Simultaneous Determination of Bromide and Chloride by Cathodic Stripping Voltammetry G. C O ~ O V OG. S ,S. ~ Wilson, and J. L. Moyers A t m o s p h e r i c A n a l y s i s Laboratory, Department of Chemistry, U n i v e r s i t y of Arizona, Tucson, A r i z . 85721
Cathodic stripping analysis of CI- and/or Br- is described. The concentration of Br- and CI- in a mixture can be determined by this method if their concentrations are comparable ([Br-]/[CI-] 0.2 lo 9). The sum of Brand CI- is determined by performing the anodic deposition at 0.33 V YS. SCE. The Br- concentration is delermined by stripping cathodically the film which was electrodeposited at 0.22 V YS. SCE. In both cases, standard solutions of about the same Br-/CI- ratio as the unknown are used for the calibration curves. Concentrations of CI- and Br- as low as 10-5Mhave been measured with a precision of about 7%. Small amounts of Br- in a chloride matrix ([Br-]/[CI-] at least as low as 0.005) can be determined also by cathodic stripping analysis. Concentrations as low as 3 X 1OV6MBr- (in have been determined with a preci[CI-] = 6.6 X sion of about 10%. The problem of interferences is discussed.
Although methods for simultaneous determination of chloride and bromide are in great demand, not many have been developed primarily because of the difficulties which arise from the very similar physicochemical properties of these two elements. Early attempts to utilize the difference in solubility of their silver salts resulted in formation of solid solutions (1) which introduced some very serious problems in the simultaneous determination of mixtures of bromides and chlorides. Photometric methods based either on the formation of halogen derivatives of a dye (2) or oxidation of a dye ( 3 ) by a halogen, conveniently generated from the corresponding halide, are usually very time consuming and/or unreliable when used on a routine basis. Neutron activation analysis (NAA) has been used extensively as a sensitive and reliable method for trace halogen mixtures. Cosgrove et al. ( 4 ) and Duce and Winchester ( 5 ) have described halogen separation and purification procedures followed by a p counting scheme applicable to the determination of trace halogens in a diverse variety of sample matrices ( e . g . , sea water, atmospheric particulate matter, geological specimens, etc.). These techniques have been used for the determination of nanogram and subnanogram quantities of bromide, chloride, and iodide. Zoller and Gordon (6) and Dams, et al. (7) have reported Present address, Science Center, R o c k w e l l I n t e r n a t i o n a l , C a l i f . 91360.
direct determination (i. e., no post irradiation separation or purification procedures) of trace halogen concentrations by high resolution y -ray spectroscopy. These high resolution instrumental techniques are somewhat less sensitive and more prone to matrix problems than are the destructive p counting techniques. The y-ray techniques are, however, less time consuming and simpler to use than the p counting techniques. While the NAA techniques do offer the advantages of sensitivity and precision for trace halogen determinations, the disadvantages of these techniques include the large expense and the availability of reactor and counting facilities. Attempts to develop electrochemical methods for simultaneous halogen determinations (8) have failed because of complications introduced by secondary reactions which take place in the vicinity of the solution electrode interface. Most of these phenomena are kinetically controlled and, therefore, seriously affect the apparent reproducibility of the method. A number of electroanalytical stripping techniques (i. e., coulometric, chronopotentiometric) have been used for the determination of diverse halides in a variety of supporting electrolytes (9-13). However, probably because of the interfacial complications, no efforts were made in these studies to do simultaneous halogen determinations. In attempting to use cathodic stripping analysis for the simultaneous determination of bromide and. chloride in various mixtures, we soon realized that additional information and a better understanding of the complicated interfacial phenomena affecting the deposition and stripping of halides a t the electrode were required. The characteristics of these phenomena have been studied and presented in a separate paper (14) and are utilized here in the development of methods for the simultaneous determination of bromide and chloride. In order to develop these methods, two important factors had to be taken into consideration. First, secondary reactions take place in the vicinity of the solution-electrode interface, which alter the composition of the original deposit and, second, chloride affects the electrodeposition of bromide a t electrolysis potentials where only the bromide should react. In this paper, detailed information about the simultaneous determination of bromides and chlorides by cathodic stripping analysis is given. This procedure is potentially useful for the simultaneous determination of trace halogens in a variety of sample matrices ( e . g . , oceanographic and atmospheric studies) and offers an alternative to the
P.O.
Box 1085, T h o u s a n d Oaks,
(1) H. A. Laitinen. "Chemical Analysis," McGraw-Hill Co., New York, N.Y., 1960, p208-11. (2) E. F. Joy, J. D. Bonn, and A. J. Bernard, Jr., Anal. Chem., 45, 856 (1973). (3) H. A. Laitinen and K . W. Boyer, Anal. Chem., 44, 920 (1972). (4) J. F. Cosgrove. R. P. Bastram, and G. H. Morrison, Anal. Chem., 30, 1872 (1958). (5) R. A. Duce, and J. W. Winchester, Radiochim. Acta, 4, 100-104 (1965). (6) W. H. Zoller and G. E. Gordon, Anal. Chem., 42, 257 (1970). (7) R . Dams, J. A. Robbins, K. A. Rahn, and J. W. Winchester, Anal. Chem., 42, 861 (1970).
(8) J. J. Lingane, "Electroanalytical Chemistry, "lnterscience, New York, N.Y., 1958. (9) I?. G. Boll, D. L. Manning, and 0. Menis, Anal. Chem., 32, 621 (1960). (10) I . Shain and S. P. Perone, Anal. Chem., 33, 325 (1961). (11) W. I . Maddox, M. T. Kelley, and J. A . Dean, J. Electroanal, Chem., 4, 96 (1962). (12) H. Specker and G. Schiewe, Fresenius' 2. Anal. Chem., 196, 1 (1963). (13) J. P. Perchard, M. Buvet, and R. Molina, J. Electroanal. Chem., 14, 5 7 (1967). (14) G. Colovos, G. S. Wilson, and J. L. Moyers. Anal. Chem.. 46, 1045 (1974). A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 8 , J U L Y 1974
1051
I
2
3
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5
6
7
8
9
IO
I1
12
13
[TOTAL HAL1DE] x IO'M
Figure 1. Dependence of the peak current on the total concentration (Br- -I-C I - )
Pre-electrolysis potential, 0.33 V vs. SCE; scan rate, 2 trode area, 2.22 f 0.07 mmz; electrolysistime, 2.0 min
mV/sec;
elec-
more sophisticated and expensive NAA methods for trace halogen mixtures. EXPERIMENTAL Instruments and Apparatus. The instrumentation used in this study has been previously reported (14, 15). Care was taken not to expose the working electrode to direct sunlight which can cause decomposition of the mercurous halide. Reagents. All chemicals employed were reagent grade and were used without further purification. De-ionized distilled water was used throughout this study. Synthetic samples were made by proper dilution of 10- 2M halide stock solution with supporting electrolyte (1.8M HzS04). Halide stock solutions, 10- ZM,were prepared daily by proper dilution of 1M halide solution with water. The samples were prepared by addition of proper amounts of sulfuric acid to bring the acidity to 1.8M HzS04. The halide contamination of the supporting electrolyte was well below the limits of detection (- 10- 7M) by cathodic stripping voltammetry. A 2 mV/sec scan rate was generally used for obtaining the stripping curves. Procedures. A . Simultaneous Determination of Chloride and Bromide. All the experiments were performed in 15-ml aliquots of supporting electrolyte. To determine total halide (bromide plus chloride), the solution was electrolyzed for 2 minutes at 0.33 V us. SCE, under stirring, and at the end of this period both stirring and electrolysis were stopped. After 10 seconds, the electrolysis potential was applied again (without stirring) for 20 seconds and then the stripping process was started by cathodic scanning of the potential. When a pronounced broad peak, indicative of a high chloride to bromide ratio, appears before the bromide peak, the solution should be stirred for an additional 2 minutes at open circuit after the electrolysis step to allow time for the complete conversion of the deposited mercurous chloride to bromide (14). The same procedure was repeated at least three times and the average peak current was used to determine the total halide (bromide plus chloride) concentration. The determination of the bromide alone was carried out in the same solution by using an identical technique changing only the electrolysis potential from 0.33 to 0.22 V us. SCE. No additional stirring of the solution is necessary in this case since no chloride film is formed at this potential (14). The approximate bromide to chloride concentration ratio was estimated from ratio of the peak currents of the stripping curves obtained from electrodeposition at 0.22 and 0.33 V us. SCE, respectively. From this estimation, standard solutions of about the same halide ratio were prepared for construction of the calibration curves. From the standard curves, the concentration of bro(15) G. Colovos, G . S. Wilson, and J. L. Moyers, Anal. Chim. Acta, 64, 457 (1973).
1052
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mide and the total bromide plus chloride concentration were estimated. The necessary data for the construction of the calibration curves were obtained by applying the same procedures as in the case of the sample. The bromide to chloride molar ratios and the total bromide plus chloride concentration range used in this investigation was to 1.0 x 10- 4M, respectively. 0.2 to 9 and 1 x B. Determination of Small Amounts of Bromide in Chloride. The same preparative steps as in procedure A were applied. The value of the pre-electrolysis potential depends on the concentration of chloride in the sample. For a chloride concentration of 6.6 X 10-4M, a potential of 0.190-0.195 V us. SCE was used. The optimum value of the pre-electrolysis potential can be determined by applying an increasingly anodic potential, under stirring conditions, to a chloride solution (bromide-free) of the same chloride concentration as the unknown. A pre-electrolysis potential corresponding to 0.1 p A anodic current (electrode area 2.22 f 0.07 mmz) was found to be sufficient for formation of the optimum amount of mercurous chloride film which in turn facilitates the electrodeposition of bromide from rather dilute solutions. A 2minute electrolysis followed by 2 minutes of stirring at open circuit and subsequent stripping was performed as in procedure A. The standard addition method was applied for determination of the bromide concentration in the samples. Bromide t o chloride ratios in the range of 0.005 to 0.04 and bromide concentrations ranging from 3 X 10- to 1.6 X 10- 5M Br were studied. RESULTS AND DISCUSSION Under certain conditions, the electrodeposition of a mixture of bromide and chloride on a mercury electrode results in a single peak cathodic stripping curve. The peak potential of this current-voltage curve coincides with the potential of the bromide peak, and the peak current (peak height) or the charge (peak area) represents the total amount of the two halides and not the amount of bromide alone. This is attributed to the substitution reaction which takes place simultaneously with the electrochemical reaction in which the mercurous chloride deposited on the electrode is displaced by bromide to form the less soluble mercurous bromide (14). This reaction' usually is fast enough to assure complete transformation within the time limits of the experiment and, therefore, the absence of the chloride peak from the current-potential curve can be easily explained. Only in cases of relatively low bromide concentration is additional time required for completion of the exchange reaction. In most instances, 2-minute stirring a t open circuit is sufficient for complete conversion of the mercurous chloride to bromide. The electrodeposition and the subsequent stripping of a halide film from the electrode depends very much on the applied pre-electrolysis potential (14). At 0.33 V us. SCE the response of the electrode to both bromide and chloride is almost identical and, therefore, in the case of binary mixtures of the above ions, the resulting current-voltage curve represents the total amount of halide present. Under such conditions, the peak current is directly proportional to the total halide concentration in solution and, therefore, makes feasible the determination of the sum of bromide and chloride by cathodic stripping analysis. In Figure 1, the average peak current, obtained by performing stripping analysis in solutions of different total bromide and chloride concentration, is plotted us. the sum of the concentration of these two halides. Each point is the average peak-current obtained from solutions of the same total halide concentration but different bromide to chloride ratios. The non-zero intercept of this plot should be attributed primarily to the solubility of the mercurous halide salt which prevents film formation on the surface of the mercury electrode. The precision is generally within f7%. In the case of solutions with high chloride to bromide ratios and low total halide concentration, the ob-
+
Table I. Determination of Total Halide (BrC1-) and Bromide Concentrations by Cathodic Stripping Analysis [Br-] X lO5M
[total] X 105M Taken
1.66 1.66 3.33 3.33 3.33 3.33 5.00 5.00 5.00 5.00 6.66 6.66
10.00 10.00
Found
% Error
Taken
1.53 1.55 3.45 3.19 3.49 3.20 5.30 4.88 4.76 4.75 6.70 6.72 10.70 10.50
-7.8 -6.3 -3.6 -4.2
0.83 1.00 0.66 1.66 2.00 2.50 2.00 2.50 3.00 4.00 5.33 5.00 7.50 8.00
+4.8 -3.9 +6.6 -3.6 -4.7 -5.0 +0.6 +0.9 +7.0 $5.0
Found
0.90 1.07 0.64 1.60 2.09 2.40 1.83 2.42 3.17 3.75 5.20 5.30 7.60 8.15
yo Error
+8.4 +7.0 -3.0 -3.6 $4.5 -4.0 -8.5 -5.3 +5.8 -6.2 -2.4 f6.0 f1.3 +1.9
tained peak currents are usually about 10% lower than expected. This can be explained by the fact that the efficiency of electrodeposition of chloride in this concentration range is lower than that of bromide and this produces low results because of the large contribution which the chloride makes to the total current. By matching molar ratios and halogen concentrations of standards and samples, this phenomenon does not present a serious problem. The reproducibility for a solution of average concentration ( 5 x 10-5M) and 1:l bromide to chloride ratio was found to be about 5%. Under the experimental conditions applied in this study, total concentrations higher than 1.2 x 10-4M cannot be used because the stripping of the resulting deposit is incomplete a t the scan rate employed and, therefore, the increase in the peak height or peak area is not proportional to the increase in concentration. The determination of bromide in a solution containing a comparable amount of chloride was based on the observation that there is a potential range in which bromide but not chloride can be electrodeposited on the mercury electrode (14). I t was expected the electrodeposition of bromide should not be affected by chloride a t pre-electrolysis potentials where the latter alone does not react with the electrode. However, a small but yet significant, from the analytical point of view, effect of chloride on the electrodeposition of bromide resulting in higher peak currents was observed. The dependence of the peak current on the concentrations of both halides is complicated. In the range of concentrations used in this study, the linearity of the ip us. [Br-] plot is not af€ected by the presence of chlorides if the bromide to chloride molar ratio remains constant. Figure 2 shows the dependence of the slope of the lp us. [Br-] plot on the molar ratio of the two halides. It can be seen that variation of the bromide to chloride ratio by a factor of 3 (three-fold increase in chloride) increases the height of the stripping curve by only 13-19%. Therefore, for small changes of the [Br-]/[Cl-] molar ratio the change of the peak height is very small and insignificant in comparison to the error of the method (about 7 % ) . Thus, reference solutions of approximately the same bromide to chloride molar ratio of the samples ( i . e . , within *25%) can be used for the calibration curves without affecting the precision of the method. This is the basis of the analytical approach used in the present study for determination of bromide in presence of chloride. The approximate molar ratio of the two halides can be estimated from the peak currents of the current-potential curves obtained by stripping the electrodeposited films
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[Br-]
X
IO'M
9
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13
Figure 2. P e a k c u r r e n t vs. b r o m i d e c o n c e n t r a t i o n for two differe n t b r o m i d e to c h l o r i d e r a t i o s ( A ) B r - / C I - = 1; ( B ) ) B r - / C I - = 3. Pre-electrolysis potential, 0.22 V vs. SCE; scan rate, 2 mV/sec; electrode area, 2.22 f 0.07 mm2; electrolysis lime, 2.0 min
formed at 0.22 and 0.33 V us. SCE, respectively. Standards of the same molar ratio were used for the calibration curves from which the sum of bromide plus chloride and the bromide concentrations in the sample can be determined Table I gives the results of the application of this method for the determination of these two halides in synthetic samples. The average error of analysis is approximately 7% for both the total halide and bromide concentrations. Under the experimental conditions used in this study 1 x 10-5M halide is the lower concentration limit for which the method functions properly. Lower concentrations can be detected, but the error of the analysis increases as the concentration decreases. Higher sensitivities probably can be obtained under different experimental conditions and examples of higher sensitivities for individual halides can be found in literature. This is usually achieved either by using faster scan rates (13) or by changing the nature of the solvent medium so that the solubility of the film becomes lower and, therefore, electrodeposition can take place at lower halide concentrations (10). It was found, however, that small amounts of bromide in the presence of relatively high concentrations of chloride can be determined by taking advantage of both the exchange reaction and the effect of chloride on the bromide peak. By this method, the bromide ion concentration can be datermined in a medium where the chloride concentration is two hundred times greater. This determination is carried out a t pre-electrolysis potentials (0.1900.195 V us. SCE for 6.6 X 10- 4M C1- ) where only a small anodic current, due to the deposition of chloride, can be observed in absence of bromide. Upon addition of bromide, the anodic current becomes larger indicating simultaneous electrodeposition of both halides. After allowing time for the exchange reaction to take place, the peak currents obtained by stripping the deposit cathodically represent the amount of the bromide present in the solution. Figure 3 shows the current-voltage curves obtained by applying the above procedure to solutions containing 6.6 X 10-4M C1- and varying concentrations of bromide. The stripping curve obtained in the bromide-free solution represents the dissolution of the chloride deposit from the surface of the electrode. The peak potential of a stripping curve for the removal of a bromide film under these conA N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO.
8, JULY 1974 * 1053
Table 11. Determination of Small Amounts of Bromide in Chloride, [CI-] = 6.6 X [Br-] X 106 M
Taken
Found
3.30
3.75 5.70 9.5 14.5
6.60 10.0 13.3
Error
$13.6 -13.6 -5.0
+11.2
C
I
19
0.09
0.19
0.09
0.19
0.09
0.19
0.09
,
0.19
009
I
E, VOLTS vs SCE
Figure 3. Synergistic effect of chloride on the electrodeposition of bromide [CI-] = 6.6 X 10-4M;the bromide concentration for A, B, C, D, and E is 0, 3.3 X 6.6 X 1.3 X and 1.66 X 10-5M,respectively: pre-electrolysis potential, 0.22 V vs. SCE: scan rate, 2 mV/sec; electrode area, 2.22 f 0.07 mm*; electrolysis time, 2.0 min
ditions is'more than 100 mV more negative than this. Depending on the conditions ( i e . , pre-electrolysis potential and chloride concentration), the sensitivity of this version of cathodic stripping analysis for bromide determinations is a t least three times greater than for the simultaneous halogen determination procedure described above. This increase in sensitivity is due to the synergistic effect of chloride on the electrodeposition of bromide. The standard addition method is best suited for bromide determinations of this kind since the reproducibility of the stripping curves from sample to sample is not sufficiently good to allow the use of calibration curves. Table II gives the results of the application of this method to the determination of bromide in synthetic samples. The error of the method is a little over 10%. It should be pointed out that since this method depends very much on the concentration of chloride, determination of bromide in solutions of differing chloride concentration results in dissolution curves of unequal peak currents for the same bromide concentration. Interference by diverse ions in cathodic stripping analy-
sis of halides are caused by species capable of forming insoluble mercury salts. Metallic ions such as copper, lead, and anions such as phosphate have no effect on the deter-, mination of bromide and/or chloride. Sulfide can seriously interfere with the determination, causing severe distortion of the stripping curve. Iodide can also interfere with the determination; however, this interference depends primarily on the halide composition of the sample. In the case of samples rich in chloride, iodide has a negative effect on the chloride peak which is indicative of destruction of the mercurous chloride deposit (14). When the bromide is the predominant species of the sample, iodides cause a displacement reaction similar to the one observed with bromide-chloride binary mixtures (14), and the procedures described above should also be applicable to the determination of trace iodides and bromides in various mixtures. To date, only measurements of bromide and chloride have been performed by this method. However, similar measurements are possible for iodide and bromide mixtures. Additionally, it should be possible to extend this method to certain mixtures of all three halides.
ACKNOWLEDGMENT We are indebted to James Fennessy for his assistance during the course of this work. Received for review December 7, 1973. Accepted March 18, 1974. This work was supported through a grant to the Atmospheric Analysis Laboratory by the Arizona Mining Association.
Immobilized Enzyme Electrode for the .Determinationof Arginase H. Edward Booker and John L. Haslam D e p a r t m e n t of Chemistry, University of Kansas, L a w r e n c e , Kan. 66044
A dual and sequential enzyme catalyzed reaction is utilized for the determination of arginase, an important metabolic enzyme. The progress of the enzymic reaction is monitored by a cation selective electrode responsive to NH4+, a product of the reaction. Experiments indicate a precision of about 3% with a detection range of about 1.6 to 16 units of arginase an'd an average time requirement of less than 10 minutes per analysis. The change in potential with time is measured, and the amount of arginase 1054
A N A L Y T I C A L CHEMISTRY, V O L . 46, NO. 8, J U L Y 1974
is determined from a previously prepared calibration curve. The utility of this procedure is exemplified by its use to study several parameters which affect the catalytic efficiency of arginase. The results of studies on the effect of pH, temperature, substrate concentration, and immobilized enzyme concentration using this new potentiometric method for arginase are presented. The possibility of extended application is likewise discussed.
