Elimination of Copper-Zinc Intermetallic Interferences in Anodic Stripping Voltammetry T. R. Copeland,' R. A. Osteryoung,2 and R. K. Skogerboe2 Department of Chemistry, Colorado State University, Fort Collins, Colo. 8052 7
A method of eliminating interference effects in anodic stripping voltammetry due to the formation of a copper-zinc intermetallic has been devised. The elimination involves utilization of specific procedures which include the addition of gallium to preferentially form a copper-gallium intermetallic rather than the copper-zinc compound. The use of gallium for this purpose has led to a preliminary study of the internal standard method as a means of quantitating anodic stripping results. Both gallium and thallium have been applied as internal standards for the determination of cadmium, copper, lead, and zinc. The results indicate that several advantages may be realized through the use of the internal standard method.
Anodic stripping voltammetry (ASV) has been established as a sensitive analytical method. Reports have also shown that pulse voltammetric stripping analysis offers improved sensitivity in comparison to linear scan techniques particularly when thin film mercury electrodes are used (1-7). Like the majority of analytical techniques, however, interference effects which cause errors may be operative. The extent of such effects is determined by the composition and the compositional variability of the analytical matrices. Among the interference effects that can occur are those due to the formation of intermetallic compounds in the mercury electrodes employed (7-13). Intermetallic compound formation is particularly liable to be significant in thin film mercury electrodes because of the large change in concentration (cn. lo6) which can take place on deposition of the metals into the film. While several reports (8-10) have dealt with the wellknown copper-zinc intermetallic formation, this interference effect has not been widely recognized in the ASV applications literature. Copper and zinc are reasonably ubiq-
'
Present address, Department of Chemistry, Indiana University, Bloomingtori, Ind. 47401. e Authors to whom requests for reprints should he sent. (1)J. B. Flato, Anal. Chem., 44 (ll),75A (1972). (2)H. Siegerman, G.O'Dom. and J. B. Flato, in "Electrochemical Contribu-
tions to Environmental Protection." T . R. Beck, Ed., The Electrochemical Society, Inc., Princeton, N J.. 1972,p 76. (3)H. Siegerman and G. O'Dom. Amer. Lab., 4 (6), 59 (1972). (4)Ei. P. Parry and D. H. Hern, Joint Conference on Sensing of Environmental Pollutants, Palo Alto, Calif., Nov. 8-11, 1971,Paper 71-1119. (5)G. D. Christian, J. Nectroanal. Chem., 23, 1 (1969). (6)T. R. Copeland, J. H. Christie, R. A. Osteryoung, and R. K. Skogerboe, Anal. Chem., 45, 995 (1973). (7)/bid., p 2171. (8) R. S.Rodgers, Ph.D. Thesis, Clarkson College of Technology, Potsdam, N.Y.. 1970. (9)A . I. Zebreva and M. T. Kozlovskii, Zhur. Fiz. Khim. 30, 1553 (1956): Chem. Abstr., 51, 2421 (1957). (10)W. Kemula, Z. Galus, and Z. Kublik, Bull. Acad. Pol. Sei., Ser. Sci. Chim., Geol. Georgr., 6,661 (1958);Chem. Abstr., 54, 17116 (1960). ( 1 1 ) 0.S. Stepanova, lzv. Tomsk. Politekh. Inst., 151, 14 (1966);Chem. Abstr., 67,87214 (1967). (12)0.S. Stepanova, M. S. Zakharov, L. F. Trushina, and V. I. Aparina, lzv. Vyssh. Ucheb. Zaved. Khim. Khim. Tekhnol., 7 (2),184 (1964);Chem. Abstr., 61, 11620 (1964). (13)M. S. Zakharov, Zh. Anal. Khim., 18, 450 (1963);Chem. Abstr., 59, 4793 (1963).
uitous elements; both are found in a diversity of analytical sample types particularly those of environmental interest. Consequently, the formation of the intermetallic poses a serious problem in the determination of either element by ASV unless certain analytical conditions are utilized. This report presents results which indicate the extent of the interference problem, summarizes the analytical conditions required for the determination of copper by ASV, and presents a simple, reliable means for eliminating the effect to permit the determination of zinc. This elimination is based on the addition of gallium to preferentially form a Cu-Ga intermetallic (11-14) inst,ead of the Cu-Zn compound. Consideration of this procedure has also lead to the investigation of the applicability of the internal standard method to ASV measurements. Thus, both gallium and thallium have been investigated as internal standards for ASV. These preliminary studies have shown that the internal standard approach offers a simplified means for quantitating ASV measurements.
EXPERIMENTAL Apparatus. All experiments were performed using a Princeton Applied Research (Model 174) Polarographic Analyzer. modified and optimized as previously described ( 6 ) ,in conjunction with a rotating electrode (Beckman, Model 1885OlW). The ciirrent/potential curves were recorded on a Hewlett-Packard s - y recorder (Model 7030AM). T h e cell was a 100-ml Berzelius beaker held with a nylon cell holder (Leeds & Northrup). T h e reference electrode was AglAgC'1 (0.1M NaCI) isolated from the sample solution hy a porous glass plug (Corning Glass, Vycor No. 7930). The counter electrode was platinum foil. The working electrode was a thin film mercury rlectrode on a vitreous carbon substrate. T h e preparation of these electrodes has been reported (6'). Reagents. All metal standards were prepared from high purity metals dissolved in nitric acid doubly distilled from quartz and diluted with distilled, deionized water to maintain a minimum acid concentration of 1M. The acetic acid/potassium acetaw (1.7M; 1.25M) supporting electrolyte buffer was prepared from A.R. grade glacial acetic acid primary stardard grade K&O?, and distilled, deionized, quartz redistilled water. Exhaustive electrolysis was used to purify the supporting electrolyte. Procedure. Natural waters, EPA water standards, and aqueous metal standards were prepared for analysis by the addition of 2 ml of the acetate buffer to 25 ml of the sample solut,ion. The whole blood and NBS leaf and liver samples analyzed with thallium as the internal standard were freeze dried. A weighed aliquot was ashed by starting a t room temperature and slowly raising the oven temperature to 400 "C over a period of 3 hours to avoid loss of trace elements. Sample solutions were deoxygenated with prepurified nitrogen while in position in the cell assembly. T h e initial potential (-1.3 V) was set and electrode rotation of 3600 rpm initiated. Plating was carried out for 2 minutes. After plating, the nitrogen stream was diverted to purge the cell space above the solution, rotation was stopped, and the potential scan (5 mV/sec) was started following a 10-second delay. Each scan was halted at -0.5 V a t which time electrode rotation and nitrogen bubbling were resumed. Microliter additions of the internal standard used were then made to the solution to obtain concentrations of 40 and 80 ng/ml gallium or 4 or 8 ng/ml thallium. After a brief period (-1 minute) t o allow (14)M. S. Zakharov. 0. S. Stepanova, and V. I. Aparina, Izv. Tomsk. Poiitekh. lnst., 128, 36 (1964).Chem. Abstr., 64,13761 (1966).
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n
Table I. 65ZnActivity of the Mercury Film under Different Plating and Stripping Conditions
IloM
Percent activity lost from the solution %percent Zn in film Plating procedure
Shipping procedure
None 10 min at 1.3 V 10 min at -1.3 V 10 min at -1.3 V
None None 5 min at -0.9 V 5 min at -0.1 V
N o Ca
80 n g / m l Ga 4
3
4.50 2.85
0 4.90 0
0
0
2
0
I
mixing of the added solution, the plating and stripping procedures were then repeated. The initial analysis without internal standard present provides a reliable measure of the background, verifies the absence of the internal standard element a t a measurable level, and allows a preliminary examination of possible peak overlap problems. If the background were known and the ahsence of the other problems could be reliably assumed, the internal standard could be added initially.
RESULTS AND DISCUSSION
Previous reports from this laboratory have described improved ASV methods for the reliable determination of cadmium and lead in a variety of sample types (6, 7). In extending these methods to include the determination of copper and zinc in natural water samples, spurious results were often observed for both elements. Erroneously low concentrations were frequently observed for zinc, and calibration by the commonly used method of standard additions often produced nonlinear curves. Concomitantly, the copper results were typically high and nonlinearity in response was also common. These observations implied that the operative interference effect involved both copper and zinc and that it might be attributed to formation of Cu-Zn intermetallic (8-10). Because the method of standard additions did not compensate for the observed effect, it was determined that the internal standard method might be used as a means of elucidation. Said method had been used extensively in spectrochemistry to overcome matrix problems (15, 1 6 ) . The pilot ion technique has been similarly used in polarography (17). In both, an element not of analytical interest is added to the samples to obtain a known concentration. The internal standard element is selected on the premise that it will be affected by the matrix in the same way and to the same extent observed for the analytical species. If the analyte and internal standard reaction properties are appropriately matched, the matrix effects are eliminated; quantitation may be based on the ratio of the analyte response to that of the internal standard; a single (universal) calibration curve is applicable; and standard additions calibration is not required. While the expectation of equivalent elemental reaction properties for ASV may not be entirely realistic, results presented below indicate that some real advantages accrue from the use of the internal standard method. The element initially chosen as the internal standard was gallium. In the supporting electrolyte used, gallium exhibits a well defined stripping peak a t -0.975 V. Thus, significant overlap with either zinc (-1.15 V) or cadmium (-0.745 V) does not occur at mutual concentrations for the three elements at least up to 0.1 Ng/ml in the electrolyte solution. Each of a series of synthetic standard solutions containing varying amounts of zinc, cadmium, and lead in di(15) W. Gerlach. Z.Anorg. Allgern. Chern., 142, 383 (1925). (16) E. F. Scribner and M. Margoshes in "Treatise on Analytical Chemistry," J. M. Kolthoff and P. J. Elving, Ed., Wiley-lnterscience,New York, N.Y., 1965, Part 1, Vol. 6, Chap. 64. ( 1 7) L. Meites, "Polarographic Techniques," 2nd ed.. Wiley-lnterscience, New York, N.Y., 1965, p 404. 2094
-12
.IO
.B
.6
-4
-2
* Au/AKl Figure 1. Stripping curves from a natural water sample illustrating the effects of intermetallic formation VQTS
Curve 1, Initial platelstrip cycle in absence of Ga. Curve 2, Second plate/ strip cycle in absence of Ga. Curve 3, Plate/strip cycle following addition of 40 ng Ga/ml. Curve 4, Platelstrip cycle after adjusting Ga concentration to 80 ng/rnl
verse ratios was run by ASV. Gallium (40 ng/ml) was added to each solution and the ASV procedure repeated. Analytical curves were generated using the ratio of the analyte to gallium peak currents as the response parameters. These curves were linear for all three elements over the entire range investigated (0 to 100 ng/ml) and their use in the analysis of other synthetic standards produced the correct results. Thus, it was concluded that mutual interference effects between these four elements were not significant. When copper was added as the fourth analyte, the results indicated that the previously mentioned interference effect between copper and zinc could be alleviated by the presence of gallium. The effects are illustrated in Figure 1 which presents tracings of four consecutive plate/strip cycles on the same sample. The formation of the Cu-Zn compound causes a reduction in the zinc peak (-10%) and an enhancement of the copper peak (-10%) between runs 1 and 2. The enhancement of the copper peak is due to the fact that the Cu-Zn intermetallic strips at a potential which cannot be resolved from that of copper. Repetitive platehtrip cycles such as these imply, however, that the removal of the intermetallic from the thin film is not complete. Frequently, several such cycles are required before the zinc and copper peaks reach constant levels indicating that the prevailing equilibria have been established. Upon the addition of gallium (40 ng/ml), the zinc peak was restored and the copper peak reduced (run 3). Doubling the gallium concentration caused no further change in the zinc peak, further reduction of the copper peak, and a disproportionate increase in the gallium peak (run 4). The two latter changes are due to the formation of a Cu-Ga intermetallic (11-14). The observed behavior is explained by the fact that the Cu-Ga compound has a larger formation constant than the Cu-Zn intermetallic; gallium consequently preferentially alloys with the copper. This type of behavior has been observed for other intermetallics involving gallium e . g . , (If ). GaAu
+ Cu
-
GaCu
Au
To verify this postulate, a series of experiments was performed using a solution 10 ng/ml in zinc, 30 ng/ml in copper, and spiked with carrier-free radioactive 6jZn. A 1-ml aliquot of the solution was initially counted to determine the zinc activity and returned to the electrochemical cell. A summary of the experiment and the results is presented in Table I. An electrode was plated at -1.3 V for 10 minutes;
ANALYTICAL C H E M I S T R Y . VOL. 46. NO. 1 4 , D E C E M B E R 1974
Table 11. Effect of Copper on Zinc Determination Using the Method of S t a n d a r d Additions
Table 111. Comparative Analytical Results for the Determination of Elements in Water Samples
Zinc concn, n g / m l Copper concn, nglml
2.4
12.4
42.4 42.4 + 400 ng/ml Ga
Actual
6.3 16.3 26.3 46.3 8.9 12.3 18.9 22.3 48.9 8.9 18.9 42.4 16.4 36.4 46.4 66.4
Found
6.3 15.8 25.1 47.4 7.6 6.9 12.8 13.7 43.5 2.4 12.6 11.6 16.4 36.1 45.8 96
Concentration, n g / m l Percent error
Pb
Cd Sample
0 -3.1" -4.6" +2.4" -14.6 -44.0 -32.4 -37.4 -11.1 -73.0 -33.5 -73.0 0 -0.8" -1.3" +44.6
~
NO.
CSU
EP-A
csu
u.4
csu
1 2" 3" 6b
1.9 1.5 7.1 1.7
1.8 1.6 7.3 1.7
25 8.8 27 19
28 9.2 35 20
10 7.7 38 4.7
Zn
_
_ PA
10 7.9 37 5.0
CC
~
_
EPA
9.0 6.7 31.4 7.3
ci Samples diluted 1:10 for analysis. Samples diluted 1:25 for analysis.
Errors within experimental error and therefore not significant.
another aliquot of the solution was withdrawn, counted, and replaced. The electrode was then stripped by a differential pulse scan as would be done in a normal analysis. The electrode was then replated for 10 minutes a t -1.3 V and the potential was changed to -0.9 V and held for 5 minutes. At -0.9 V, no zinc should remain in the electrode since this potential is well anodic of the zinc stripping potential (-1.15 V). Another aliquot was counted, returned and the plating procedure repeated. This time, the potential was held a t -0.1 V for 5 minutes before an aliquot was counted. This potential is well anodic of the copper stripping potential. The entire procedure was repeated after the solution had been made 80 ng/ml in gallium. Any reduction in counts in the solution indicates the presence of zinc on or in the electrode film. In the absence of gallium, some zinc remains on or in the electrode until potentials anodic of the copper stripping peak are reached supporting the theory of a copper-zinc intermetallic which strips a t or near the copper potential. In the presence of gallium, no zinc was present in the film a t potentials anodic of the zinc stripping potential supporting the theorized function of the gallium. Clearly, the existence of the zinc-copper intermetallic poses severe limitations on the determination of either of these elements by stripping voltammetry. This is especially true for thin film electrodes where the preconcentration of metals in the film is higher than for a hanging mercury drop, thereby permitting more extensive intermetallic formation. Copper may be determined by plating a t potentials where zinc is not reduced. However, if the plating potential is then changed to determine the zinc present, the measured responses for both copper and zinc will be affected to an extent dependent on their respective concentrations in the sample. The magnitudes of the error due to this effect that can be encountered in zinc analyses are indicated in Table 11. Here, several zinc solutions were analyzed by the standard additions method in the presence of differing copper concentrations. The presence of minimal copper concentrations (?IO ng/ml) generates large errors in zinc analyses. Errors of over 70% were observed. With excess gallium present, however, the analysis error is remarkably reduced
until zinc and gallium concentrations are reached a t which peak overlap becomes significant ( 3 0 0 ng/ml of each element). While the zinc-copper intermetallic formation is most prominent in mercury thin films, experience has shown that it is also significant for hanging mercury drop electrodes (HMDE). T o establish this, a zinc solution (0.2 wg/ ml) was plated and stripped with an HMDE having an area of 0.025 cm2. Copper was added in microliter amounts to that solution to obtain a concentration of 0.017 pg/ml without significant change of the zinc concentration, and the plate/strip cycle was repeated. The zinc peak height was suppressed 12.6% relative to that without copper present. A further copper addition to increase the concentration to 0.033 Mglrnl resulted in an overall zinc peak suppression of 15.4%. Thus, the effect was of significant magnitude for an HMDE of typical size even a t these relatively low copper concentrations. While an interest in the present work was the possible use of gallium as an internal standard for the analysis of copper, the formation of the Cu-Ga intermetallic clearly restricts its utilization. Copper can be determined, however, by plating a t a potential above that for zinc; then sufficient gallium can be added to completely tie up the copper and permit the use of gallium as the internal standard for the other three elements. The procedure adopted in this study was to add sufficient gallium to the sample to obtain a well defined gallium stripping peak. A t this concentration, x, all the zinc was released from the intermetallic and most of the copper was tied up as an intermetallic. After recording the stripping peaks for this solution, an additional increment of 40 ng/ml gallium was added to ensure that the initial amount added was sufficient to prevent formation of the zinc-copper intermetallic and the complete formation of the copper-gallium compound. The plate/strip cycle was then repeated. The resultant increase in the gallium peak height ( i i x + 4 0 ) - icXJ was divided into the respective peak currents for zinc, lead, and cadmium. These ratios were used to obtain calibration curves. These procedures were used for the analysis of a series of standard water samples from the Environmental Protection Agency ( 1 8 ) . These. samples contained copper concentrations roughly commensurate with the zinc content. A summary of results is presented in Table 111. The agreement between the measured results and those given by the EPA is generally better than 10%. I t may be inferred that gallium serves adequately as an internal standard under the conditions outlined above. The zinc-copper intermetallic complications encountered for zinc analysis even a t relatively low copper concentra(18) Environmental Protection Agency, "Standard Methods of Water and Waste Analysis." May 1971.
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NO. 14, DECEMBER 1974
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Table IV. Comparative Analytical Results for the Determination of Elements Using Thallium as a n I n t e r n a l Standard
4
Concentration, n g / m l
-
Sample type and No.
EPA Waters
OONCWTRATKU ,qlL
Urines
Figure 2. Analytical curves obtained with thallium as the internal
C adni iim
_
_
EPA
CSU
2.1 1.6
1.8 1.6 7.3 1.9' 6.8'
32.0 9.2 34.6 9.1 10.4
1 2a 3"
7.6
1 2
2.5 6.6
Lead
_
CSU
(19) T. R. Copeland, Ph.D Thesis, Colorado State University, Fort Collins, Colo , July 1973.
Whole Blood A 0.2 . . . Tomato Leaf NBS 2.7 . . . Bovine Liver NBS 0 . 2 8 0.27
~~
EP\
CSU
28.0 9.2 35.0 7'
9.8 8.1 30.5
rp.\
10.0
7.9 36.7
gh __
tions appear impossible to avoid without addition of sufficient gallium. Because gallium was not universally applicable as an internal standard but did produce improved results generally indicative of potential advantages of the internal standard approach, further studies were carried out using thallium as the internal standard. Thallium is well behaved electrochemically and is not strongly complexed by most common anions. The stripping potential (-0.670 V) is intermediate between cadmium and lead and minor overlap of these peaks was observed. The thallium peak height, however, was found to be independent of lead or cadmium present in standard solutions in 20-fold excess of the thallium concentration. Each of a series of standard solutions containing zinc, cadmium, and lead was plated and the stripping peaks were recorded. All solutions contained 80 ng/ml gallium to minimize zicc-copper intermetallic formation. Thallium was added (4 or 8 ng/ml) and the thallium stripping peak current was obtained. Calibration curves, presented in Figure 2, of peak current ratios (metalhhallium) us. metal ion concentration, were linear over the 0 to 50 ng/ml range investigated. Using these calibration curves, several water standards and urine samples were directly analyzed. Analyses were also performed on acid digests of NBS tomato leaf and bovine liver standards, and a freeze-dried blood sample. The results presented in Table IV quite consistently agree with the specified concentrations within 10%. The zinc value for the bovine liver sample is in error due to the inordinately high copper content of the sample (193 pg/g). The precision of the method was estimated by repetitive analyses of the same sample solution. The results were reproducible to within 1%as estimated by the relative standard deviation. Ten successive analyses of the same water sample for lead, done over a one-week period, yielded a relative standard deviation of 8%. The precision values obtained in the same manner using the standard additions technique for lead were 1%and 2.9%, respectively (19). I t is worthwhile a t this point to compare the internal standard and standard additions methods. The basic requisites for the proper utilization of the method of standard additions are three-fold. The analyte spikes should experience the same matrix interactions as the natural analyte species; the background a t the analyte stripping potential must be accurately and precisely measureable; and the analytical response must be linear with concentration. Adherance to these conditions has been shown for certain analyses (6). For others, however, these criteria are not satisfied and errors result. Such sources of error may be categorized on the basis of a limiting reagent concept as determined by the interactive concentrations of a matrix component(s) or
2hc
~
Concentration,
standard
2096
_
1.37 4.0 0.38
q / ( j
1.37" 17.2 3.9 29 30 0.34 32 130
'
a Samples diluted 1 : l O for analysis. Comparative data by carbon rod AA. CComparative data from a 3-laboratory roundrobin analysis (18).
the analyte itself. Variations in matrix components can cause changes in plating parameters such as diffusion coefficient values, the viscosity of the analytical solution, or the extent of adsorption of non-analyte species on the mercury surface. Such changes will be reflected by changes in the analytical response unless truly exhaustive electrolysis methods are used. Matrix constituents such as complexing agents, precipitating agents, or intermetallic forming elements may also bind a fraction of the analyte, thereby making that fraction undetectable. In these examples, the matrix components are the limiting reagents. Alternatively, the addition of an analyte spike may induce precipitation, complexation, or intermetallic formation between the analyte and a matrix constituent. A spike addition may also cause the solubility of the analyte in the mercury to be exceeded. The analyte is the limiting reagent for these examples. T o avoid either category of interference, the reactions must be reversible within the time scale of the measurement(s). Several examples can be cited that show a lack of adherence to this requirement. In considering these two general classes of interferences and the use of the method of standard additions where an appropriate range of analyte concentrations is examined, one may deduce which type of effect is operative by examination of the standard additions curve. If a matrix constituent is limiting, a positive deviation from linearity will be observed; if the analyte is limiting the deviation should be negative. Although nonlinearity in ASV measurements is considered by some investigators to be rare, its occurrence indicates non-aderence to one or more of the requisites to calibration by the method of standard additions and that another means of quantitation is necessary. The conditions which must prevail to permit the use of the internal standard method have been outlined above. In considering the application of this method to anodic stripping, it was apparent that it would frequently alleviate or eliminate a t least two of the example interferences discussed above. If the addition of analyte spike(s) could be avoided, the introduction of interferences due to the analyte being limiting could be prevented. For those cases for which the concentration ratio of the analyte to a matrix component determines the magnitude of an interference effect, carrying out an analysis a t a constant analyte-to-interferent ratio might also be advantageous. Further, plating parameter interferences due to viscosity differences
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1 4 , DECEMBER 1974
and the adsorption of non-analytical constituents on the mercury should be obviated. In view of these considerations, the internal standard method offers a t least some advantages that should recommend its use. Certainly such usage does not require extensive additional measurement steps. I t is clear from the data presented herein that the use of the standard additions method for the simultaneous determination of zinc and copper is a dangerous procedure unless the methods outlined above are utilized. When employed in an internal standard procedure, the gallium method was accurate to within 10% in the low copper samples analyzed. Thallium, which is not strongly complexed and forms no intermetallics, also provided results within 10% of the specified concentrations. The time required for multielement analysis is greatly reduced through the use of the internal standard method rather than standard additions procedures. Although the method does not necessarily compensate for all chemical interactions between the matrix and the analyte (it does so only if the standard and the analyte undergo the same reactions with the matrix), it does alleviate problems of analysis which arise from the added spikes necessary for standard addi-
tions, e.g., amalgam saturation. Clearly the method requires investigation over a wider range of analytical circumstances. than considered herein before its full utility can be assessed. The current results indicate that appropriately selected internal standards can be used in at least limited cases as means for simplifying the quantitation of anodic stripping results. Certainly, internal standards can be conveniently used supplementary to the method of standard additions to detect or alleviate the occurrence of the types of errors discussed above. The use of gallium to circumvent the zinc-copper interference in the lower concentration ranges extends the general analytical utility of anodic stripping with mercury thin films and should find wide acceptance. The copper-zinc intermetallic also forms in mercury drop electrodes. The magnitudes of the resultant errors depend on the concentrations of copper and zinc electrolyzed into the drop. Again, the procedures outlined above eliminate the problem. RECEIVEDfor review December 12,1973. Accepted July 22, 1974. Research supported by the National Science Foundation under Grants GI-34813X and GP-31491X.
Reversible MetaVSalt Interfaces and the Relation of Second Kind and All-Solid-state” Membrane Electrodes “
Richard P. Buck and V. Rogers Shepard, Jr. The William Rand Kenan Laboratories of Chemistry, The University of North Carolina, Chapel Hill, N.C. 275 14
On the basis of the electrochemical potential concept, interfacial and overall cell potentials are derived for “all-solidstate,” ionic contact, and second kind electrodes made from metal halide, sulfide, and mixed salt membranes. Results depend upon the different potential determining (electronic or ionic) processes provided by the conducting contact, its reactivity, and solid state properties, primarily elemental defect activities of the membrane. All-solid-state electrodes are predicted and found to show different standard potentials from the other two types unless the salts are equilibrated with component metal in the solid state sense of “complete” equilibrium. Experimental E o values for silver bromide of controlled nonstoichiometry and numerous experimental results for other salts are cited in support of the theory. Justification of known stabilizing procedures for second kind electrodes is found from the theory.
When silver wire is completely imbedded in a AgX or Ag2S crystal, pressed pellet, or cast membrane, a so-called “all-solid-state” selective membrane electrode is formed (1-3) whose response is reversible to solution activities a (Ag+) and a (X-) or a ( S - ) . Inert electronic conductors and metals whose salt formation free energies are positive relative to silver, (e.g., C, Hg, and Pt) behave similarly (4(1) J. Vesely and J. Jindra, Acta lM€KO(Int. Meas. Conf.) Proc., 4th 66-77 (1968). (2) G. M.Farren and J. J. Staunton, Ger. Patent 1,940,353 (Feb. 19, 1970). (3) J. J. Staunton, Ger. Patent 2,002,676 (Nov. 5 , 1970). (4) J. Vesely, 0 J. Jensen. and 6.Nicolaisen, Anal. Chim. Acta., 62, 1 (1972).
8). Electrodes of second kind which consist of lightly-plated porous AgX film on the surface of silver wires also respond reversibly to ionic activities and serve as classical reference electrodes (9, 10). The latter electrodes need not even be plated with salt since constant standard potentials are observed simply when silver wire is dipped into stirred, deaerated solutions, saturated with AgX or AgzS powders. For reasons that are clear from the thermodynamic analysis, second kind electrodes reach equilibrium most rapidly by procedures leading to saturation of the salts with silver and uniform solution activities within the electrode pores. These procedures include current reversal during anodization, thermal-induced-partial decomposition of the salt, and spontaneous self electrolysis among shorted electrodes (9, 10). All-solid-state membrane electrodes, on the other hand, do not allow direct contact between bulk metal and solution; and observed standard potentials can differ significantly from electrodes of the second kind using apparently the same materials (11). This unusual property is not restricted to metal-contacted, single salt membranes, but occurs with the AgpS-based electrodes such as commercially available PbS, CdS, and CuS-Ag2S mixed sulfide, cationselective compositions (8, 11-14), the selenide- and telluA. Marton and E. Pungor, Anal. Chim. Acta., 54, 209 (1971). J. Ruzicka and C. G. Lamm, Anal. Chim. Acta., 53, 206 (1971). J. Ruzicka and C. G. Lamm, Anal. Chim. Acta., 54, 1 (1971). M. Koebel, Orion Research, Inc. private communication, 1973. D. J. G. lves and G. J. Janz, “Reference Electrodes,” Academic Press, New York, N.Y., 1961, Chap. 4. (10) G. J. Janz and D. J. G. Ives, Ann. N.Y. AcadSci, 148, 210 (1968). (11) G. Trumpler, Z.Phys. Chern., 99, 9 (1921). (5) (6) (7) (8) (9)
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