628
ANALYTICAL CHEMISTRY
From these results, it was concluded that the errors were due to one or both of the following side reactions, occurring a t the silver indicator electrodes: Ag
+
Anode I- = AgI
+ e-
02
+
Cathode 2H+ 2e- = H? 4H+ 4e- = 2Ha0
++
(1)
(2)
In an acetate buffer, with access to air, a t a p H of 5.5 (it was calculated that Reaction 1 would not proceed to a significant extent a t this pH) satisfactory titrations were performed. It seems probable that the high indicator currents prior to the end points are due primarily to the side reaction involving reduction of hydrogen ion (Reaction 1). The applied potential increases the driving force behind this reaction, and increases its rate, with a given electrode area. Because of the buffer the hydrogen ion concentration is essentially constant throughout the titration. The over-all potential in favor of Reaction 1 does not change appreciably until the iodide concentration decreases rapidly near the equivalence point. The behavior of the indicator current is consistent with the proposed reactions. Reaction 2 appears to be less probable because the following electrode reactions could take place in the absence of iodide: Anode Ag = Ag+
+ e-
0 2
+
Table I.
Cathode 4H+ 4e- = H20
+
If such were the case, it would not be possible to obtain the observed, stable indicator currents of only a few microamperes in the presence of oxygen. Alternative Procedure. It was also possible to run titrations of iodide in strongly acidic solution (0.02F in sulfuric acid), if the side reaction was not allowed to proceed to a significant extent, Preliminary work showed that the use of small indicator anodes, or small indicator potentials, prior to the end point reduced the amount of side reaction. ,4 procedure was developed in which a very small indicator anode, consisting of a 1 X 1 mm. silver wire, A , was used prior to the end point. The indicator cathode consisted of a 5 X 5 mm. electrode, B , and a 1.5 X 2.0 cm. electrode, C, both silver foil, connected together. .I small indicator potential (98 mv.) was applied, which provided a small
a
b
Coulometric Titration of Iodide in Acetate Buffer., *DH 5.5
Titration Iodide, XlgNo. of Rate, Taken Found Titrations pa (r, E n o r 0.2828 0.2811 7 1 -0.60 0.7786 0,7789 3 10 +o 04 1.948 . 1 .9505 10 10 t0.10 3.896 3.897 4 10 +0.03 7.792 7.796 3 10 +0.05 7.792 7.800 2 25 +0.10 3 50 7.792 7.800 +0.10 15.58 15.58s 8 50 0.0 22.64 22.66 4 50 +0.09 1.950,1.948,1.949, 1.948.1.949.1.950,1.948,1.950,1.946,1.946, 15.56,15.59.15.61, 15.58,15.57,15.57,15.58,15.57.
indicator current (10 to 15 pa.). A decrease in this current indicated the approach of the end point. After the end point, B was indicator anode, C was indicator cathode, A was disconnected, and the indicator potential was 230 mv. Using this procedure, samples of about 3 mg. were titrated with an average error of about 0.1%. CONFIRMATORY TITRATIONS
Table I contains data obtained from confirmatory titrations carried out in acetate buffer solutions. Equally satisfactory titrations can be performed by the alternative procedure, but the manipulations are more complicated. LITERATURE CITED
Kolthoff, I. M., and Lingane, J. J., “Polarography,” 2nd ed., p. 887, New York and London, Interscience Publishers, 1952. Lingene, J. J., and Small, L. A , , ANAL.CHEM.,21, 1119 (1949). MacNevin, W. M., Baker, B. B., and McIver, R. D., Ibid., 25, 274 (1953).
Meier,‘D. J:, Myers, R. J . , and Swift, E. H., J . Am. Chem. Soc., 71, 2340 (1949). Nakanishi, M., and Kobayashi, H., BuU. Chem. SOC.Japan, 2; 394 (1953).
.
Ramses. W. J.. Fafrington. P. S., and Swift, E. H., ANAL.CHEM 22, 332 (1950).
Szebelledy, L., and Somogyi, Z., 2.anal. Chem., 112, 313 (1938). Wooster, W. S., Farrington, P. S., and Swift, E. H., A x ~ L . CHEW.,21,1457 (1949). RECEIVEDf o r review Xovember 2, 1953. Accepted February 2. 1984
Coulometric Titrations with Externally Generated Chlorine, Bromine, and Iodine JAMES N. PITTS, JR., DONALD D. DEFORD, THOMAS W. MARTIN, and EDWIN A. SCHMALL’ Department of Chemistry, Northwestern University, Evanston,
Although chlorine, bromine, and iodine have been prepared by internal generation methods, only iodine has heretofore been prepared by external generation. The work reported in this paper was undertaken to investigate the possibilities of preparing all three of these halogens by external generation. A new type of single-arm cell has been designed for this purpose, and successful generaiion of all three halogens at currents of 200 ma. or higher has been accomplished. The cell described should also prove useful for the generation of other reagents. High precisions and accuracies have been realized in the titration of arsenite samples with externally generated halogens.
T
H E general technique of performing coulometric titrations with externally generated reagents has been described in a previous communication (8) from this laboratory. This communication reported the successful generation of hydrogen ion, hydroxyl ion, and iodine a t currents of 250 ma. in a two-arm
111.
electrolysis cell in which the anode and cathode products were completely separated from each other.’ These are the only reagents prepared by external generation which have been reported to date. On the other hand, many titrations with internally generated halogens have been reported. Szebelledy and Somogyi (f 7), in their classical papers on coulometric titrations, employed electrolytically generated bromine for the titration of thiocyanate, hydrazine, hydroxylamine, sulfite, bisulfite, and hydroxide on a macro scale. Swift and his coworkers (2-4, IO, 15, $0)have employed electrolytically generated bromine for the titration of arsenic, antimony, iodide, thallium, aniline, and thiodiglycol. Shaffer, Briglio, and Brockman (16)developed an instrument employing electrolytically generated bromine for the continuous automatic titration of mustard gas in air. Austin, Turner, and Persy ( 1 ) have described an instrument employing electrolytically generated bromine for the continuous automatic determina1 Present address, School of Chemistry, University of Minnesota, Minneapolis, Minn.
V O L U M E 2 6 , NO. 4, A P R I L 1 9 5 4
\
+RUBBER
629
STOPPER
M
GENERATOR ELECTROLYTE
I
nn
SI
,T
14/20 JOINT
.I mm. CAPILLARY STOPCOCK BODY FRITTED GLASS DISK PLATINUM QAUZE DISKS
PLATINUM FOIL
7 12/30 X)INT
I
TO SAMPLE
Figure 1. Generator Cell tiori ~f sulfur compounds in gases. Carson ( 5 , 6 ) determined 8quinolinol and uranium by bromometric titration. Swift and his coworkers determined iodide, thallium, and arsenic with electrolytically generated chlorine (3, 9, 20) and arsenic and thiosulfate with electrolytically generated iodine (1l , 14). Wise, Gilles, and Reynolds (19) employed electrolyticalIy generated iodine for automatic titrations of arsenic. Tutundzic and hlladenovic (18) have described the titration of thiosulfate with electrolytically generated iodine and recommend this method for the standardization of thiosulfate solutions. This paper describes the results of studies on the external generation of chlorine, bromine, and iodine a t currents of 200 ma. or more. A general purpose single-arm cell for the generation of these reagents is described; this cell offers several didtinct advantages over the two-arm type for the production of halogens. APPARATUS
Generator Cell. A cross-sectional view of the general-purpose single-arm cell which was employed in these titrations is shown in Figure 1. I t is constructed from a 1-mm. capillary stopcock body appropriately fitted with two standard-taper joints. The cathode compartment consists of a standard-taper 14/20 joint sealed to a 24-em. length of 10-mm. borosilicate glass tubing. The bottom of this joint is ground flat and sealed with -4piezon wax to a 40- to 60-mesh fritted-glass disk about 1 mm. thick. The fritted disk together with the rubber stopper which seals the top opening of the cathode chamber prevent diffusion of the anodically generated halogens into the catholyte. The disk acts as a n effective hysical barrier, without significantly increasing cell resistance, w&le the stopper causes a downward displacement of catholyte by trapping the hydrogen evolved from the cathode. The volume of the cathode chamber is sufficiently large so that more than 1 meq. of halogen can be generated without refilling. The cathode consists of a 20-cm. length of 24-gage (B-S) platinum wire wound in a helix about 5 cm. long and sealed through the tube with Apiezon wax. The construction of the anode section is especially important. The top of a standard-taper 12/30 joint is ground so that, on insertion into the stopcock body, there is a gap slightly greater than 2 mm. between the top of the joint and the fritted disk. This gap comprises the anode compartment and is symmetrically spaced just above and below the capillary outlets. In constructing the anode itself, one platinum foil and six platinum gauze disks are cut to fit the diameter of the anode compartment. One of the gauze disks is then laid on top of the platinum foil and the
two are welded together by first heating them to red heat in a gas flame and then striking lightly with a hammer. Each of the remaining gauze disks is successively flame welded in this way to the top of the preceding disk. The bottom of the foil is then welded to a platinum lead wire. The anode foil is 0.003 inch thick, the gauze disks are approximately 50 mesh and are made of 33-gauge (B-S) wire, and the lead wire is 24 gage (B-S). This anode unit, which fits snugly in the anode chamber, is carefully sealed t o the top of the standard-taper 12/30 joint with Apiezon wax. The over-all anode design features good electrical contact throughout, maximum anode surface, minimum holdup volume, and efficient anode flushing a t low flow rates. The total holdup volume, including the delivery arm, is only about 0.3 ml. In assembling the cell, all joint surfaces are lubricated with heavy Celvacene Hi-Vac grease (Distillation Products Co.). Extreme care must be taken to avoid getting excess grease on any part of the anode surface. This is especially critical when generating chlorine a t currents above 100 ma. A thin layer of glass wool is placed directly on top of the anode and is tucked lightly around its sidrs in order to obtain more uniform flow through the cell. Power Supplies. A line-operated, constant-current ower supply, similar to the instrument described by Reilley, &oke, and Furman ( l a ) , was employed for all titrations in which the current did not exceed 205 ma. The current was checked a t frequent intervals with a built-in potentiometer. Generation currents in the range from 205 t o 630 ma. were obtained from an unregulated high voltage supply; these currents were continuously monitored during titrations by potentiometric measurement of the I R drop across a standard 1.0000-ohm resistor. The current limiting resistors of the source were manually adjusted as necessary in order to maintain a constant current. Titration Assembly. The sample to be titrated was placed in a 250-ml. beaker and the indicator electrodes and the delivery tip of the generator cell were immersed in the solution. Vigorous stirring was provided by a magnetic stirrer. The generator electrolyte was supplied to the cell by hydrostatic pressure from a separatory funnel placed about 3 feet above the cell. The funnel was connected through a Tygon sleeve to a stopcock which was used to control the flow of electrolyte. This stopcock was in turn ronnected by Tygon tubing to the influent capillary arm of the cell. The cell was mounted with a 3-finger clamp. Prior to a titration, the cathode compartment of the cell was filled by removing the rubber stopper and placing a finger over the delivery tip. When the compartment was almost full, the stopper, moistened to ensure a tight fit, was again replaced and the delivery arm was washed with water. In all cases the flow rate of generator electrolyte through the cell was maintained a t a value not less than 0.1 ml. per second (about 1 drop per second). The flow rate was always adjusted before submerging the delivery tip. End-Point Detection System. Each titration was followed potentiometrically with a platinum-saturated calomel electrode system and a Beckman Model G pH meter. REAGENTS
All chemicals used were reagent grade. The arsenite samples which were employed were aliquots of standard solutions prepared in the conventional way from weighed quantities of arsenic trioxide. The generator electrolyte which was employed for the generation of chlorine a t currents from 200 to 410 ma. was 2M hydrochloric acid. At lower generation currents 1.OM hydrochloric acid was satisfactory. For the generation of bromine the electrolyte was a solution 0.2M in potassium bromide and 1,011 sulfuric acid. For the generation of iodine a solution 0.3M in potassium iodide, 0.1M in boric acid, and 0.5211 in sodium sulfate was employed. EXPERIMENTAL PROCEDURE
.
When any one of the generator electrolytes is subjected to electrolysis, water is reduced a t the cathode to yield hydrogen gas and hydrouyl ion. The acid present in each of the electrolytes neutralizes the base formed and prevents the effluent solution from becoming basic. In addition to the desired oxidation of the halide ions to halogen a t the anode, it is possible also that water may be oxidized. In order to determine the current efficiency for generation of the halogens, known quantities of arsenite were titrated with the halogen generated a t the anode. Since arsenite is not readily oxidized by oxygen except in strongly basic media, the current efficiency for the generation of halogen could be
readily calculated from the coulombs of electricity required to titrate the sample. I n each case the titration was allowed to proceed without interruption to within a few seconds of the expected end point. Thereafter the generation was carried out in increments of 0.2 to 0.3 second, with sufficient time between increments to permit the indicating electrodes to reach equilibrium. I n order to achieve greater precision in the location of the end point, the final incremental portions of the titrations employing currents of 410 and 630 ma. were usually carried out with a current of 204.0 ma. The potential break a t the end point was sufficiently pronounced in all cases to permit visual determination of the end point without resort to plotted titration curves. I n all titrations with iodine the sample solu-
I
I
I
I
I
I
I
I
I
1000-
800
-
-
-
BM)-
a, B?,
r
/-
-
~
J
-
0'2
-zoo0
I
60
I
100
I 110
I
200
I ex,
I
300
I
W
I
I
400
410
s60
V O L U M E 26, NO. 4, A P R I L 1 9 5 4 the titration of other samples has so far met with little success. The reaction between chlorine and oxalate is very slow and is unsuitable as the basis of a titrimetric determination. Because of the volatility of chlorine, it is not possible to heat the sample to increase the velocity of the reaction. Titrations of iodide t o the iodine monochloride end point have likewise been unsuccessful. Iodine was lost by volatilization from the open beaker employed for the titration and results were always low. It might be possible to perform this titration satisfactorily in a closed system. The reaction between ferrous iron and chlorine is too slow t o permit a successful chlorimetric determination of iron without a catalyst. Thus i t appears that chlorine will find only limited applicability as an oxidimetric reagent because of the generally slow rate of reaction with reducing agents in acid solution. However. since chlorine is a reasonably Qtrongoxidizing agent and since it can be prepared from a very inexpensive electrolyte, it is attractive as a reagent in those cases in which the rate of reaction is sufficiently rapid t o permit a successful titration. Standard solutions of hypochlorite have found some use in the past as an oxidimetric reagent for the titration of such substances as urea, sulfide, nitrite, and hydrogen peroxide in basic solution. Although no titrations of this type Bere attempted in thip work, there is no reason t o believe that they could not be performed satisfactorily. I t would be necessary, of course, in such titrations to add a sufficient quantity of base to the sample solution t o neutralize the acid delivered from the generator cell. Automatic titrations of arsenite samples with both iodine and bromine have been performed successfully with amperometric indication of the end point. The apparatus used for these titrations vias identical with that described previously ('7) for potentiometric titrations except that the galvanometer, Kith suitable shunts, was employed as a microammeter to indicate the current flowing between the indicator electrodes. As the apparatus used for these titrations is now obsolete, the details of these titrations are not reported here. An improved apparatus for automatic amperometric titrations is now being developed. An apparatus for automatic amperometric titrations has also been descrihrd by Richter and Swift ( l a ) . The advantages and disadvantages of external generation of the halogens as opposed to internal generation are worthy of ('omment. In general, the external generation technique is applicable only to titrations involving relatively large samples in which the introduction of traces of impurities and the dilution of the sample by the relatively large volumes of generator electrolyte are not critical. Tlie internal generation method is, therefore, to be preferred for microanalysis and for trace analysis. Liken-ise the internal generation method is superior for mast batch titrations on a macro scale. The large electrode area which is possible with the internal generation technique, together with the depolarizing action of the sample, permit the use of higher currents than can be employed in an external cell. For example, the authors have found that chlorine can be generated internally with 100% efficiency a t currents of 200 ma. in the titration of arsenitr hamples which are made 0 . 2 M in hydrochloric acid For external generation of chlorine a t 200 ma the concentration of hydrochloric acid must be increased tenfold to achieve 100% eficiencv. The generator anode which was employed for the internal generation studies was a cylindrical platinum gauze electrode of the type anal) sia commonly employed as the cathode i~ ~lec~trodeposition
K
631 The cathode was a spiral of platinum wire plaoed in B borosilicate glass tube with a fritted glass disk a t the bottom. The cathode was concentric with the anode so that the generation field was entirely confined within the anode cylinder. This technique made it possible to follow the titration potentiometrically with electrodes placed outside the anode cylinder. For batch titrations, the method of external generation is probably to be preferred over the internal method only in those cases in which the latter method is not applicable. Such cases include those in which the conditions for the generation of the reagent and for the performance of the titration are not compatible and those in which some constituent in the sample undergoes an undesired reaction a t the generator electrode. For example, the external method must be employed for chlorimetric titrations in basic solution since chlorine can be generated only in acid solution. The external generation method is also the preferred method for many continuous automatic titrations in flow processes. The generator cell which has been described should prove useful for the generation of other reagents in those cases in which mixing of the cathode and anode electrolysis products can be tolerated. ACKXOW LEDC\I EYT
The authors are grateful to the Research Corp. for financial assistance which made this work possible. T. W. Martin is also indebted to the United States Rubber Co. for a graduate fellowship. LITERATURE CITED Austin, R. R., Turner, G. K.. and Persy, L. E., Instruments, 22, 588 (1949). Brown, R. A , . and Swift, E. II., J . A m . Ciiern. SOC.,71, 2717 (1949). Buck, R. P., Farrington, P. S., and Swift, E. H., ANAL.(:HEM.. 24, 1195 (1952). Buck, R. P., and Swift, E. H., Ibid., 24, 449 (1952). Carson, W. N., Jr., I b i d . , 22, 1565 (1950). Ibid., 25, 466 (1953). DeFord, D. D., Johns, C. ,J., and Pitts, 3. K,,Ibid., 23, 941 (1951). DeFord, D. D., Pitts, J. S . , and Johns, C. J., Ibid., 23, 938 (1951). Farrington, P, S.,and Swift, E. I€.,Ibid., 22, 889 (1950). Neyers, R. J., and Swift, E. H., J . Ani. Pitern. SOC.,70, 1047 (1948). Ramsey, W. J., Farrington. P. S., arid Swift, E. H., Axar.. CHEM.,22, 332 (1950). Reilley, C. iY., Cooke. W ,D., and I k m i a n , N. H., Ibid., 23, 1030 (1951). Richter, H. L., and Swift, E. I I . , .%l)stracte of 123rd Meeting. p . 12B, .ha CHEW SOC., L o a .%ngelrs. Calif., March 16-19, 1953. Rowley, K., and Swift, E. H.. AX.^.. (IHEXI., 26, 373 (1954). Sease, J. W., Kiemann, C . . and Swift, E. €I., I b i d . , 19, 197 (1947). Shaffer, P. A,, Jr., Briglio, A , , .TI.., and Brorkman, J. A , , Jr., Ibid., 20, 1008 (1948). Ssebelledy, L., and Somogyi, Z., 2. anal. Chem., 112, 313, 323. 332, 385, 391, 395, 400 (193s). Tutundaic, P. 9.. and Mladrnovii., S., Anal. Chim. Acta, 8 , 184 (1953). Wise, E. K.,Gilles, P. W., and Reynolds, C . A . , Jr., ANAL. CHEY.,25, 1344 (1953). Wooster, W. S.,Farrington. P. S., and Swift, E. H., Ibid., 21, 1457 (1949). K E C E I TE n for review November 20, 1953. Accepted January 8, 1954.
