Advantages of Rapid Direct Current Polarography in the Presence of Analytically Undesirable Phenomena Associated with Anodic Mercury Waves D. R . Canterford and A. S. Buchanan Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
A. M. Bond Department of lnorganic Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
Analytical polarographic methods based on anodic waves corresponding to the formation of mercury compounds are common. However, at normal drop times, abnormal phenomena associated with t h e presence of reaction products on t h e electrode surface may give rise to adverse effects such a s maxima, erratic drop behavior, or nonlinear calibration curves, and therefore may prevent collection of useful analytical data. An investigation of t h e anodic behavior of chloride, iodide, sulfide, sodium diethyldithiocarbamate, and dithiothreitol u n d e r rapid polarographic conditions has shown that analytically undesirable behavior d u e to s u c h phenomena may be simply eliminated by using short controlled drop times. At a drop time of 0.16 sec well defined waves were obtained in each c a s e and limiting current-concentration plots were linear over a wide concentration range. The added advantage of t h e much shorter time required to record a ' polarogram makes the rapid d c technique particularly suitable for routine analysis of such systems.
One of the disadvantages of conventional polarography, compared with some other analytical techniques, has been that the natural drop times of the dropping mercury electrode (DME)-usually between 2 and 8 seconds-necessitate reasonably slow scan rates of potential, and thus the time required to record a polarogram is correspondingly long. The use of short controlled drop times permits the application of fast scan rates of potential. In routine analytical applications of polarography, the resultant shorter recording times possible with this rapid technique may be of considerable advantage if a large number of analyses are to be performed. However, a fast scan rate of potential is by no means the only advantage of the rapid technique, and as was pointed out in a recent review ( I ) , the method has probably yet to receive the attention it deserves. Cover and Connery (2), using a vibrating dropping mercury electrode (VDME) with drop times as short as 5 msec, observed that maxima of the first and second kinds were suppressed without the addition of surfactants, and that catalytic and kinetic waves could be minimized or eliminated. Inhibition of DME response by adsorption of species on the electrode surface may prevent collection of useful analytical data with polarography. Adverse effects of such inhibition often include nonlinear response to concentration of electroactive species, erratic drop behavior, or polarographic waves so distorted that meaningful current mea(1) A . M . Bond,J. Electrochem. SOC.,118,1588 (1971). (2) R. E. Cover and J. G. Connery, Anal. Chem., 41,918 (1969).
surement is prevented. Elimination, or a t least minimization, of these effects is obviously desirable. Connery and Cover (3) examined, with their VDME, a number of systems where adsorption phenomena were present a t the DME. They considered several cases of adsorption of electroinactive species and two examples where the product of the electrode reaction was adsorbed, and showed the superiority of the VDME over the DME as an analytical tool for these systems. Another phenomenon which inhibits DME response, and with which is often associated analytically undesirable behavior similar to that described above, is the formation of insoluble reaction products on the electrode surface (4); a situation which is particularly relevant to anodic polarographic waves corresponding to the formation of mercury compounds (5, 6). Abnormal behavior associated with this important class of electrode process was not examined by Connery and Cover. In view of the fact that this group includes the halides, sulfide, and many organosulfur compounds (e.g., dithiocarbamates), an investigation of the feasibility of using the rapid technique to overcome problems due to the formation of mercury compounds on the electrode surface was undertaken. Endeavors to improve analytical procedures for the determination of dithiocarbamates, for example, are particularly desirable (7) because of their use in pesticides and as vulcanization accelerators and anti-oxidants in the rubber industry. Since the sensitivity of dc polarography decreases as the drop time is shortened, it was also of interest to establish whether or not it was necessary to use drop times in the millisecond range to obtain analytically useful data in these systems. The shortest drop time used in the present work (0.16 sec) has previously been shown to result in only a slight decrease in the sensitivity of dc polarography (8). However, a t a drop time of 5 msec, used by Connery and Cover (3), a significant decrease in sensitivity would be expected. The species studied were chloride, iodide, sulfide, sodium diethyldithiocarbamate (NaDtc), and dithiothreitol (DTT).
EXPERIMENTAL C h l o r i d e , iodide, a n d s u l f i d e s o l u t i o n s were p r e p a r e d f r o m rea g e n t grade chemicals. S u l f i d e s o l u t i o n s were p r e p a r e d a n d s t o r e d
(3) J. G. Conneryand R. E. Cover,Anal. Chem., 41, 1191 (1969). (4) R. W. Schmid and C. N. Reilley, J. Amer. Chem. SOC., 80, 2087 (1 958). (5) I. M. Kolthoff and J. J. Lingane. "Polarography," 2nd ed., Interscience, New York, N . Y . , 1952. (6) J. Heyrovsky and J. Kuta, "Principles of Polarography," Academic Press, New York, N . Y . , 1966. (7) D. J. Halls, A. Townshend, and P. Zuman, Analyst (London), 93, 219 (1968). (8) A. M. Bond and D. R. Canterford, Anal. Chem., 44, 721 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8 , JULY 1973
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Ir . ’ 10
Ib1 nvd
i
02
v5
Ag/A@
Figure 1. Conventional and rapid dc polarograms of 1 . 2
30
X
10-3M chloride in 1M NaC104 (ai drop time = 2.9 sec. ( b ) drop time = 0 16 sec
under argon to prevent aerial oxidation. BDH LR-grade sodium diethyldithiocarbamate and Calbiochem A grade dithiothreitol were used without further purification. All solutions were prepared using triply distilled water. Supporting electrolytes used were 1M NaC104 (chloride, iodide, and sulfide); a borate buffer solution of p H 9.2 (NaDtc); and a phosphate buffer of p H 7 (DTT). All measurements were made at 25.0 & 0.1 “C and solutions were deaerated with oxygen-free argon. Polarograms were recorded with a PAR (Princeton Applied Research) Model 170 Electrochemistry System using a three-electrode cell. All potentials are reported relative to a silver/silver chloride (1M NaC1) reference electrode, connected to the polarographic test solution by a salt bridge containing 1.44 NaC101. Tungsten or platinum wire sealed in glass was used as the third (auxiliary) electrode. Short controlled drop times of 0.16, 0.24, or 0.32 sec were obtained with a Metrohm Polarographie Stand E354.
RESULTS AND DISCUSSION Chloride. Oxidation of mercury in the presence of chloride has classically been regarded as a reversible process (5, Si, yielding insoluble Hg2C12 according to 2Hg + 2C1- F. Hg2C12 + 2e (1) VlEek (Si, when investigating the dc polarographic behavior of chloride ion, observed a small but distinct “prestep” on the anodic wave which he attributed to the formation of a monomolecular layer of calomel in the form HgC1. He also noted that the limiting current was not a linear function of bulk chloride concentration and that at high chloride concentrations polarograms showed three or even four separate waves. These observations were confirmed in the present work when using conventional dc polarography with a drop time of 2.9 sec. Figure l a shows the conventional dc polarogram of 1.2 x 10-3M chloride, in which the presence of a pre-wave can be readily distinguished. The broad maximum on the diffusion current plateau, presumably associated with a film of reaction product on the electrode surface, makes accurate evaluation of the limiting current impossible. Erratic drop behavior in the region of this maximum is also evident. The corresponding polarogram recorded a t a drop time of 0.16 sec is shown in Figure l b . Although the pre-wave is still evident, the limiting current plateau is well defined. (9) A. A . Vlcek. Coilect. Czech. Chem. Cornmun.. 19, 221 (1954).
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io3
Figure 2. Concentration dependence of the limiting current of dc chloride waves under rapid polarographic conditions ( t = 0.16 sec.)
3’4
03
VOLT
20
[CI-Ix
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
At chloride concentrations up to 3 x 10-3M, only two waves were observed with the rapid technique (although three were evident a t normal drop times), and the total i L 2 ) varied linearly with bulk chlolimiting current ( i l l ride concentration up to this level (Figure 2). The height of the pre-wave (ill) became independent of concentration above about 1 x 10-3M. Although two waves were still present a t a drop time of 0.16 sec, the rapid technique is certainly superior to convential dc polarography in an analytical sense and can be used for the determination of chloride over the range 1 x lO-4M to a t least 3 x lO-3M. The fact that the detection limit for chloride is so much higher than that usually associated with polarography is a result of the close proximity of the chloride wave to the “free dissolution” wave of mercury (5, 6), and is in no way related to the use of a short drop time. The other species studied gave rise to waves a t considerably more negative potentials ( i e . , well removed from the dissolution wave of mercury) and were therefore detectable a t much lower concentrations. Iodide. The dc polarographic behavior of iodide ion has been investigated by Kolthoff and Miller (IO), who found that the anodic limiting current was poorly defined and fluctuated irregularly above 5 X 10-4M. They concluded that these abnormalities were caused by a film of mercurous iodide on the electrode surface and were able to show that addition of gelatin improved the shape of the diffusion plateau. The appearance of a faint pre-step on the anodic iodide wave has subsequently been reported (11). A comparison of conventional and rapid dc polarograms of 1.2 x 10-3M iodide (Figure 3) illustrates the analytical advantage of short drop times for this system. The erratic behavior at normal drop times prevents evaluation of the limiting current, whereas a t a drop time of 0.16 sec the limiting current plateau is very well defined. The previously reported pre-step on the iodide wave is still evident under rapid conditions. Although the slightly irregular behavior seen on the diffusion plateau of the rapid polarogram (Figure 3 b ) became worse with increasing iodide concentration, reproducible values of the limiting current could be obtained up to a t least 2 x 10-3M iodide, without resorting to the use of gelatin. Under rapid conditions, the limiting current was a linear function of concentration up to about 1 x l O - 3 M
+
(10) I. M . Kolthoff and C . S. Miller, J. Amer. Chem. SOC.. 6 3 , 1405 (1941 ) . (11) I . M .Kolthoff and Y . Okinaka, J . Arner. Chem. SOC.. 83,47 (1961)
I -OB
-04
-06
VOLT
v5
-02
Ag/AgCi
Figure 4. Conventional and rapid dc polarograms of 1.2 X 1OC3M sulfide in 1 M NaC104 ( a ) drop time = 2.9 sec. f b ) drop time = 0.16 sec L 2-T
is
A9/CgCI
Figure 3. Conventional and rapid dc polarograms of 1 . 2 X
10-3Miodide in 1M NaC104 ( a i drop time = 2.9 sec. fbi drop time = 0 16 sec
iodide. Deviation from linearity a t higher concentrations coincided with the appearance of irregularities on the diffusion plateau. Comparison of polarograms recorded with drop times of 0.32, 0.24, and 0.16 sec indicated that this irregular behavior probably could be eliminated by using even shorter drop times. Sulfide. In a recent investigation of the dc polarographic behavior of sulfide, Julien and Bernard (12) reported the presence of three anodic waves. Using the autoinhibition hypothesis of Laviron and Degrand (13), they explained their observations in terms of the deposition of successive layers of HgS on the electrode surface. In the present work, four dc waves were observed under conventional conditions for sulfide concentrations above 1.1 X 10-3~. With the rapid technique, polarograms were simplified considerably (compare Figures 4a and 46), with only two waves being observed up to 2 x 10-3M sulfide. Figure 5 shows the concentration dependence of the limiting cur~ ) of the total limiting current rent of the first wave ( , i L and for both the conventional and rapid techniques. In contrast with the other species studied, the total limiting current remained a linear function of concentration under conventional conditions. Sodium Diethyldithiocarbamate. In 1953 Zuman et al. (14) recognized that the anodic waves of dithiocarbamates were due to mercury compound formation. More recently, the rather complicated polarographic behavior of a number of dithiocarbamates has been rationalized in terms of “adsorption” of reaction products on the electrode surface (7, 15-18). For the purposes of the present work, only one of these compounds, sodium diethyldithiocarbamate [(C2H5)2NCSSKa = NaDtc], was selected for investigation under rapid polarographic conditions. (12) L. Julien and M. L. Bernard. Rev. Chim. Miner.. 5, 521 (1968). (13) E. Laviron and C. Degrand. in “Polarograohy, 1964” G . J. Hills, E d . , Macmillan. London, Vol. 1. 1966, p 337. (14) P. Zurnan, R . Zumanova. and B. Soucek, Chem. Listy. 47, 1522 (1953). (15) W . Stricks and S. K . Chakravarti, Ana/. Chem., 34, 508 (1962). (16) 0 . J. Halls, A. Townshend, and P. Zuman, Ana/. Chim. Acta. 40, 459 (1968). (17) D. J. Halls. A . Townshend, and P.Zurnan, Ana/. Chim. Acta. 41, 51 (1968). (18) D J. Halls, A . Townshend, and P. Zuman, Ana/. Chim. Acta. 41, 63 (1968).
./
, 1
0
5
io lS21x l o ‘
15
~
20
Figure 5. Concentration dependence of the limiting current of dc sulfide waves under conventional and rapid polarographic conditions ia) drop time = 2.9 sec. ibi drop time = 0.16 sec
Halls et al. (17) studied the dc polarographic behavior of NaDtc in great detail and confirmed the conclusion of Stricks and Chakravarti (15) that a one-electron oxidation process was followed by rapid disproportionation according to
They observed an “adsorption controlled” pre-wave and found that the total current was not a linear function of concentration. They also noted irregular behavior in the region from -0.4 V to +0.2 V ( u s . SCE) which was interpreted as the formation of an “adsorption” layer which differed in properties from the film which gave rise to the pre-wave. Regular build u p and break down of this film, resulting in periodic fluctuations in the current, was shown by the shape of current-time curves. Addition of surfactants such as gelatin or ethanol suppressed this irregular behavior and gave a linear calibration curve up to ANALYTICAL
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Figure 8. Conventional and rapid dc polarograms of 8 X 10-4M DTT in phosphate buffer ( p H 7) Figure 6. Conventional and rapid dc polarograms of 9 NaDtc in borate buffer (pH 9.2)
X
10-4M
( a ) drop time = 2.9 sec. (bi drop time = 0.16 see
[NaDtcl x IO4
Figure 7. Concentration dependence of the limiting current of
NaDtc waves under conventional and rapid polarographic conditions (ai drop time = 2.9 sec. (bl drop time = 0.16 sec
1 x 10-3M, and they recommended a 60% ethanolic 0.1M NaOH media for analytical measurements. Figure 6a shows the conventional dc polarogram of 9 x 10-4M NaDtc. In agreement with previous work (17), the total current showed considerable deviation from linearity when plotted against concentration (Figure 7 a ) . On the corresponding rapid polarogram, the diffusion plateau is very well defined without any of the irregular behavior seen on the conventional polarogram (compare Figures 6a and 6 b ) . The height of the first wave was independent of concentration above 6 x 10-4M and the total limiting current varied linearly with concentration up to 1.6 X 10-3M (Figure 7 6 ) . Thus, with the rapid technique, it is possible to obtain analytically useful waves over a wider concentration range than possible with conventional polarography in the presence of 60% ethanol or gelatin. The improved polarographic behavior under rapid conditions is not unexpected since Halls et al. ( I 7) found that the periodic fluctuations in current in the -0.4 V to +0.2 V region occurred in the latter two-thirds of the drop life. 1330
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
( a ) drop time = 2.9 sec. ibi drop time = 0.16 sec
By using short drop times, the film which gives rise to this abnormal behavior is not permitted to build up. Although only one dithiocarbamate was studied, it is expected that the rapid technique could be used to advantage for the routine analysis of many other members of this important group of compounds. Dithiothreitol. Dithiothreitol (DTT), the threo isomer is an extremely useful of 2,3-dihydroxy-1,4-dithiolbutane, reagent in biochemical studies. It is capable of maintaining monothiols completely in the reduced state and of reducing disulfides quantitatively (29). In a recent study of the rheologically important thiols and disulfides of wheat dough, Phillips (20) developed a polarographic method of determining unreacted DTT, based on previously unreported oxidation waves. Presumably oxidation of mercury is involved in the electrode process as is the case with many other organosulfur compounds. A preliminary investigation of the anodic behavior of DTT suggests that the two waves observed by Phillips a t higher DTT concentrations (20) result from a film of the reaction product on the electrode surface. Comparison of Figures 8a and 8 b shows how, a t high concentrations, the complicated conventional dc polarograms (not unlike the behavior of NaDtc) can be greatly simplified by using the rapid technique. Theoretical Rationalization of Observations. For systems inhibited by adsorption on the surface of the DME, the improved polarographic behavior under rapid conditions has been attributed to the decreased drop time and the increased rate of surface area formation which both operate to minimize the extent of surface coverage during detector life 13). Similar arguments hold for the present situation. The appearance of a second dc wave indicates inhibition of the electrode process when the surface of the electrode is completely covered by a film of the reaction product. Because of the higher rate of surface area formation at short drop times, the concentration of depolarizer a t which total coverage of the electrode surface occurs, would be expected to be higher under rapid conditions. This prediction was confirmed by observing the concentration at which the second wave appeared as a function of drop time. For example, for sulfide the second wave appeared under conventional conditions ( t = 2.9 sec) at 3 x l 0 - 4 M but did not become evident a t a drop time of 0.16 sec until the concentration was increased to 7 x 10-4M (compare Figures 5a and 5 b ) . (19) W. W. Cleland. Biochem.. 3, 480 (1964). (20) J. W. Phillips, B.Sc. (Hon.) Report. Department of Biochemistry, University of Melbourne, 1971
Of course, if short enough drop times were used, only one wave would be observed over the entire concentration range. Examination of current-time curves for mercury drops of natural life illustrates clearly why the rapid technique provides considerably simpler polarographic behavior. Figure 9 shows some current-time curves measured on the sulfide system. When the overall electrode process is diffusion controlled the current is proportional to t l l 6 (Figure 9a), but in potential regions where films of the reaction product inhibit the electrode process the curves deviate markedly from this shape (Figures 9 b and 9c). However, it can be clearly seen that very early in the drop life, where rapid polarographic measurements are effectively made, almost normal shape is observed (i.e., i 0: t 1 I 6 ) irrespective of the behavior later in the drop life.
CONCLI.k3IONS This investigation has demonstrated the advantages of employing short controlled drop times for the polarographic analysis of systems in which the reaction product, a compound of mercury, is formed on the electrode surface. Abnormal behavior associated with such systems, which a t conventional drop times is often severe enough to prevent collection of analytically useful data, can be simply eliminated or minimized by using short drop times. This is more convenient than the usual, rather empirical, approach of adding gelatin or some other surfactant. I t has been established that a drop time of 0.16 sec, readily achieved with a commercially available mechanical drop timer, is sufficiently short to overcome many problems arising from mercury compound formation in the systems studied. Although a second wave was observed a t higher concentrations under these conditions (i.e , t = 0.16 sec), this is not considered a serious inconvenience to the analyst, as the total limiting current plateau was always very well defined and the total limiting current was linearly dependent on concentration. Since the sensitivity of dc polarography decreases as the drop
time Figure 9. Current-time curves for 1 X 10-3M sulfide in 1 M
NaCIO4 at various applied potentials Drop time = 2.0 sec. ( a ) -0.250; ( b ) -0.625; ( c ) -0.675 ( v s . Ag/ ASCI)
time is shortened, there appears little advantage from the point of view of analytical application of the rapid technique, in using drop times shorter than about 0.1 sec. The added advantage of the much shorter time required to record a polarogram permits us to conclude that the rapid technique is considerably superior to conventional dc polarography for routine analysis of systems involving formation of mercury compound reaction products on the electrode surface.
ACKNOWLEDGMENT The authors acknowledge the assistance of J. W. Phillips for providing the sample of dithiothreitol. Received for review August 21, 1972. Accepted December 18, 1972.
Ultrapurification of Water for Electrochemical and Surface Chemical Work by Catalytic Pyrodistillation B. E. Conway, H. Angerstein-Kozlowska, and W. 6. A. Sharp Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada
E. E. Criddle Defence Research Establishment, Defence Research Board, Ottawa, Ontario, Canada
Recently, domestic and industrial water supplies have become contaminated by organic impurities that cannot be removed by ordinary or oxidative distillation because of steam volatility of the impurities or their derivatives. The results of using a pyrocatalytic distillation system for preparation of ultrapure water for electrochemical and surface chemical work are described. Exacting electrochemical and optical criteria are defined for judging and characterizing the purity of water, with respect to organic impurities, especially with regard to their effects at Pt and H g electrodes.
In recent years, both in North America and in Europe, it is being found impossible to prepare pure water free from organic, surface-active contaminants by means of distillation, even from alkaline KMn04, although previously such a procedure was known to be quite adequate. The organic contaminants now commonly present in many domestic and industrial water supplies are steam volatile and are hence not removed by distillation; also they are not efficiently removed by a “preboil.” That residual distillable impurities might arise after permanganate treatment of water, and affect the electrochemical behavior of Pt, seems to have been envisaged by Formaro ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8 , JULY 1973
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