotential curves with the requirements to be satisfied by the titration and electrode reactions, summarized above, revealed that the experiment must be run under the following circumstances (TI ith a wide tolerance) : Limits of current density, j: 1 to 15 ma. per sq. cm. Concentration of bromide: 0.1i21, and concentration of sulfuric acid: 0.01M. The solution must he free of oxygen. Sumber of rotations per minute of anode: 600. Further increasing the speed of rotation does not noticeably improve either the upper limit of the current density or the homogenization of the generated reagent. The follon ing example indicates the quantity of electricity to be expected, the amount of current adjustment, and the titration time.
I 60
,.
I 5im'A
2b
!
*""T Rc
Ba:120
V.
l I.--.--
Einmv.vs AgAgBr
Figure 1. Current-potential curves of solutions of supporting and generator electrolyte at several concentrations and at distinct conditions at generator electrode
Figure 2.
Rotation o f anode or magnetic stirring of solution (Figure 2) 1 a. 0.01 M sulfuric acid, 600 r.p.m. 1 b. 1 M sulfuric acid, 600 r.p.m. 20. Like 1 a, but with 0.1 M potassium bromide 2b. Like 1 b, but with 0.1 M potassium bromide 2c. like 26, but with stationary electrode and magnetic stirring
A gas sample of 5 ml. with about 10% by volume of sulfur dioxide contains about 4 x 10-5 equivalent of sulfur dioxide. According to Faraday's law this amount requires roughly 4 coulombs for the generation of bromine (the exact value is 0.804 coulomb for 0.10 ml. of sulfur dioxide a t 20" C. and 760 mm. of mercury). Therefore, a titration time of, for instance, 100 seconds requires a generating current of ca. 40 ma. Indication of End Point of Titration. To keep the indication system simple (no transformation of the signal to one electrical in nature, as is the case n i t h a photometric end point detection method) and effective, use can be made of potentiometric or amperometric systems, which both have two indicator electrodes. The potentiometric system was preferred, however, because it is insensitive to stirring and the relative change in signal (potential difference betn-een the two indicator electrodes, with an impressed constant direct current of 10 pa.) is much larger. The two current-potential curves for the generator electrolyte (see Figure 3) rcpresent the situation qualitatively before (1) and after (2) the titration end point. A very small excess of bromine causes the potential difference between the two electrodes to drop suddenly from roughly 500 to 700 mv. to a few niillivolts: AE, to 0 2 (free bromine depolarizes the cathode). The indicator electrodes are constructed so that one platinum gauze electrode (see Figure 4, I n ) shields the other electrode. The influence of the field of the generating current on the potential
Apparatus for recording current-potential curves
A. Anode compartment (titration cell) An. Spiralized anode of platinum wire (diameter 1 mm.) area 10 sq. cm.; Hollow shaft (see E) i s filled with mercury for electrical contact and provided with stirring vanes E . Precision glass bearing (Jenaer Glaswerk, Schott & Gen., Mainz, West Germany: K.P.G. Lagerhulse No. 5083 with hollow Ruhrerwelle NO. 5090) lubricated with glycerol C. Cathode compartment Ca. Cathode, as An, positioned neor diaphragm E. Electronic mv.-meter for measuring potential of anode An vs. reference electrode Re In. Platinum indicator electrodes. Cylindrical gauze cathode (diameter and height 5 mm.) shields spiralized anode (diameter of 20-mm.-long w i r e
0.5 mm.)
M. Rc.
Soft iron rod, enclosed in glass (magnetic stirring) Variable control resistances in series: 100, 1000, 10,000, and 50,000 ohms to control current density a t anode, An, from 0.2 to 200 ma. per sq. cm. R1. Wire-wound limiting resistance, 50 ohms Re. Silver-silver bromide reference electrode, made b y anodic treatment of 2 sq. cm. silver-plated platinum spiral in solution of bromide (generator electrolyte) S,. On-off switch for measuring current S,. Two-position switch V. Stopcock, in closed position. Nitrogen forced throwgh solution via g W. Westan cell, can be enclosed with SI in potential measuring circuit, if potential difference exceeds span o f meter scale of E e. Escape for evolved hydrogen Inlet for nitrogen to deoxygeniza solution g. h. Inlet for nitrogen
difference can be neglected. The outer gauze electrode is polarized negatively to favor a fast indication (reduction of an excess of bromine), APPARATUS
Design of Coulometric Titrator. The main operations in the determination of sulfur dioxide-sampling, ti-
AE2
'czth.
Figure 3. Qualitative explanation of potentiometric indication system Current-potential curves of cell solution without ( 1 1 and with (2) free bromine
tration, and recording-are monitored automatically by a process timer, PI' (Figure 6). This timer permits the operation of the starting snitches, SZ and Ss> and the three Martonair valves, MV1. M V z , and MV,, which in turn monitor the pneumatically operating valves, u. 1-8. These valves are readily obtainable and are suitable for controlling both liquid and gas streams. Sampling. The pneumatically operated gas-sample delivering system (9) (SS in Figure 5, v. 7 in Figure 6) has a sample loop of 5 ml. (tubell). During sampling, the gas to be analyzed streams through this loop to the atmosphere ("exit"). I n a following stage, the content of this loop (the sample) is isolated from the gas to be analyzed and at the same time passed through the absorption vessel, D, with the aid of nitrogen a t a flow rate of 6 liters per hour. I n the meantime the sample gas stream is conducted via I to the atmosphere. The flow rate of this sample stream is 6 liters per hour. I n a following step the alkaline solution of VOL. 34, NO. 1, JANUARY 1962
139
’+
C
Figure 4. Coulometric cell with details
FY
Symbols same as in Figure 2 a. Opening for drain to v.3 b. Supply from v.5 c. Supply from v.8 d. Vent via nitrogen atmosphere f. Cylindrical piece of polyethylene preventing escape of sulfur dioxide
Figure 5. apparatus sulfite is passed quantitatively to the coulometric titration cell. Coulometric Titration Cell. This cell (Figure 4) consists of two partsthe anode compartment, A , capacity about 175 ml., with the rotating anode, An, and the cathode compartment, C (with the cathode Ca)separated from the former by a sintered-glass diaphragm. This diaphragm is necessary to prevent redox cyclization-Le., reduction of generated bromine a t the cathode, V.V. (decrease of current efficiency). I n the design according to Figure 4, mixing is fast (eliminating the need of anticipating the titration end point, the more so as the titration is tandem in character) and losses of sulfur dioxide are prevented as much as possible (cylindrical piece of polyethylene, f,nitrogen atmosphere via d), The agent (basic solution of sulfite) and generator electrolyte (acidic solution of bromide) can be introduced by valve v. 8 (via c) and valve u. 5 (via b ) , respectively. After each titration the solution is partially discharged by valve u . 3 (via a). Besides being shielded, the potentiometric indication system, In, is fixed diametrically with regard to the cathode, so that the influence of the field of the generating current can be neglected. Both compartments contain the same solution, the exchange of li uid during charging and discharging %eing kept as small as possible, because of capillary e. The cell resistance amounts to roughly 400 ohms. Electrolyte. The Mariotte flask, M I (Figure 5 ) , with a capacity of 10 liters, contains a n oxygen-free solution of about 0.005N sodium hydroxide and flask Mz,capacity 5 liters, contains a n ox gen free solution of about 0 . 0 3 d sulfuric acid and 0.3M potassium bromide. Con: sequently, the generator electrolyte in 140
ANALYTICAL CHEMISTRY
the coulometric cell has roughly the required composition: 0.01M sulfuric acid and 0.1M potassium bromide. The capacity of both flasks is such that they must be replenished only once a week, if the total duration of one titration cycle amounts to 10 minutes. Constant Current Supply and Recording of Titration Time. To perform a coulometric titration, a constant current supply is necessary. The type chosen here is a simple constant-voltage transformer-rectifier, supplying 200 volts d.c. (TR, Figure 6 ) . The product (iQ)t=c1 X tl is recorded rather than the titration time, tl, so that some drift, spread over a much longer time than tl ( t l roughly 100 seconds) does not seriously impair the accuracy, the more so as the resistance of the whole electrolysis circuit (variable with R2)is about 10 times greater than the slightly variable cell resistance, Now, during the titration time the current, iQ, is fairly constant. The feed current is free from earth to avoid electrical connection with the indicator system. At the end point of the titration the potential difference over the indicator electrodes drops under, say, 25 mv. and the generating current is interrupted by the combined action of the indicator system, In, the amplifier, AMP, for the indicator signal, and relay RL1, as a result of which Sl is opened (Figure 6). The 10-pa. d.c. feed current for the indicator system is derived from a d.c. source, available in AMP, together with a high resistance. One of the indicator electrodes is on earth potential. The recorder unit consists of a reversible synchronous motor, Mr, a Helipot, RI with a shunt resistance, R3, or adaptation to the recorder, REC, and a 5-mv.
Scheme
of
titration
D.
Oblong absorption tube (vertical path of gas bubbles about 12 cm.) with inlet tube and conical piece of sintered glass to form small gas bubbles F. Dip tube, fflled with paraffin oil (depth 5 cm.), causing overpressure if valve 6 is closed, to force all liquid from D to cell if valve 8 is open MI, Mz. Mariotte flasks, containing 10 liters of absorption liquid and 5 liters of generator electrolyte, respectively PI, Pz. Automatic pipets (communicationtype), 10 and 5 ml., respectively SS. Pneumatically operated gas-sample delivering system [Shell, Amsterdam ( 2 ) ] , produced in 18-8-2 steel alloy, capacity of sample loop I! about 5 ml., working overpressure of soft P.V.C. membranes 1.8 atm. Paths I and II each alternately (also in time) occupied by sample gas or carrier gas (nitrogen) in. Generating current. Eight numbered valves are pneumatically operated by Mortonoir valvesMV(Figure 6 )
potentiometer, provided with an automatically operating calibration device. The titration starts when relay RL2 is activated. The motor turns sliding contact S1 clockwise and a t constant speed, thus causing a potential difference across R1, which is a linear function in time. When the end point of the titration is reached, Sl is opened (deactivation of RLl) and consequently the generating current is switched off by S4 (deactivation of RL2). As soon as relay RL2 is deactivated, 81 returns. Its motion is stopped by the limit switch, Ss. Only RL2 can be deactivated if RL1 is deactivated. The input signal of R d once having dropped below 25 mv., the titration cannot be continued for the rest of the cycle. A more intricate electrical circuit, including anticipation, is not necessary, because the titration is tandem in character and mixing of the generated bromine occurs very fast, thanks to the rotation of the anode.
OPERATIONAL PROCEDURES
The sequence of the operations, together with the electrical diagram, is given in Figure 6. The total duration of one cycle is adjusted a t 10 minutes. When necessary, this time can be reduced to 5 minutes. To avoid a time lag, the apparatus is placed near the process stream. The cycle starts by opening valves 1, 2, and 3 for 1 minute. The (communication-type) pipets, PI and Pz, with a capacity of 10 and 5 ml., respectively, are now filled with the absorption and the generator solution, respectively. During the first minute, 15 ml. of generator electrolyte are discharged from the cell via valve 3, so that the contents of the cell are refreshed continuously. I n this stage the process is operated by MVI. After 1 minute, the first three valves are closed and valves 4 and 5 are opened for 2 minutes to fill the absorption tube, D, and the cell, respectively. At the same time valve 6 is opened. Pipet PI can now be discharged, because the nitrogen in D is now a t atmospheric pressure. Meanwhile the gas sample stream passes through the sample tube, I1 of SS, valve u. 7. Yowl the process is operated by MV*. From the third to the fifth minute, the contents of sample tube I I are led into vessel D , as described before. Oxygen, also present in the gas sample, does not oxidize the sulfite, in any case not to a remarkable extent, because oxygen does not dissolve noticeably in the alkaline solution. On the conma.-m@ter
trary, the alkaline absorption solution should be free from oxygen, because, otherwise, the oxidation reaction would proceed homogeneously and rapidly. From minute 5 to 5.5, valve 8 is opened by MV8 and the sample arrives at the cell as a basic solution of sodium sulfite. The nitrogen overpressure of 5 cm. of paraffin oil (see F; valve 6 is now closed) prevents liquid in D from staying behind. As the indicator potential difference, AE, is increased to some hundreds of millivolts, and so passes beyond 20 mv., RLl is activated, and thus SI is closed. I n this short time (half a minute) the loss of sulfur dioxide from the cell (the liquid is acidic) may be neglected. The titration starts a t 5.5 minutes (when Sa is closed). Bromine is now generated a t a constant rate by anodic oxidation of bromide. The starting procedure is as follows: Switch SZis closed until 5.7 minutes. When sulfite enters the cell, SIis closed as described before. However, RL2 can be activated only if 5's is closed. At this moment the titration starts and the sliding content, SI, moves on. The essential point is that RLz can be deactivated only when the titration is completed (switch S? off), and be reactivated only 5.5 minutes after the start of the following cycle (during this time Ss is closed). Otherwise, recording would become ambiguous. The value now indicated on the recorder is proportional to (&)'- t, x tl, where tl is the titration time in seconds. This value must be the same as i, x t , = Q, m-here i, is the mean generating cur-
r8"8
@ @
rent in amperes and Q is the total amount of coulombs consumed. It is evident that i, ought to be constant during one titration; however, its value may differ from one titration to another. Assuming the apparatus to be in good condition-i.e., no leakages nor defective electric contacts-calibration of the apparatus is not necessnry, because both the absorption and the current efficiency are loo%, and the potentiometer-recorder is provided with an automatically operating calibration device for the 5-mv. span. The quantity of electricity is proportional to the measured potential difference. PERFORMANCE
Essential for a good performance of the apparatus is the total reproducibility. The reproducibility was determined on three series of ten successive measurements on the same sample and under the same conditions. The standard deviations amounted to 0.7, 0.6, and 0.8%, respectively. For the random deviations, one or more of causes 3, 4, and 5, mentioned below, were responsible. The systematic variations were mainly due to causes 1, 2, 3, and 5. 1. Temperature. An alteration in temperature of roughly +3" C. caused a scale deviation of -1%. Consequently, the temperature of the gas sample had to be kept constant-e.g., a t 20' C. 2. Pressure. The effect of deviations in atmospheric pressure can in most cases be eliminated in a simple and rapid way by some correcting device. 3. Mixing Delay during Titration. The titration error, caused by a mixing delay during titration, would be positive for one separate titration. However, the measurements are carried out successively, and consequently the overlap, if constant, is eliminated. When the indicating system, In (Figure 4), was used as a bi-ampero-
4 Figure 6. Electrical diagram and scheme of sequence operations during one cycle AMP
rr4,5,6,7 v.8 5 2 s 3 A € in rnv. i inma
~-
... . . p r.... r L . P - c 0 , 5 . P-rceil,B.press. inD& .. 2
...,
.
,v.8.d+ceii.
.
simple inn.
s2. (out : 5 . 7 ' ) . S %titration starts ( 5.5').
-
!,
-1
or w,tch valve open
-
electr.connection. =rnech.connection.
of
+
RLI. Adapted Brown amplifier (Service Replacement Amplifier Assembly 357504-1 1, containing 1 0-pa. d.c. feeder for indicating system. Output of amplifier can actuate relay RLI and consequently switch SI Mr. Reversible two-phase motor for driving sliding contact SIof Helipot R1 MV. Electromagnetically operated four-way pneumatic command valves (Type S 5 5 6 / 2 2 , Martonair, Ltd., Parkshot, Richmond, Surrey, England), overpressure 1.8 o m . PT. Process timer (Type SX, A.G. Burrell & Co., Ltd., Sheffield, England) with 1 2 adjustable switches RI. He!ipotentiometer, 3 turns, 50 ohms Rz. 10,000 ohms, for adjustment of generating current io R3. About 0.1 2 5 ohm, reducing input voltage of REC REC. 5-mv. potentiometer-recorder RL2. Relay with switches SI,SZ,St, and Sl Contact switches, positioned a t ends of Helipot R1 Sg, Sg. SI. Switch, in "open" position, milliammeter in function TR. Transformer-rectifier, free from earth to avoid electrical connection with indicator system and supplying 200-volt d.c. Other symbols same as in preceding figurer VOL 34, NO. 1, JANUARY 1 9 6 2
141
Table I. Reproducibility of Results Obtained on Same Sample with Manual lodometric Procedure A and Apparatus B
(Per cent by volume sulfur dioxide) An a1y si5 Procedure A B 7.47 . .-. 7.41 7.45 7.48 7.42
7.46 . . _. 7.44 7.38 7.40 7.43 7.42 7.46 7.48 7.56 7.49
Mean values are equal for both series: % SO*. Relative standard deviation is 0.4 and 0.7 for series A and B, respectively. 7.45 vol.
metric system (potential difference between these electrodes 25 mv.), this overlap error was found to be about 3% a t i, = 50 ma. and 600 r.p.m. of the generator anode. The standard deviations of the successive overlaps are about 10%. Therefore, the ultimate
standard deviation due to the niising delay amounts to 0.3%. After each titration one twelfth of the contents of the cell is discharged via valve 3 , thus causing a systematic error of +0.25%, which can be corrected. 4. Generating Current, i,. This current should be constant during one titration, because the recorded scale indication is proportional to (i,)t-zl X tl = (i,)L-flX Q/io. The standard deviation of the quotients of (i,) l F I i , and i, is the same as those of the calculated Q value-i.e., (i,It+ x tl-and the recorded scale value-i.e., 0.3%. This was determined on three series of ten successive measurements on the same sample and under the same conditions. 5. Losses of Sulfur Dioxide. These losses do not exceed 1%. They are very constant for a given apparatus and can be expected mainly during the absorption and titration in the cell. The recorded scale values have been corrected for these losses and for the systematic discharge error mentioned under 3, by comparing them with the values obtained by the nellk n o m manual iodometric procedure (see also Table I).
RESULTS
The recorded scale values of the apparatus, compared with the values obtained by the manual iodometric procedure, are compiled in Table I. The same precision data hold for SOz percentages (by volume) ranging from 2 to 20. ACKNOWLEDGMENT
The authors are grateful to C. Bokhoven for his stimulating interest. Thanks are also due to J. A. L. Thissen and P. -19. Schmitz for their helpful suggestions in planning the electrical design and constructing the instrument. LITERATURE CITED
(1) Glass, J. R., More, E. J., ANAL. CHEV.32, 1265 (1960). (2) Hooimeyer, J., Kwantes, A,, Craats,
F. van de, “Second Symposium on
Gas
Chromatography,
Amsterdam,
1958,” p. 288, London, 1958. ( 3 ) Lingane, J. J., “Electroanalytical
Chemistry,” 2nd ed., Interscience, New York, 1958.
RECEIVEDfor review May 8, 1961. Accepted September 8, 1961.
A Potentiostat for Amperometric Kinetic Studies JOHN M. MATSEN’ and HENRY
B.
LINFORD
Department of Chemical Engineering, Columbia University, New York 27, N. Y.
b It is possible to study the kinetics of a chemical reaction in which an ion M+* is reduced to M+ b y anodically reoxidizing Mf a t such a rate as to keep the redox potential of the solution constant. The current consumed in the electrolysis is then directly proportional to the reaction rate. Advantages of this technique are discussed. An electronic instrument is described which automatically produces electrolysis current to maintain constant potential in the solution. It has been used in a kinetic study of the oxidation of coal b y Fe+3.
A
and coulometric techniques have proved to be of considerable value in analytical chemistry ( 5 ) because of their accuracy without need for standardization, adaptability to small concentrations, specificity, and ease of automation. Somewhat similar techniques may be applied to kinetic studies of certain reactions. Several authors (1, 2, 4, 6) have measured the kinetics of hydrolysis reactions by electrically generating OH- ions 142
MPEROMETRIC
ANALYTICAL CHEMISTRY
a t such a rate as to keep p H constant. With the assumption that the desired cathode reaction proceeds with 100% current efficiency, the OH--generating current is a direct measure of the rate of hydrolysis. Comparable techniques have now been applied to kinetic studies of reactions involving redox couples. If an ion AI+2 is being reduced by compound R according to the equation 2h1+’ R HzO -C 21\11 R 0 2H- (1)
+ +
+
+
the ratio LI.+2/M+ and hence the potential of the solution will tend to decrease. The If+’ion may be anodically regenerated, hon-ever, 2M+ -+ 2M+*
+ 2.9-
(2)
and if this is done in such a way as t o keep the solution potential constant, then the current passed in Reaction 2 will be directly proportional to the rate of Reaction 1. The cathode reaction corresponding to Reaction 2 is 2e-
+ 2H20
+
Hz
+ 20H-
(3)
The cathode must be separated from
the anode by a semipermeable menibrane to prevent cathodic reduction of M+’. With an anion exchange membrane and with only OH- anions in the cathode compartment, all current between anode and cathode compartments will be carried by OH- ions which m-ill neutralize the H + ions from Reaction 1. Thus the only changes in composition of the reaction mixture will be in concentration of R and R.O. R and R.0 must not react appreciably a t the 15-orking electrode, or t h e assumption of 100% current efficiency will be invalid. Such a condition was apparently encountered by Einsel, Trurnit, Silver, and Steiner ( I ) , who noted but could not explain the fact that with a large electrode (an OH--generating cathode in their case) the amount of current used was appreciably greater than the amount of acid neutralized. It seems likely that they were reducing some of the organic reactants or products of their hydrolysis reaction, such 1 Present address, Central Basic Research Laboratory, Esso Research and Engineering Co., Linden, X. J.
