(4) R. P. Buck and R. W. Eidridge, Anal. Chem., 35 1829 (1963). (5) G. A. Philbrick Researches, Inc. Boston, Mass.. "Applications Manual for Phiibrick Octal-Plug-in Computing Amplifiers," 1956. (6) J. S. Huntington and D. J. Davis, Chem. Insfrum., 2, 83 (1969). (7) R. L. Meyers and I. Shain, Chem. Instrum., 2, 203 (1969). (8) R. H. Bull and G. C. Bull, Anal. Chem. 43, 1342 (1971). (9) Chia-Yu Li, Conaid Ferrier, and R. R . Schroder. Chem. Instrum., 3, 333 (1972). (10) R. Bezman and P. S.McKinney. Anal. Chem., 41, 1560 (1969). ( 11) D. F. Untereker, T. M. Riedhammer, W. G. Sherwood, and S. Bruckenstein, submitted for publication. (12) D. 0. Jonesand S . P. Perone, Anal. Chem., 42, 1151 (1970). (13) S.0. Farwell and R. 0. Geer, Abstracts 26th Northwest Regional Meeting of the American Chemical Society, Bozeman, Mont., June 1971, No.
drop, the DME potential was set back to zero, the digital function generator advanced 5 mV, and the above process repeated 4 drops later. Figure 4 gives the average of the twelve sampled currents as a function of potential. The Tast polarograms are quite satisfactory and compare favorably with those obtained using dedicated instruments (23).
CONCLUSIONS In one mode of operation, the digitally controlled function generator yields cyclic voltammograms at solid electrodes indistinguishable from those obtained using a conventional analog generator. Replicate electrochemical experiments give an absolute voltage reproducibility limited by the voltage recording technique, rather than the function generator accuracy, which is several orders of magnitude better than that typically obtained with analog integration techniques. This feature is especially useful when signal averaged voltammograms are required. Complicated reproducible voltage programs can be readily produced under computer control by the digital function generator. In the manual mode, the function generator is easy to use and also is highly reproducible.
LITERATURE CITED (1) Dennis C. Johnson, Department of Chemistry, Iowa State University, Ames, Iowa, personal communicatlon, 1968. (2) W. L. Unerkofler and I. Shain, Anal. Chem., 35, 1778 (1963). (3) G. Lauer, H. Schlein, and R. Osteryoung, Anal. Chem., 35, 1780 (1963).
95
6'1.Connor, G.
(18) (19) (20) (21) (22) (23)
H. Boehme, C. J. Johnson, and K. H. Poole, Anal. Chem., 45, 437 (1973). J. S. Springer, Anal. Chem., 42 (8) 22A (1970). D. Napp, D. Johnson and S. Bruckenstein, Anal. Chem., 39,481 (1967). D. Untereker, Ph.D. Dissertation, State University of New York at Buffalo, 1973. M. Z. Hassan, Anal. Chem., 43, 7 (1971). W. G. Sherwood and S. Bruckenstein, unpublished work, 1973. D. F. Untereker. T. M. Riedhammer, W. G. Sherwood. and S. Bruckenstein, unpublished work. S. Brummer and A. C. Makrides, J. Nectrochem. SOC., 111, 1122 (1964). S. Cadle, Ph.D. Dissertation, State University of New York at Buffalo, Buffalo, N.Y., 1972. P. 0. Kane, J. Polarogr. Soc., 8, 10 (1962).
RECEIVEDfor review April 22, 1974. Accepted September 5, 1974. This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research USAF, under Grant No. AFOSR-74-2572.
Simultaneous Determination of Cyanide and Sulfide with Rapid Direct Current Polarography D. R. Canterford' Deparfment of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
Rapid (short controlled drop time) dc polarography provides a simple method for simultaneous cyanidehulfide analysis. The rapid method is superior to conventional dc polarography since it extends the sulfide concentration range over which these two species can be simultaneously determined. The optimum supporting electrolyte pH is In the range 9 to 10. At higher pH values, the anodic hydroxide wave interferes with the cyanide wave. Detection limits are 5 X 10-sM (cyanide) and 4 X 10-6M (sulfide). With the shortest drop time used (0.16 sec), up to 5 X 1OV2Mcyanide and 5 X 10-3Msulfide could be tolerated. With shorter controlled drop times, higher concentrations of sulfide could be tolerated. Partial or complete coverage of the electrode by a film of HgS has no effect on the cyanide limiting current. Iodide and thiosulfate interfere with the determination of cyanide.
Because of their extreme toxicity, both cyanide and sulfide are of importance in water quality control programmes. With most analytical techniques, cyanide and sulfide interfere with each other, which obviously precludes IPresent address, Research Laboratory, Kodak (Australasia) Pty Ltd, P.O. Box 90, Coburg, Victoria 3058, Australia. 88
their simultaneous determination. For example, the ionselective electrode ( 1 , 2 ) responds to both species, as do many amperometric (3, 4 ) , spectrophotometric (5, 6), and spectrofluorimetric (7, 8) methods. Distillation is usually an integral part of the procedure for the determination of both species (9). Although distillation removes many interfering materials it does not separate cyanide and sulfide from each other. For cyanide analysis, the standard method of removing sulfide is by precipitation as a heavy metal sulfide (e.g., PbS) (9).However, an appreciable amount of cyanide can be occluded by the precipitate, resulting in low cyanide values ( 1 0 ) . This procedure should therefore be avoided if possible. Cyanide and sulfide both give anodic polarographic waves corresponding to mercury compound formation. The rapid polarographic technique, which is based on the mechanical dislodgment of mercury drops from a capillary at short time intervals, possesses a number of advantages over conventional direct current (dc) polarography for such processes (11). A t normal drop times, sulfide produces up to four dc waves spread over a very large potential range. Under rapid conditions, however, fewer sulfide waves are recorded, which reduces the possibility of interference from other anions which also give anodic waves. The determination of sulfide in the presence of cyanide was used to illustrate this advantage of the rapid technique (12).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
Karchmer and Walker (10) reported that conventional dc polarography could be used to determine cyanide in the presence of sulfide only if there was no more than a 10-fold excess of sulfide and if the sulfide concentration did not exceed 5 X 10-4M. T o overcome these restrictions, they proposed a very complicated procedure which involved the precipitation of the bulk of sulfide by titration with a slightly less than stoichiometric amount of silver nitrate, followed by the application of an empirical correction factor to account for the cyanide occluded by the silver sulfide precipitate. Previous work (12) suggested that the rapid polarographic technique would provide a simple alternative to this procedure and, as an obvious extension, could be used for the simultaneous determination of cyanide and sulfide over wide concentration ranges. The results of an investigation of this aspect of the rapid polarographic method are reported here.
Reagent grade chemicals were used without further purification. Concentrated cyanide and sulfide stock solutions were prepared using de-oxygenated, triply distilled water. Sulfide solutions were stored under argon. Small aliquots of stock solution were transferred by microsyringe to a known volume of supporting electrolyte which had previously been deaerated with oxygen-free argon and thermostated at 25.0 "C. Argon was passed over the surface of the solution while polarographic measurements were being made. Short controlled drop times of 0.16 and 0.32 sec were obtained with a Metrohm Drop Controller E 354 S. Polarograms were recorded with a Metrohm Polarecord E 261. A silver/silver chloride (1MNaC1) reference electrode and a platinum wire auxiliary electrode were used. A n EIL Model 46A pH meter was used to measure supporting electrolyte pH values.
RESULTS AND DISCUSSION The direct current polarographic behavior of sulfide ion in several supporting electrolytes has been discussed elsewhere (13).Kolthoff and Miller (14) investigated the oxidation of mercury in the presence of cyanide ion. According to them, the electrode process in dilute sodium hydroxide (0.01 to 0.lM) was
+
+
2e
(1) However, Tanaka and Murayama ( 1 5 ) subsequently reparted that a t high pH (e.g., in 0.1M NaOH) the reactions Hg Hg
+ +
2CN-
Hg(CN)2
3CN- e Hg(CN)34CN-
==
I
a
6
12
10
PH
Figure 1. Dependence of cyanide half-wave potential ( E T l 2on ) pH
EXPERIMENTAL
Hg
i
+
2e
Hg(CN)42- + 2e
(2)
(3) must also be considered, with the three reactions being in dynamic equilibrium a t the surface of the electrode. These authors also showed that the electrode process is described by Equation 1 alone, only at lower pH values where formation of Hg(CN)3- and Hg(CN)d2- is negligible because of the low activity of cyanide ion. As with sulfide, the potential of the cyanide wave depends not only on its concentration but also on the pH of the supporting electrolyte. Furthermore, hydroxide ion itself gives an anodic wave, the potential of which depends on its concentration (14). Thus, before considering the simultaneous determination of cyanide and sulfide, it was necessary to establish the optimum supporting electrolyte in which to record polarograms. Optimum Supporting Electrolyte. With a direct method of determining cyanide and sulfide, such as the present one based on anodic polarographic waves, it is important that the pH of the sample be maintained a t as high a value as possible during any manipulation of the solution and while the polarogram is being recorded. This is to ensure that loss of cyanide and sulfide as volatile HCN and H2S is minimized. With sulfide, this presents no difficulty. As pre-
Closed circles, data from Tomes ( 76): open circles, this work (potentials reiative to Ag/AgCI)
1
-w
U
41
VOLT
vs
41
rb!
re/&&l
Rapid dc polarograms of 4 X l @ M cyanide in ( a ) 0.1M NaOH, and (b) NaOH/H3B03solution (pH 9.75) Figure 2.
Drop time = 0.24 sec. Polarograms recorded in supporting electrolyte only are shown in each case
viously shown (12), the anodic sulfide wave(s) occur at a much more negative potential than the hydroxide wave and the total limiting current for sulfide is independent of p H over a wide range. Thus, if a distillation procedure was used with the sulfide being collected in 0.1 or 1M NaOH, sulfide analysis could be carried out directly in such media. The cyanide wave, however, is closer to the hydroxide wave and the possibility of interference must be considered. Tomes (16) studied the reduction of Hg(CN)P ( i e . , the reverse reaction) in a number of supporting electrolytes. His results for the dependence of cyanide half-wave potential on pH, together with some data collected in the present work with the rapid technique, are plotted in Figure 1. These results indicate that the potential of the cyanide wave is independent of p H above pH 9 or 10. Figure 2a shows the polarogram of 4 X 1OAsMcyanide in 0.1M NaOH. For comparison, the polarogram recorded in the supporting electrolyte alone is also shown. Contrary to the claim of Kolthoff and Miller (14), the limiting current plateau for cyanide is not well defined before the start of the hydroxide wave and this medium is obviously unsuitable for the determination of low cyanide concentrations. By lowering the p H to 9.75 with boric acid (Figure 2b), the hydroxide wave is shifted to a considerably more positive potential without changing the position of the cyanide
ANALYTiCAL CHEMISTRY, VOL. 47, NO. 1 , J A N U A R Y 1975
89
-c
-03 VOLT
YS
1
AglAgC
i
Figure 3. Rapid dc polarograrns of (a) 6 X 10-6Mcyanide, and (b)
w
supporting electrolyte only
i
I
Drop time = 0.32 sec Q1
45
VOLT
9.9
10.8
0.185 0.648 1.11 0.200 0.700 1.20 0.208 0.729 1.25
0.799 2.75 4.74 0.836 3.00 5.11 0.913 3.12 5.35
.a
,
Ao/neCi
Figure 4. Conventional dc polarograrns of cyanide and sulfide
Table I. Dependence of Cyanide Limiting Current ( i c y - ) on Cyanide Concentration ( C C- ~) and pH PH= C C ~ , r>u ‘CN-, J l A zch- c r y - , I/ 2 1mhf-l
9.4
4!
4 VI
(a) 1 X 10-3Mcyanide, (6)2 X 10-3Msulfide, X 10-3Msulfide. Drop time = ca. 3 sec
4.32 4.24 4.27 4.18 4.29 4.26 4.39 4.28 4.28
(c)1 X 10-3Mcyanide and 2
i
0 Supporting electrolyte consisted of 0 1M NaOH and 0 5M NsB03 in tarlous racios
J i
10pr
I
(C)
wave. Under these conditions, the limiting current plateau becomes well defined and the wave is much more suitable for analytical purposes. Karchmw and Walker ( I O ) recommended a pH of 10.8 for determination of cyanide with conventional dc polarography. In the present work, it was observed that better separation of the cyanide and hydroxide waves could be obtained if the pH was somewhat lower than this-e.g., in the range 9 to 10. Since the hydroxide wave shifts by about 30 mV in a positive direction for a ten-fold decrease in hydroxide concentration ( I 4 ) , it would be predicted from the results in Figure 1 that maximum separation of the cyanide and hydroxide waves would occur in this pH range. Prior to polarographic analysis of cyanide, Karchmer and Walker adjusted the pH of their supporting electrolyte to within f 0 . 2 unit by titration with boric acid in the presence of a glass electrode ( I O ) . Since it has been established that the limiting current for cyanide is independent of pH, at least over the range 9.4 to 10.8 (Table I), it appears that it is not necessary to control the pH so carefully. Table I also shows that the limiting current is linearly dependent on cyanide concentration. In the remaining work, a supporting electrolyte of pH 9.75 was used. This was prepared from 0.1M NaOH and 0.5M H3B03 in the ratio of 4:l.At this pH, the activity of cyanide ion is probably sufficiently high for Equations 2 and 3 to contribute to the overall electrode process. Figure 3b shows the background current recorded in the presence of the supporting electrolyte. This polarogram 90
ANALYTICAL CHEMISTRY, VOL. 47,
4
’
I7
u
4 VOLT
VI
41
4!
AV-1
Figure 5. Rapid dc polarograrns of cyanide and sulfide Concentrations as in Figure 4. Drop time = 0.32 sec
was obtained at the maximum sensitivity setting of the instrument and with no recorder damping. Also shown is the polarogram recorded under identical conditions for 6 X 10-6M cyanide (Figure 3a). The smoothness and lack of “noise” on these polarograms should be noted. This is generally found with the rapid technique where the drop time is controlled precisely, as long as the apparatus is free from external vibrations. Often, the detection limit of a technique is defined as the concentration that produces a signal equivalent to twice the magnitude of the fluctuations in the background ( I 7). More rigorously, the detection limit can be defined as the smallest quantity or concentration of a substance that can be detected at a chosen probability level (18). Such definitions are useful for atomic absorption, Xray, and radiochemical techniques where there is usually a large amount of random background “noise.” However, the difficulty in applying them to the rapid polarographic method will be appreciated from the behavior shown in Figure 3. In the present case, the cyanide concentration was gradually increased until a definite wave could be observed by comparison with the polarogram recorded for the supporting electrolyte only. Using this procedure, the detection limit was estimated to be about 5 X 10-6M ( i e . ,approx. 0.1 mg/l.). The useful quantitative range extended
NO. 1 , JANUARY 1975
Table 11. Effect of HgS Film on Cyanide Limiting Current ( ~ c N - ) cs
2 3
‘\I
... 1.0 x 3 . 0 X lo-” 1.25 x 1.50 x loe3 a
Number of sullide w a v e s .
D
.
one one two two
Coverage of electrode by HyS f i l m
i CN -, u~~
... partial partial complete complete
2.56 2.61 2.62 2.60 2.57
Average of three measurements on 1 X 10- SM cyanide.
down to about 1 X 10-5M cyanide. For sulfide, as shown previously (12),the detection iimit, with the rapid method is 4 x 10-6M with the minimum concentration for quantitative analysis being about 7 X 10-6M. Simultaneous Cyanide a n d Sulfide Determination. Comparison of Figures 4 and 5 illustrates clearly how the sulfide concentration range over which cyanide and sulfide can be simultaneously determined is extended by using the rapid polarographic method. Under conventional conditions, for this particuiar sulfide concentration, the cyanide wave occurs before the total limiting current plateau for sulfide is reached and determination of either species is not possible (Figure 4). With the rapid technique, however, fewer sulfide waves are recorded and both species can be readily determined from the same polarogram (Figure 5 ) . Previously ( I 2 ) ,the highest sulfide concentration investigated with the rapid technique was 2 X 10-3M. It has now been observed that at controlled drop times of 0.32 and 0.16 sec, a third sulfide wave appears a t concentrations above 3.5 X 10-3 and 5 X lO-3M, respectively. The total limiting current remained linearly dependent on concentration up to a t least 10-2M sulfide in both cases. The appearance of this wave indicates that the sulfide concentration is sufficiently high for the formation of two monomolecular layers of HgS on the electrode surface during drop life. Since the third sulfide wave occurs a t about the same potential as the cyanide wave, 5 X 10-3M sulfide is the maximum concentration that can be tderated for sirnultaneous cyanide/sulfide anaiysis with the apparatus used in this work. Higher concentrations of sulfide could be tolerated by using even shorter drop times, such as those in the millisecond range employed by Cover and Connery (19), since the concentration at which the third sulfide wave first appears increases with decreasing drop time. The maximum cyanide concentration that can be tolerated is 5 X 10-2M. Above this level, the cyanide wave, because it shifts to more negative potentials with increasing concentration, begins to overlap the sulfide waves. The presence of a second sulfide wave ( e g . , Figure 5b) indicates that from the plateau of the first wave onward, the electrode is covered by a monomolecular layer of HgS (13). In effect, therefore, cyanide analysis in Figure 5c is being carried out a t a mercuric sulfide coated mercury electrode rather than a t a conventional mercury electrode. The results in Table I1 show that partial or complete coverage of the electrode surface by HgS has no effect on the limiting current for cyanide. On the plateau of the second sulfide wave, the electrode process is limited by diffusion of sulfide (S2or HS-) to the electrode surface rather than on transport of “mercury” through the layer of HgS (13, 20). If any cyanide is present, then the overall process is limited by diffusion of cyanide to the electrode surface. Strictly, the term “diffusion” should not be used when discussing the rapid technique, since it has recently been observed that solution stirring results from capillary vibration with this method (22).However, in the present context
Figure 6. Rapid dc polarograms of 1 X 10-4M cyanide in (a) presence and ( b )absence of 1.5 X 10-3Msulfide Drop time = 0.16 sec
L
___ [CN-1
x
ia’i M
figure 7. Calibration curve for low cyanide concentrations in the presence of 1.5 X 10-3Msulfide Drop time = 0.16 sec
it is used in a broad sense to distinguish the type of electrode process under discussion from others such as kinetically controlled or adsorption controlled processes. Although the processes studied in this work are not strictly diffusion controlled under rapid conditions, this does not prevent the waves from being used for anaiytical purposes. Meites (22) has discussed the important and general problem of determining a minor constituent (A) in the presence of a major constituent (B) where the wave of A is preceded by that of B. In this case, it becomes difficult to distinguish and measure the small increase in current due to A from the large current due to B, even when the wwes are well separated. According to Meites, the most satisfactory solution to this problem is to choose a supporting electrolyte in which the order of the waves is reversed--i e., wave of minor constituent precedes wave of major constituent. Under these conditions, the minor constituent can be readily determined. (These comments, of course, refer to dc polarography. With derivative-like techniques such as ac polarography (23),the problem is not as serious since the current generally returns to the base value after each peak). With simultaneous determination of cyanide and sulfide, the order of the waves cannot be changed. In all supporting electrolytes, the anodic sulfide wave occurs a t a more negative potential than the cyanide wave. Therefore, the determination of small concentrations of cyanide in the presence of excess sulfide was investigated in more detail. Figure 6 shows the wave for 1 X 10-4M cyanide recorded in the presence and absence of 1.5 X 10-3M sulfide. Both polarograms were obtained at the same drop time and without recorder damping. This diagram shows that al-
ANALYTICAL CHEMISTRY, VOL. 47, NO. l , JANUARY 1975
91
fide also give anodic polarographic waves corresponding to mercury compound formation. The possibility of interference by a number of these species on the determination of sulfide with the rapid method has been investigated previously (12). The effect of these anions on the cyanide wave has now been studied. The I- and Sz032- anodic waves are at sufficiently negative potentials to overlap the cyanide wave and prevent its determination. However, the other anions tested produce waves at more positive potentials and cyanide can be determined in the presence of a t least a 10-fold excess of Br- or s03'-, 100-fold excess of SCN-, and 200-fold excess of C1-. Any I- or Sz03*- present in a sample could be separated by distillation.
I
E-=
Figure 8. Rapid dc polarograms of 2 X 10-5M cyanide in (a) absence and (b) presence of 1.5 X 10-3Msulfide
CONCLUSIONS This investigation has shown that rapid dc polarography provides a very simple method of simultaneous cyanide/ sulfide analysis. This technique could be used to determine these species either directly or in conjunction with the standard distillation procedure, provided the pH of the solution is adjusted to be within the range 9 to 10. Although this p H is lower than that suggested by Karchmer and Walker (IO),loss of cyanide and sulfide by volatilization is not considered a problem since it requires only about 40 seconds to record a complete polarogram such as that in Figure 5c. As previously discussed (121, rapid dc polarography is not as sensitive as some of the other techniques developed for sulfide analysis. The same applies to the determination of cyanide. However, since the rapid technique reduces the need for chemical separation of cyanide from sulfide, a procedure which is both time consuming and liable to introduce errors, it is suggested that this method may prove of use in many practical situations, for example, the monitoring of industrial effluents.
Drop time = 0.16 sec
Table 111. Reproducibility of Cyanide Limiting Current in the Presence a n d Absence of Sulfide Relative mean deviation
CCN-, M
1 x 10-3 8
X
Cs2-,Y
ofiCN-, s a
...
0.90
1 . 5 x 10-3
1.1
1 . 5 x 10-3
0.95 3.4
...
10-'
Five replicate runs.
though the sulfide and cyanide waves are well separated and therefore do not interfere directly, the shape of the cyanide wave is influenced by the presence of this large concentration of sulfide. In the absence of sulfide (Figure 6 b ) , the base line (or residual current) and limiting current plateau are parallel. However, in the presence of sulfide (Figure 6a), they are no longer parallel and therefore the current value obtained depends on the extrapolation procedure used to measure this parameter. For this reason, it is recommended that for accurate determination of low concentrhtions in the presence of excess sulfide, standards containing the appropriate concentration of sulfide, or, more conveniently, the addition method should be used. Figure 7 indicates how a linear calibration curve can be obtained down to low cyanide concentrations in the presence of a large excess of sulfide (1.5 X 10-3M), provided a standard procedure is adopted for measurement of the wave height. For high concentrations of cyanide, this species can be determined by direct comparison with cyanide standards whether or not sulfide is present (see Table 11). Another factor to be considered when determining cyanide in the presence of sulfide, which gives a preceding wave, is the reproducibility of the cyanide wave. Figure 8 shows rapid dc polarograms of 2 X lO-5M cyanide in the presence and absence of 1.5 X 10-3M sulfide. Again, both polarograms were obtained without recorder damping. A much higher noise level in the presence of sulfide is evident. This is not surprising in view of the large current flowing prior to the cyanide wave in this case. Some reproducibility data for cyanide is shown in Table 111. These results show that for 8 X 10-5M cyanide the relative mean deviation of five replicate runs increased from 0.95% in the absence of sulfide to 3.4% in the presence of 1.5 X 10-3M sulfide. However, for 1 X 10-3M cyanide, reproducibility was little affected by the presence of sulfide. Interferences. Many anions other than cyanide and sul-
92
*
LITERATURE CITED (1) R. Naumann and C. Weber, fresenius' 2. Anal. Chem., 253, 111 (1971). (2) J. Vesely, 0. J. Jensen, and B. Nicolaisen, Anal. Chim. Acta, 62, 1 (1972). (3) J. A. McCloskey, Anal. Chem., 33, 1842 (1961). (4) G. W. Miller, L. E. Long, G. M. George, and W. L. Sikes, Anal. Chem., 36, 980 (1964). (5) S. Komatsu, T. Nomura. and T. Ito, Nippon Kagaku Zasshi, 90, 171 (1969). (6) R. E. Humphrey and W. Hinze. Anal. Chem., 43, 1100 (1971). (7) L. S. Bark and A. Rixon, Ana/yst (London), 95, 786 (1970). (8) F. Vernon and P. Whitham, Anal. Chim. Acta, 59, 155 (1972). (9) "Standard Methods for the Examination of Water and Wastewater." 13th ed., American Public Health Association, New York, N.Y., 1971. (IO) J. H. Karchmer and M. T. Walker, Anal. Chem., 27, 37 (1955). (11) D. R. Canterford, A. S. Buchanan, and A. M. Bond, Anal. Chem., 45, 1327 (1973). (12) D. R. Canterford, Anal. Chem., 45, 2414 (1973). (13) D. R. Canterford and A. S. Buchanan, J. Electroanal. Chem., 45, 193 (1973). (14) I. M. Kolthoff and C. S. Miller, J. Amer. Chem. SOC.,63, 1405 (1941). (15) N. Tanaka and T. Murayama, Z. Physik. Chem., (Leipzig) 11, 366 (1957). (16) J. Tomes, Collcct. Czech. Chem. Commun., 9, 81 (1937). (17) J. Ramirez-Munoz, "Atomic-Absorption Spectroscopy," Elsevier, Amsterdam, 1968. p 221. (18) R. Gabrieis, Anal. Chem., 42, 1439 (1970). (19) R. E. Cover and J. G. Connery, Anal. Chem., 41, 1797 (1969). (20) R. D. Armstrong. D. F. Porter, and H. R. Thirsk, J. Phys. Chem., 72, 2300 (1968). (21) D. R. Canterford, unpublished work, 1973. (22) L. Meites, "Polarographic Techniques," Interscience, New York, N.Y., 1965, p 162. (23) B. Breyer, F. Gutmann, and S. Hacobian, Aust. J. Sci. Res., Ser. A, 3, 567 (1950).
RECEIVEDfor review April 18, 1974. Accepted August 5, 1974.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
