1988
ANALYTICAL CHEMISTRY
results above, duplicate blanks from a given reagent solution are reproducible to about 0.02 absorbance units. For this reason, the maximum sensitivity claimed for this determination is about 5 X 10-OM R-11, which gives an absorbance of about 0.1. The sensitivity of the method can be increased by concentrating the carbonyl compound in the iso-octane solvent before performing the analysis. This procedure is being used to determine concentrations of 1 X 10-eJ4 R-11 in milk by extracting 4 volumes of milk with one of iso-octane. The data plotted in Figure 3 show that the calibration line for R-11 is independent of the reagent solution as long as the simultaneous blank determination is also made. Consequently, a single calibration is all that is required for the analysis of any number of samples over any period of time. The work described is concerned with only one compound, and the general applicability of this procedure to all carbonyl compounds has not been experimentally verified. However, a number of semiquantitative experiments with other aldehydes and ketones indicate that extraction by iso-octane under the conditions of this method is probably a general property of 2,4-dinitrophenylhydrazones. In adapting this procedure t o the determination of a differeht carbonyl compound, a new calibration is necessary, because the absorption maxima and molar absorptivities of the 2,4-dinitrophenglhydrazones vary with the structure of the carbonyl compound ( 4 ) . The rate of
reaction may also vary and the reaction time should be determined when calibrating for a different carbonyl compound. ACKNOWLEDGMENT
The authors wish to express their appreciation to the Phillips Petroleum Co. for making possible the publication of this paper. NOTE:
Since the presentation of this paper, it has been found that in some instances-for example, the determination of traces of acetone in hydrocarbons-it is possible to use 1 t o 2 phosphoric acid alone in the aqueous phase, thus eliminating the need for repurified alcohol. The rate of the reaction may be decreased somewhat by this change, so the mixing time should be checked to be certain that the reaction is complete in the allotted time. LITERATURE CITED
(1) Hillyer, J. C., Swadesh, S., Leslie, M. L., and Dunlap, A . P., Ind. Eng. Chem., 40,2216-20 (1948). (2) Lappin. G. R., and Clark, L. C ,ANAL.CHEM.,23, 541 (1951) (3) Pool, 11.F..and Klose, A. A . , J . Am. Oil Chemists' Soc., 28, 215 (1951). (4) Roherts, J. D., and Green, C., J . A m . Chem. SOC., 68,214 (1946).
RECEIVED for review May 12, 1955. Accepted September 6. 1955.
Sixth Annual Pittsburgh Conference on Analytical Chemistry a n d .Ipplied Spectroscopy, Pittsburgh, P a . , March 1953.
Sensitivity of Bromine-Bromide Potentiometric End Point WILLIAM C. PURDY, EUGENE A. BURNS, and L. 8. ROGERS Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge 39, Mass.
The sensitivity of the bromine-bromide potentiometric end point has been investigated with respect to the factors contributing to the reagent blank in a coulometric titration. The size of the blank often decreased with an increase in the surface area of the indicator electrode, an increase in the rate of stirring the solution, and a decrease in the current passed by the potentiometer, thereby indicating that polarization was taking place. The blank also depended upon other experimental factors such as the concentrations of reagents, the electrode material, and the electrode pretreatment. The size of the potentiometric blank could be diminished by proper choice of the experimental conditions so that it closely approximated the corresponding amperometric blank.
A
(4)and amperometric (9) methods have been employed in coulometric titrations, the former have the inherent advantage of being more readily adaptable to successive determinations of a number of substances in a mixture by observing in a single titration separate end points for each, On the other hand, potentiometry is usually limited to systems containing a reversible half-cell reaction whereas amperometry is not. The relative sensitivities for the two methods have not been compared using a reversible half-cell reaction, although it is interesting to note that one group of investigators switched from classical potentiometry ( d ) , to a polarized potentiometric system when higher sensitivity was required ( 3 , 11, 1 2 ) . It has long been known that potentials for solutions more dilute than about l O - 7 J f frequently do not correspond to those calculated with the aid of the Kernst equation ( 5 ) . In fact, the LTHOUGH both potentiometric
observed potential has usually been found to be independent of the concentration of the substance in question. Though factors such as losses by adsorption onto walls of the container, hydrolysis, and colloid formation may contribute to the end result, control of the electrode potential may often pass over to competing reactions because polarization occurs during the measurement. The potential of a system, although presumably measured with no current flowing, is probably rarely ever measured under ideal conditions, because of the limited sensitivity of the galvanometer and the finite number of turns on the slide-wire used for balancing the bridge. In instruments of the vacuum-tube voltmeter type, a small grid current must be passed to obtain a signal. The purpose of the present study, therefore, was to examine the factors that might influence the sensitivity of a classical potentiometric end point-Le., the magnitude of the blank-on the assumption that such a potentiometric measurement would involve the passage of a small current. Then, under the same conditions as for the potentiometry, the amperometric sensitivity was determined for the same electrode, and the results were compared. EXPERIMENTAL DETAILS
Reagents and Solutions. Stock solutions of 1M sulfuric acid tvere prepared by dilution of the 96% reagent. A stock solution of 2M sodium bromide was made by dissolving the weighed amount of sodium bromide in 1 liter of distilled water. These stock solutions were used in preparing the 0.1M sulfuric acid and 0.2M sodium bromide ordinarily used as the electrolyte. A standard 0.05143M solution of trivalent arsenic was prepared by dissolving 1.2718 grams of arsenic trioxide in 1M sodium hydroxide, neutralizing with 1.V sulfuric acid, and diluting to 250 ml. Apparatus. A cell of the type reported by Myers and Swift
1989
V O L U M E 2 7 , NO. 12, D E C E M B E R 1 9 5 5 (9) was employed with the following changes: A 150-ml. beaker was substituted for the weighing bottle; an external cathode, in contact with 0.liM sulfuric acid, was used in the generating circuit; and an external saturated calomel electrode was used as the anode in the indicating circuit. Both the external generator cathode and the indicator anode were connected to the cell by means of potassium chloride-agar salt bridges after preliminary runs had shown no measurable effect in substituting them for sulfate bridges. Platinum indicator electrodes of 11.54, 1.31, 0.201, and 0.0152 sq. cm. area (geometric) were employed a t different times, but most of the work was done with the 0.201-sq. cm. wire. Electrodes of gold, graphite, tungsten, silver, palladium, tantalum, and platinized platinum of about the same size as the 0.201-sq. , cm. platinum wire were also used. Amperometric end points were detected by means of a Fisher Elecdropode; potentiometric end points, by means of a Rubicon potentiometer (Catalog S o . 8702) having a 1.5-volt scale which could be read to the nearest 0.5 mv., a Leeds and Sorthrup pH meter (Model 7664), and a Beckman Model G p H meter. The sensitivity of the galvanometer on the Rubicon potentiometer was about 5 X lo-* ampere per mm.; currents passed by the pH meters, according to each manufacturer, were 2 x 10-12 ampere for the Leeds and Northrup, and 1 X ampere for the Beckman. Ordinarily, the Leeds and Korthrup instrument was operated as a continuous indicator, whereas the others were actually in the circuit only for the few seconds required to make the meawrement.
mined number of seconds, generation was stopped for 30 seconds and a measurement of potential or current then made. Ordinarily the solution was stirred vigorously during both the generation and the measurement. Each curve was obtained at least three times in order to ensure the reliability of the result. In no case was a definite trend noted within a series though it was often observed that the first determination on a particular day did not agree with the others. Behavior of an electrode was checked each day by making one or more runs under known conditions before investigating new variables. RESULTS
Pretreatment of Electrode. During the early stage. of the study it was frequently observed that the curves could not be reproduced, especially in day-to-day comparisons. This led to an examination of the effect of various pretreatment procedures on the reproducibilities and shapes of the curves. As shown in Figure 1, preanodized indicator electrodei, and those washed n i t h concentrated nitric acid, produced flat lines until about 10-7 equivalent of bromine had been generated, following which the slope changed to a value corresponding to that for the bromine-bromide couple (0.03 volt per tenfold change in concentration of bromine). In several instances there n-as a slight decrease in potential between the initial flat portion of the curve and the beginning of the bromine-bromide slope. On the other hand, when the indicator electrode had been precathodized, an S-shaped curve was obtained which, hoxever, attained the bromine-bromide slope at the same point as the preanodized electrode. Deaeration of the bromide-sulfuric acid mixture prior to the generation of bromine produced an initial potential m-hich was less noble than otherwise. In addition, a somewhat larger number of equivalents of bromine had to be generated hefore attaining the bromine-bromide slope. Longer periods of precathodizing produced the same two effects as deaeration though the increase in the blank was very slight and may not hare been significant.
Equivalents Bromine Generated
Figure 1.
Effect of pretreatment on reproducibilities and shapes of curves
Obtained using Leeds and Northrup p H meter a n d a 0.201 s q . ~ 1 1 1 .platin u m indicator electrode. Generation current, 100 pa. t o 10-6 erlui\alent a n d 1.00 ma. beyond Indirntor plertrodp .~.. .-. .~ 1. Precathodized 5 minutes in 0.1.M sulfuric acid 2. Pretreated with bromine solution from a prior blank run 3. Preanodized 5 minutes in O.lh.1 sulfuric acid 4. Precathodized 5 minutes in 0.1.1.1 sulfuric acid, and sulfuric acidbromide solut,ion deaerated 30 minutes with Durified nitroeen before initial generation of bromine ~~~
100
Current for generating the bromine \?-as fed from an electronic supply (IO)through a parallel circuit so that currents from 1 pa. to 150 ma. could be selected. The current was determined by measuring the potential drop across a Leeds and Sorthrup decade box connected in series with the electrolytic cell. Procedure. Ordinarily the generator anode and the indicator cathode were pretreated simultaneously by a 5-minute electrolysis in 0.lM sulfuric acid using a current of 3 ma. The electrodes were then mashed by filling the cell with distilled water and stirring the solution vigorously for 45 seconds. Three successive washings were performed in this way. This washing piocedure was always used regardless of the pretreatment employed. Additional washings had been found to produce little effect and were therefore abandoned early in the study. A solution on which a blank mas to be determined nas prepared by adding an amount of distilled water to the empty cell so that addition of appropriate volumes of the stock solutions of sulfuric acid and sodium bromide brought the total volume to 100 ml. In each case a pipet was used to ensure reasonable reproducibility. Bromine was usually generated at currents from 100 pa. to 2 ma., which, from polarographic studies, were known to be in the range where one could expect 100% current efficiency for the electrode in question (about 2-sq. cm. area). After a predeter-
5.
I
1
I
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1
1
, ,
, / I l l
Rubicon potentiometer and 11.5 sq. cni. electrode. a , b , and c are concentrations xvhere large platinum foil, small p l a t m u l l foil, and platinum TTire, respectively, first gave "true" readings
When pretreatment consisted of the generation of equivalent of bromine, followed by three regular washings with distilled water, an S-shaped curve mas also obtained. Hom-ever. the initial potential was approximately halfway between those for the preanodixed and the precathodized indicator electrodes. Area of Electrode. As shown by the representative curve> in Figure 2, the curves obtained with electrodes of different areas were found to depend upon the instrument employed for the
1990
ANALYTICAL CHEMISTRY
measurement. For that reason, the results obtained with each instrument are discussed separately. In using the Beckman Model G, the two large foils and the large wire electrode were found to require the same number of equivalents of bromine to reach the point where the brominebromide slope began. However, the smaller the electrode, the greater the drift in the readings at lower concentrations of bromine. Thus the larger foil was steady at the initial point, before any bromine had been generated. However, measurements with the small foil drifted slowly toward larger potentials until about 4 X 10-8 equivalent of bromine had been generated; those with thewireelectrode, until 1.4 X 10-7 equivalent had been generated. The smaller wire (button) electrode produced readings which not only drifted but also moved erratically upward and downward in going from one concentration to the next; measurements did not stabilize until 5 X 10-6 equivalent of bromine had been generated. In contrast to the drift usually obtained with the Beckman instrument, the Leeds and Xorthrup always produced comparatively steady readings. Figure 2 shows that the two-foil electrodes gave curves which were essentially identical and which agreed with those obtained with the Beckman instrument. On the other hand, the wire electrode produced a curve which started out the same as those for the two foils, but, although it reached the bromine-bromide slope at the same point as did the foils, its potentials beyond that point were about 0.06 volt lower than those for the two foils. This resulted in an apparent shift of the inflection point toward higher concentrations of bromine. The button electrode was distinctly different from the other three. I t began about 150 mv. lower than the rest, showed a very slight increase (approximately 0.04 volt) in potential in the region where the other electrodes showed a large change, and at still higher concentrations of bromine showed an irregular trend toward higher potentials. It is obvious from this last example that the reliability of a potentiometric measurement should not be based upon its stability.
define arbitrarily a “true” reading as one in which a 1-mv. change in either direction from the null point on the scale of the potentiometer produced a 1-mm. deflection on the galvanometer. If more than a 1-mv. change was required to produce such a deflection, a potential M-as defined as “indeterminate.” The amounts of bromine which had to be generated in 100 ml. of solution in order to obtain the first true readings were 6 X lo-*, 2 X 10-7, 3 X 10-8, and 2 X equivalent for the large and small foils, the wire, and the button, respectively. At all lower concentrations for the respective electrodes the readings were indeterminate. Once a particular electrode had begun to produce true readings, its values fell on a curve shared by the larger electrodes. The curves for the large foil did not agree N-ith the corresponding curves for the Beckman or Leeds and Northrup instruments either in the inflection point or in the point at which the bromine-bromide slope was first attained. Rate of Stirring. Solutions were usually stirred with a glass paddle at 600 r.p.m. This represented the maximum rate at which one could stir without introducing large numbers of bubbles into the solution. It was recognized that stirring the solution might not lead to results as reproducible as those one might have obtained had the electrode itself been rotated. However, stirring was felt to be the better choice because measurements were to be made with large foils as well as with wires.
I
I
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01 0.8
Figure 4.
:
1
I I I I I I I I I 10-6 Equimlenls Bramine Generated
1 1 1 1 1 1 1
107
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1
I I I I I I 105
Effect of initial concentration of bromide on potentiometric curve
Obtained using Beckman Model G pH meter, 0.201-sq. cm. platinum electrode, and generation current of 100 pa. Electrolyte contained 0.1M sulfuric acid in addition t o sodium bromide. Curves 1-6 represent tenfold decreases in bromide concentration from 0.2M t o 0.000002M
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1 1 1 1 1 110’ 1
I I I I I I 0.6
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Equivalents Bromine Generated
Figure 3.
Effect of rate of stirring on potentiometric curve
Obtained using Beckman Model G p H meter, 0.201-sq. em. platinum electrode, and generation current of 100 Fa. 1. Solution stirred 1 minute after generation had been stopped: then stirring atopped for 1 minute before measuring potential Stirring rates of 2. 150 r.p.m. 3. 300 r.p.m.
4:
450 r.p.m. 600 and 900 r.p.m.
The Rubicon potentiometer appeared to be “dead” when attempts were made to measure solutions containing little or no bromine. This behavior was similar to that encountered when a high resistance was being measured, though, in this case, the resistance of the circuit was known to be less than 1000 ohms. As bromine was generated, the dead region gradually diminished until it finally disappeared. Hence, the decision was made to
Stirring rates of 600 and of 900 r.p.m. gave essentially identical results, despite the large number of air bubbles stirred into the solution at 900 r.p.m. However, as shown in Figure 3, rates of 450, 300, and 150 r.p.m. required progressively larger amounts of bromine to reach the bromine-bromide potential. The extreme is shown by curve 1, for which the solution was stirred 1 minute after generation of bromine had stopped, and was then allowed to stand another minute before the potential was read. Under such conditions the initial potential for the titration curve was always significantly lower, and the concentration of bromine required to reach the bromine-bromide slope was much higher. Hence, the rate of stirring had a marked effect on the apparent size of the blank. Composition of Electrolyte. One of the main reasons for studying the effect of the composition of the electrolyte was to determine whether or not the presence of small amounts of impurities in the reagents was contributing to the blank. A study was first made using different concentrations of bromide in tenfold dilutions from 0.2M to 2 X IO-eM. Figure 4 shows that between 0.2M and 2 X 10-4M the predicted change of about 0.06 volt for each tenfold dilution was obtained in the region where the curves
1991
V O L U M E 2 7 , NO. 12, D E C E M B E R 1 9 5 5 followed the theoretical bromine-bromide slope. However, the 2 x 10-6M solution produced only half the predicted change, while the 2 X 1 O - c X gave very irreproducible results from run to run. In the last case bromine may not have been generated a t 100% current efficiency, but this factor was completely overshadowed by the effect of concentration polarization. When the Leeds and Sorthrup instrument was operated continuously in the normal fashion, results similar to those reported above for the Beckman instrument were obtained, except that erratic curves were produced in 2 X 10-6.M bromide rather than in 2 X 10-6M. On the other hand, when the leads to the indicator electrodes were disconnected, except for the few seconds required to make a measurement, the polarization phenomena were decreased so that the curves were in essential agreement with those obtained using the Beckman instrument. The blank was essentially constant regardless of the concentration of bromide. -4s a result, it was concluded that the impurity in the bromide reagent made an insignificant contribution to the blank.
!
!
I l , l / l
10.'
1
presence of bromine. The value of the potential (-0.125 volt) was very close to the formal value predicted for silver in 0.2M bromide solutions. The tantalum electrode also gave a constant potential a t about 75 mv., but its value could not be interpreted in terms of known half-cell reactions. The tungsten electrode was apparently undergoing attack, because a measurable potential using the Leeds and Yorthrup instrument could not be obtained until a concentration of 6 X 10-7 equivalent of bromine had been generated. At higher concentrations of bromine the potential gradually increased, reaching 160 mv. after 9 X equivalent had been generated. A study was made with the platinized-platinum electrode, because it was felt that an increase in the microscopic or true area of the electrode might decrease the polarization effects. (The platinizing solution contained only chloroplatinate-i e., ions such as lead, which are frequently added to improve the deposit of platinum black, were absent.) I t was interesting to find, therefore, that such an electrode produced stable reading before bromine had been generated in the solution even when the Rubicon potentiometer was employed. However, its potential was comparatively insensitive to bromine, maintaining a relatively stable potential at about 400 mv., and showing only a small Sshaped increase in potential after 5 X 10-6 equivalent of bromine had been generated. If, instead of the usual precathodization, the platinized electrode had been anodized for 15 minutes, a flat line was obtained at 740 mv. over the entire range of bromine concentration that was usually examined.
1 1 1 1 ! 1 1 1
10 6 Equivoienfs Bromine Generated
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Figure 5. Effect of type of supporting acid and of concentration of sulfuric acid on potentiometric curve Obtained usine Beckman Model G pH meter, 0.201-sq. om. platinum electrode, a n d generation current of 100 pa. Electrolyte contained 0.221 sodium bromide in addition t o acid 1 0.1.M sulfuric acid 2 . 0.1M hydrochloric acid 3. 0.1M perchloric acid 1. 0.1M phosphoric acid .a. 1.OM sulfuric acid I3. 0.01M sulfuric acid
3Y''
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IO-, Equivalents Bromne Generated
Figure 6.
In Figure 5 are shown the curves obtained with different acids and with different concentrations of sulfuric acid. Perchloric, hydrochloric, phosphoric, and sulfuric acids had essentially the same blank when present in comparable molar concentrations. More important, a tenfold increase in the Concentration of sulfuric acid from 0 1 to 1 . O X resulted in less than a twofold increase in the blank (and a decreasc in the bromine-bromide potential of about 0.03 volt), whereas a tenfold decrease in the concentration produced no significant change in the blank. As a result, it can safely be concluded that the 0.1M concentration of sulfuric acid usually employed did not make a major contribution to the overall blank. Electrode Material. Electrodes having about 0.2-sq. cm. geometric area were fashioned from platinized platinum (no lead in the platinizing solution), palladium, tantalum, tungsten, silver, gold, and carbon (pencil lead). The gold electrodcproduced a very interesting curve which had the same shape and the same size blank as that of the platinum, but had potentials 150 mv. more noble throughout the titration. This shift is in the direction expected if the gold were acting somewhat as an indicator electrode for its own ions in a solution of bromide. The silver electrode, which also acted as an indicator for its own ions in a bronlide medium, produced a stable potential unaffected by the
1
10-8
1119
,
8
1
,
8
,
10-6
Effect of pretreatment upon potentiometric curves
Obtained during generation of bromine and upon subsequent curves obtained b y dilution of those solutions, using 0.201-sq. em. platinum indicator electrode and generation current of 100 pa. 1. Generation cycle with usual precathodizing procedure 2 . Dilution cycle for curve 1 using 0.124 sulfuric acid, 0.1M sodium bromide. or a mixture of 0.1.M sulfuric acid and 0.1.M sodium bromide a s diluent 3. Generation cycle using a n electrode t h a t had been "prehrominated" 4. Dilution cycle for Curve 3 using sulfuric acid-sodium bromide rniutnre ~
5 and 6. Generation a n d dilution (with sulfuric acid-sodium bromide mixture) cycles, respectively, using a n indicator electrode t h a t had been preanodized 1-4. Obtained with Leeds a n d Northrup instrument 5 and 6. Obtained with Beckman
Interesting results were also obtained with the palladium electrode. It was observed that hydrogen was not evolved during precathodization, even though oxygen was being evolved at the anode. When such an electrode was used to measure the potential of the bromine, a stable potential a t -0.225 volt was obtained throughout the usual range of bromine concentrations. However, after prolonged anodizing, the electrode started out at about 200 mv. and maintained essentially a constant potential until the point where the gold and platinum electrodes had reached the bromine-bromide slope at which point it proceeded to increase slowly. Though it is obvious from these results that the palladium electrode cannot be used for measuring bromine, its
1992
ANALYTICAL CHEMISTRY
avidity for hydrogen and the resulting stable potential lead to the belief that it could be used as an arbitrary, but very useful indicator electrode for acid-base titrations. The carbon electrode gave a curve very similar to that for platinized platinum, except that its initial potential was about 0.05 volt higher, and the point a t which the curve reached a slope approximating that of the bromine-bromide half-cell was closer to that obtained with smooth platinum, being about 3 X lo-' equivalent. In this case, the larger blank may have been due to reaction of bromine with the electrode, particularly with any organic binders that might have been present Position of Generating Cathode. The question of the effect of the position of the generating cathode was investigated briefly, because it was felt that the hydrogen generated in the solution might be a factor contributing to the potential on the indicator electrode. Though curves for internal and external generator cathodes n ere essentially the same, the decision was made to use an external cathode to avoid the possibility of introducing variations as a result of the presence of hydrogen in the solution. DISCUSSION
From polarographic studies, it is known that the presence of films on electrode surfaces requires the passage of a measurable number of microcoulombs in order to remove them, and thence to obtain a more nearly true value for the polarographic process alone (6, 8). In addition, Wagner (IS) has suggested that the current required to charge a double layer on the surface of a 1-sq. cm. electrode would also be appreciable when generating and measuring 10-7 equivalent or less. From those considerations it can be predicted that coulometric generations and detection of trace amounts of bromine would be more accurate using smaller areas for the generation and the indicator electrodes, because of the smaller amounts of current required to condition the electrode surfaces. Another reason for using small indicator electrodes in amperometry was encountered by Lothe ( 7 ) in preliminary studies on the bromine-bromide system using the exact setup of Myers and Swift (9) He observed that at generation rates of the order of 10 pa., the slope of the current versus the microequivalents of excess bromine was much smaller than when currents of 100 to 1000 pa. were employed. Furthermore, after generation had been stopped, a noticeable decrease in current occurred which could, holTever, be eliminated entirely by removing the externally imposed potential from the large amperometric indicator electrode. Apparently, appreciable consumption of the reagent by the indicator electrode (area of about 20 sq. cm.) was taking place. Therefore, the use of a smaller indicator electrode should, in principle, be desirable as a means of decreasing the blank for both potentiometric and amperometric end points. However, the present study showed that nrith present equipment, polarization effects became noticeable when small electrodes were employed in potentiometric measurements whereas amperometric currents obtained with the button electrode were too small to be measured accurately. The fact that the Rubicon, which draws a larger current than the other two instruments, was found t o have larger blanks; that the blank decreased with an increase in electrode size in measurements made with the Leeds and ?-orthrup instrument; that the results with the wire and with the button obtained with the Leeds and Northrup were in the direction of consumption of bromine; that stirring was effective in reducing the magnitude of the blank; and that the operation of the Leeds and Sorthrup as a continuous reading instrument, as opposed t o an interrupted measurement, produced larger blanks-all point t o the existence of polarization. There is no question that polarization alone cannot account for the entire blank. It is likely that oxidizable impurities were present in the reagents. However, the experiments involving different concentrations of bromide and of sulfuric acid indicated that these sources together probably did not account for a major
portion of the blank. Nevertheless, in an effort to obtain more information concerning this point, experiments were run on a solution in which 1.6 X 10-7 equivalent of bromine had been generated, following which 25 cc. of the solution had been removed by pipet, and 25 cc. of another solution substituted for it, -4s shown in Figure 6 repetitive dilutions, using an indicator electrode which had been preanodized, resulted in a continuation of the bromine-bromide slope to smaller concentrations than had been found to obey the Nernst relationship when bromine was being generated. The fact that the dilution was carried out with 25 cc. portions of a stock nlixture of bromide in sulfuric acid indicated that the impurities in these two substances were not of major proportions. However, it is interesting t o note that after an electrode had been precathodized or had simply been used to generate bromine in another aliquot of the stock solution of bromide-acid, the dilution curve tended to retrace the S-shaped generation curve. Dilution curves for the precathodized electrodes may indicate the presence of small amounts of hydrogen which diffused out of the platinum to establish a mixed potential, whereas those for the preanodized surfaces may indicate the presence of a film either of oxygen gas or a sorbed layer ( 2 ) with sufficient rapacity to counteract the impurity added by the dilution.
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800L
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200-
L
90-e
1
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I I I I I I 10-7
Equivalents
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I 1 IIlJ 1u5
Bromine Generated
Figure 7. Effect of waiting different lengths. of time before measuring potential of 0.201-sq. cm. platinum indicator electrode with Leeds and Northrup pH meter Generation current was 100 #a. 1-4. 0, 15, 30, a n d 60 seconds of waiting, with generatlon current off. before measuring potential
From these dilution studies it would appear. by inference, that the chief source of impurities might be the distilled water. However, brief studies with water from different sources, including doubly-distilled water and distilled water stored for various times, did not result in significant differences which would justify additional experiments along this line. Another factor considered as a possible contributor to the blank was that of incomplete stripping of bromine from the generator anode. As shown in Figure 7 , the magnitude of the blank was indeed a function of the length of time that nas waited before measuring the potential. However, a delay longer than 30 seconds before measuring the potential did not appear to produce a significantly smaller blank. The question of whether or not the formation of a film and/or the charging of a double layer can account for the blank has already been mentioned briefly. One of the most important arguments, which would appear to indicate that the formation of such a film is not a major contributor to the blank in the present study, is the fact that in measurements with the Beckman instrument, where polarization effects were minimized, blanks for the wire and for the two foils were essentially identical. I t must be concluded, therefore, that if film formation is indeed necessary, the amount of current required was too small to be detected by the technique employed in the present study.
1993
V O L U M E 2 7 , NO. 1 2 , DECEMBER 1 9 5 5
B moderate amount of information has been accumulated which indicates that the initial portions of the titration curves, prior to the points beyond iyhich the bromine-bromide slope is obeyed, are mixed potentials. Evidence for this is shown in the differences between the precathodized and the preanodized indicator electrodes. K i t h each a-ashing of a precathodized electrode, the potential progressively rose, whereas successive washings with a preanodized electrode gave progressively lower potentials; furthermore, the longer the precathodization, the loner the initial potential. Also, deaeration of the solution prior to generation of bromine resulted in loner potentials for precathodized electrodes. Other evidence may he found in the curves for electrode materials
Figure 8. Variation of blanks with size of electrode using potentiometric and amperometric measurements Potentiometric curves obtained using Leeds and Northrup pH meter. Generating current was 100 p ~ 1 and 2 . Amperometric and potentiometric curves for 0.201 sq. cm. wire 3 a n d 4. Amperometric and potentiometric curves for 11.54-sq. cm. platinum foil
.
other than smooth platinum. The displacement of the curve of the gold electrode below that of smooth platinum is perhaps the most convincing, although the behavior of the other electrodes is also in accord with this conclusion. The fact that platinized platinum required a larger blank may have been because of the presence of more hydrogen on the surface, but it was more probably the result of the fact that the exchange current for the hydrogen reaction was higher on such a surface with the result that higher concentrations of bromine were required in order to make themselves felt by “taking over” control of the potential of the electrode. In general, slower reactions having smaller exchange currents would require higher concentrations of the species before they could control the potential of the electrode, or could minimize polarization effects to the same extent as a faster reaction (1). In view of the fact that the literature often implies that the amperometric end point is more sensitive than the potentiometric, it appeared desirable to examine this conclusion using a single electrode under conditions that were maintained rigidly the same for both sets of measurements. As shown in Figure 8, this was carried out tvith two different sizes of electrodes, the potentiometry being carried out as described heretofore and the
amperometry by imposition of a potential of +0.41 volt on the indicator electrode by means of a Fisher Elecdropode. The ordinates for the two sets of amperometric titrations were selected in such a way as to normalize them approximately in terms of the geometric areas of the electrodes. The conclusions from these comparisons are most interesting. With both electrodes, amperometric indication of “excess bromine,” if considered in terms of the first deviation from a linear extrapolation from the first two or three points of the amperometric curve, nearly coincides with the beginning of the rapid rise of the S-shaped portion of the potentiometric titration curve. For both electrodes, however, the first deviation on the amperometric curve appears to precede slightly the beginning of the rapid rise of the values on the potentiometric curve. The difference seems to be greater for the larger electrode, but it cannot really be said from the limited data on hand whether or not this difference is significant. Secondly, although the inflection point of the potentiometric curve (on a linear time-scale or on a logarithmic time-scale) cannot be determined with high precision, it appears to fall at the same concentration as the amperometric end point. Thirdly, the concentration at which the current first appears to fall on a straight line beyond the amperometric end point is the concentration a t which the potentiometric curve first begins to folloi? the bromine-bromide slope. The cloee correspondence of the amperometric and the potentiometric curves would appear to leave little to choose betryeen them. However, it is likely that the first indication of excess bromine might be detected less ambiguously by the amperometric method. Furthermore, excess reagent might be detected sooner under conditions where an appreciable portion of the blank was not contributed by the surface. The presence of an imposed potential should precondition the electrode surface, thereby decreasing the magnitude of the blank. Figure 8 also shows the decrease in slope beyond the end point that n-as obtained when larger electrodes are employed. This decrease was attributed to the consumption of bromine by the amperometric indicator electrode. If still larger electrodes were used, the bromine was consumed faster (7‘) and produced noticeable curvature even though the potential was imposed on the indicator electrode only for the few seconds required to obtain a measurement of the current. In vieu of the conclusions reached above, that the relative sensitivities of the amperometric and potentiometric end points were essentially the same for the electrodes used in the present study, it was of interest to compare the results of amperometric and potentiometric determinations of arsenic when they, too, were obtained under comparable conditions. As shown in Table I, the precision and accuracy of both potentiometric and amperometric determinations of 77.0 y of arsenic(II1) were in essential agreement. Trial calculations showed that it did not appear to matter which of the three corresponding pairs of points was selected as the end point, providing the comparable position on the curve for the other method was selected in accordance with the discussion given above. However, when the amount of arsenic was reduced tenfold, the results were high, by both methods, although theamperometric method appeared to be some&hat more accurate and more precise than the potentiometric. The advantage was slight, but unambiguous a t this level of concentration. I t seems likely that the advantage might have been increased a t still lower concentrations of arsenic in accordance with the reasoning outlined above. However, the present study wa8 not extended to smaller amounts of arsenic, because the precision even for the amperometric technique was rather poor even at a level of 7 y of arsenic. It appears that such a study might best be carried out using a very small electrode for the reasons mentioned earlier. -4more sensitive galvanometer would then be required for amperometric measurements and for detecting the null point in a potentiometric measurement. Alternatively, a vacuumtube voltmeter requiring the passage of a much smaller current
1994
ANALYTICAL CHEMISTRY
might be useful in potentiometry. However, such measures will not decrease the apparent potentiometric blank if the relative magnitudes of the exchange currents for the desired reaction and for a competing one are such that the latter is already controlling the potential of the electrode. ACKNOWLEDGMENT
The authors wish to thank Isadore .4mdur and James J. Lingane for many helpful suggestions and criticisms. The authors are indebted to the Atomic Energy Commission for partial support of this work. LITERATURE CITED
Bockris, J., O’M., “Modern Aspects of Electrochemistry,” pp. 212,213,Academic Press, New York, 1954. Ibid., p. 227. Cooke, W. D..Reilley, C. W., and Furman, N. H., ANAL.CHEM., 23, 1662 (1951). Furman, N.H., Cooke, W. D., and Reilley, N, H., Ibid., 23,945 (1951). Haissinsky, M., “Electrochimie des substances radioactives et dessolutionsextr@mement dilubes,” Herman et Cie, Paris, 1946. Kolthoff, I. M., and Tanaka, N., ANAL.CHEM.,26, 632 (1954). Lothe, J. J., private communication, 1953. Marple, T. L.,and Rogers, L. B., ANAL.CHEM.,25, 1351 (1953). Myers, R. J., and Swift, E. H., J. Am. C h n . SOC.,70, 1047 (1948). ReiUey, C . Tu’., Adams, R. N., and Furman, N. H., ANAL.CHEM., 24, 1044 (1952).
Table I. Comparisonof Potentiometric and Amperometric End Points for Coulometric Determination of Arsenic (Using 0.201 sq. em. platinum electrode in vigorously stirred solutions. Bfter each period of bromine generation, intervals of 30 seconds for 77.0 y and 45 seconds for 7.70 of arsenic were interposed before connecting the indicator leads for t%e few seconds required to obtain a reading) Arsenic Arsenic Found, y DeterTaken, mination y Amperometric potentiometric^ Single b 77.0 77.8 *0.60 76.7 f O . Q c Successived 77,O 77.3 f0.3 77.3 f 0 . 5 Single b 7.70 8.51 f0.28 9.30 f0 . 3 8 Successived 7.70 8 . 2 8 f 0.18 8.92 &0.35 Singled Blank 9 . 8 i 0 . 2 X 10-8 equiv. 8 . 9 f 0 . 2 X 10 a equiv. a Inflection point on linear-time basis used as end point; Leeds and Northrup instrument. b Three determinations starting with fresh stock solution of bromide and acid, and with pretreatment consisting only of three regular washings t o remove bromine from previous run. c Av. dev. d Three determinations run in series using a single portion of bromideacid solution. 6 Quadruplicates.
(11) Reilley, C. N.,Cooke, W. D., and Furman, N. H., Zbid., 23, 1223 (1951). (12) Ibid., p. 1226. (13) Wagner, C., private communication, 1952.
RECEIVED for review October 8, 1954. Accepted August 24, 1955. Pittsburgh Conference on Analytical Chemistry, Pittsburgh, Pa., March 1955.
Coulogravimetric Determination of Zinc and Cadmium WILLIAM M. MAcNEVINand R.
D. MclVER1
McPherson Chemical Laboratory, The Ohio State University, Columbus, Ohio
m = number of milliequivalents (Zn
The principle of coulogravimetric analysis, an example of double measurement, has been shown to be applicable with the mercury cathode. Mixtures of zinc and cadmium are deposited electrolytically and weighed. The number of coulombs required is also determined and results are obtained by solution of simultaneous equations.
m = -wt. of Zn 0.03269 Wt. of Zn =
C
OULOGRAVIMETRIC analysis or the simultaneous application of coulometric and gravimetric measurements was described earlier (6) in its application to the analysis of mixtures of chloride and bromide. Chloride and bromide, which cannot be separated by electrodeposition, were deposited together on a silver cathode and weighed. The number of coulombs required to deposit both was also determined, and the solution of simultaneous equations gave values for the amounts of chloride and bromide. At about the same time DuRose and Hutchison ( I ) reported use of the same principle in determining the composition of commerciai lead-tin alloys plated from fluoborate electrolytes. This paper reports the extension of the method to the mercury cathode, using zinc and cadmium as the pairs of metals. The mercury cathode in electrolytic analysis of individual metals has been studied by Lingane (2). In the work described, the potentials suitable for depositing both metals on the mercury cathode were determined polarographically. The increase in weight of the cathode, w , and the total number of milliequivalents, m, of zinc and cadmium were determined from coulometric measurements. The weights of zinc and cadmium were obtained from the following considerations: w = weight of zinc weight of cadmium (1)
+
1
- wt. of Zn - coulombs used -96500 0.03269
Present address, U. S. Army, Box D , Lorraine, Kan.
+
w
- wt. of
Wt. of Cd = w
(0.05621 m
(2)
wt of Cd +
0.05621
Zn
0.05621
::::$?; ~
+ Cd)
w)
- wt. of Zn
VP m = - (2.141 X T
(3) (4)
(5) (6)
where
V
P
T
= = =
volume of gas in milliliters corrected pressure in mm. of mercury absolute temperature APPARATUS
A Lingane-type potentiostat (3) was used and the contact8 of the galvanometer relay were adjusted to give control to f 0 . 0 1 volt. The cell used is shown in Figure 1. It consisted of a 180ml. tall-form electrolytic beaker which held the supporting electrolyte. The cathode support, E, was constructed from 15-ml. borosilicate glass tubing which insulated the platinum or tungsten contact and copper lead from the electrolyte. The anode, A , was a coil of silver wire or a coil of copper wire heavily plated with silver. Stirring was provided both by a magnetic stirrer, H,J, and a propellor-driven stirrer, C. Contact with the calomel electrode, K , was made with a saturated potassium chloride-4% agar salt bridge, F , and outgassing was done with nitrogen through the bubbler tube, G, D. The cell was covered in order to maintain an atmosphere of nitrogen over the electrolyte. A 100-ml. hydrogen-oxygen coulometer of the Lingane type ( 4 )
