Electroadsorptive Removal of Impurities from Perchloric, Sulfuric, and Phosphoric Acids, and Sodium Hydroxide M. 2. Hassan' and Stanley Bruckenstein Department of Chemistry, State University of New York at Buffalo, Buffalo, N. Y. 142 14
A method for the purification of acids and alkali based upon adsorptive and electroactive properties of a column of platinum sponge held at a fixed potential has been developed. This column rapidly removes various cationic and anionic impurities. This flow-through nature of the electrode minimizes the diffusion path of impurities to the electrode surface and shortens preelectrolysis time considerably. The results of purity tests performed with a rotating platinum disk electrode (RPDE) on different supporting electrolytes before and after passing them through the platinum sponge purification column are discussed. The efficiency of the method is illustrated by comparing RPDE data obtained during hydrogen adsorption, oxygen reduction, isopotential point experiments, and experiments involving the formation and/or reduction of oxidized platinum.
In this paper, we report the investigations of the electrochemical purification of various supporting electrolytes, using the rotating platinum disk electrode (RPDE). The presence of supporting electrolyte impurities complicates electrochemical experiments, particularly a t solid electrodes. Long time preelectrolysis has been routinely recommended (1-12) to remove electroactive impurities. Also, physicochemical techniques, such as distillation (13: 1 5 ) , recrystallization (16, 17), oxidation with hydrogen peroxide (18, 19) and adsorption on activated charcoal, platinized platinum, or silica gel (11, 12, 20-23) have often preceded, or followed, preelectrolysis to remove non-electroacPresent address, E a s t e r n Research Center, Stauffer Chemical Company, Dobbs Ferry, N.Y. 10522. G. N. Lewis and R. F. Jackson, 2. Physik Chem., 56A, 193 (1906). J. O.'M. Bockris and B. E. Conway, Trans. Faraday SOC., 45, 989 (1949). L. Meites, Anal. Chem., 27, 416 (1955). J. N. Butler, J. Phys. Chem., 70, 2312 (1966). S. Gilman, Electroanal. Chem., 2, 117 (1967). A. Damjanovic. M. A. Genshaw, and J. O.'M. Bockris, J. Electrochem. SOC.,114, 466 (1967). 0. A. Petrii, R . B. Marvet, and Zh. N. Malysheva, Elekrokhimiya, 3, 851 (1967). I. I. Labkovskaya, V. I . Lukyanycheva. and V. S. Bagotzky, Sov. Elecfrochem.. 5, 535 (1969). A . H. Taylor, R. D. Pearce, and S. B. Brummer, Trans. Faraday Soc., 66 2076 (1960). T. Biegler. D. A . J. Rand, and R. Woods, J. Electroanal. Chem., 29, 269 (197 1). V. S. Bagotzky, Yu. B. Vassiliev, and I. I. Pyshnograeva, Electrochim. Acta, 16, 2141 (1971). S. D. James, J. Electrochem. Soc., 114, 113 (1967). T. Biegier, J. Electrochem. SOC., 116, 1131 (1969). E. B. Asatkin, K. I. Rozental, and V. I. Yakovieva, Electrokhimiya, 5, 136 (1969). A. Mizuike, "Separation and Preconcentration" in the monograph, "Trace Analysis: Physical Methods," G. H. Morrison, Ed., Interscience, New York, N.Y., 1965, p 109. A. H. Taylor and S. E. Brummer, J. Phys. Chem., 72, 2851 (1968). A. H. Taylor and S. B. Brummer. J. Phys. Chem., 73, 2397 (1969). D. A . Vermilyea, J. Electrochem. SOC.,105, 286 (1958). W. Visschar and M. A. V. Devanathan, J. Electroanal. Chem., 8, 127 (1964). G. C. Barker, U.K. At. Energy Res. Estab. Harwell, C/R(UK), 1563 (1957). N. P. Berezina and N. V. Nikolaeva-Fedorovich, Elektrokhimiya, 3, 3 (1967). (22) S. Levina and V. Sarinaky, Acta Physicochim., 6, 475, 491 (1937). (23) B. Ershler and M. Proskurnin, Acta Physicochim., 6, 195 (1937).
1962
tive impurities. Purification of activated charcoal is tedious and time consuming. HC104 has been purified by fuming (24) and vacuum distillation (14), but its explosive nature requires the use of special equipment and care. The present study was initiated to develop a safe and convenient method for purifying perchloric acid solutions, and was later extended to the purification of other electrolyte solutions. Our method is based on the adsorptive and electroactive properties of a column of platinized platinum sponge held a t a fixed potential. Such a column rapidly removes various cationic and anionic species. Long preelectrolysis times are eliminated by using a flow-through electrode, which minimizes the diffusion path of impurities to the electrode surface. Impurities can be divided into three categories: those that adsorb, those that are oxidized or reduced, and those that adsorb concurrently with, or are followed by, electron transfer in the potential range of interest. Because impurities affect certain electrode processes a t platinum electrodes in different ways, these processes can be used to evaluate the degree of electrolyte contamination. A summary of the ones which we used follows. 1. Inhibition of Hydrogen Adsorption. A monolayer of hydrogen is adsorbed on a platinum electrode a t potentials more anodic than bulk hydrogen evolution. Using cyclic voltammetry, current peaks are observed that have been ascribed to the differences in the energy of the adsorption sites. If some other species can adsorb on platinum in the same potential region, H-adsorption is decreased. Therefore, the amount of H-adsorption, QH, can be used to indicate the extent of electrode coverage, (1 - QH), by impurities. A procedure for determining QH has been described earlier (25). 2. Oxygen Reduction Inhibition. Oxygen is reduced on a reduced platinum electrode at a diffusion-controlled rate, and the limiting current should be independent of time. However, it was reported recently (26) that in 1.OM H2SO4, prepared from freshly-opened Malinckrodt reagent grade sulfuric acid and triply distilled water, the oxygen reduction limiting current decreased appreciably with time a t a RPDE. This decrease was ascribed to the adsorption of impurities (26-29) and can be used to estimate supporting electrolyte contamination. 3. Isopotential Points. The appearance/disappearance of isopotential points has been used to establish the presence or absence of adsorbable impurities and to characterize the electrochemical properties of species on the electrode surface (25, 30). 4. Inhibition of the Formation of Oxidized Platinum or Its Reduction. Under controlled electrolysis conditions (24) F . C. Anson, mentioned while visiting our laboratory in 1971 (25) M. Z. Hassan. D. F. Untereker, and Stanley Bruckenstein, J. Electroanal. Chem., 42, 161 (1973). (26) D. C. Johnson and Stanley Bruckenstein, Anal. Chem., 43, 1313 (1971). (27) G. W. Tindall, S. H. Cadle. and Stanley Bruckenstein, J. Amer. Chem. Soc., 91, 2119 (1969). (28) A . Damjanovich. M. A. Genshaw, and J. O.'M. Bockris, J. Nectrochem. Soc., 114, 466 (1967). (29) S. Gilman, "Electroanalytical Chemistry," Vol 2, A. J. Bard, Ed., Marcel Dekker. New York, N.Y., 1967, p 117. (30) D. F. Untereker and S. Bruckenstein, Anal. Chem., 43, 1858 (1971).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
in impurity-free supporting electrolytes, the charge required to oxidize and reduce a Pt electrode is constant. In the presence of adsorbable impurities, the electrode oxidation may be inhibited, resulting in both a decrease in oxide reduction charge, and a shift in the peak reduction current to more anodic potentials when cyclic voltammetry is used. The total anodic charge in the absence of impurity adsorption will usually be greater than that in their presence, provided the adsorbed impurities inhibit electrode oxidation. In the presence of adsorbed electroactive species, it is possible that the anodic charge could equal that in their absence. However, in such a case the shape of the i-E curve would be changed if the adsorbed impurities inhibit the platinum oxide formationheduction and can also be oxidized. The anodic charge will always be greater if the adsorbed impurities can be oxidized without inhibiting Pt oxide formationheduction. Thus, the changes observed in the family of iD-En curves as a function of the time the RPDE spends a t potentials where impurity adsorption takes place, can be used to estimate the amount of supporting electrolyte contamination. The results of purity tests performed on different supporting electrolytes before and after passing them through the Pt sponge purification column are given below. The efficacy of our method is illustrated by comparing data obtained using methods 1through 4.
EXPERIMENTAL A p p a r a t u s a n d Chemicals. Platinum sponge was prepared (31) by a) dissolving scrap platinum in aqua regia t o make HzPtCls. 6H20, b) converting HzPtCls. 6H2O to (NH4)2PtCls, and c) igniting (NH4)2PtCls to make the sponge. Physically, the "sponge" appears t o be a powder, and can be poured as needed. The P t sponge surface area was increased after the initial experiments by platinization using H2PtCls in 4.OM HCl. All chloride was removed by repeatedly washing the sponge with triply distilled water, and it was always stored under triply distilled water. The all-Pyrex preelectrolysis cell is shown in Figure 1. Purified platinized platinum sponge was transferred to the column to form a Pt bed 3-4 cm deep. Four Pt wires were sealed through Co-glass beads into the column. One Pt wire was welded to a Pt gauze that was embedded in the Pt sponge. The sponge served as the indicator electrode in a conventional 3-electrode potentiostatic circuit (32). Two other Pt wires were connected through a 5-K resistor to a 6-V battery. Hydrogen was evolved a t one and served as a polarized hydrogen electrode (PHRE) (33). The fourth P t wire was the counter electrode. T h e effective surface area of the Pt sponge bed, as determined hy electronic integration of the platinum reduction peak, was IO4 cm'. Fresh P t was deposited once each week on the platinum sponge from lO-'M Pt(S04)2 solution in 1.OM H2S04. Pt(S04)z was obtained from K & K and used without further purification. When P t was deposited at potentials 100 mV anodic to hydrogen evolution. no Pt(IV), or P t ( I I ) ,was detected in the 1.OM HzS04 solution coming out of the column. The electrochemical experiment performed to test for Pt(1V) and/or Pt(I1) using a rotating gold disk electrode has been described elsewhere ( 3 4 ) . Other experimental conditions used in testing impurities with a RPDE, were the same as reported earlier (25,35,36). Reagent grade chemicals and triply distilled water were used. Purification a n d Testing Procedure. Two liters of the supporting electrolyte was prepared. A portion (-500 ml) of it was transferred to the R P D E cell and the purity tests mentioned above were performed before any preelectrolysis.
i
cs
\w
x*
Figure 1. Supporting electrolyte purification apparatus (A) Gas washing bottle: (B) Gas wash tube, 12 EC; (C) Standard taper inner and outer joints. 3 34/28: (D) Exit for gases; (E) Ball-and-socket joints, 18/9. connected with pinch clamp, No. 18; (F) Reservoir cap assembly with tubes for (a)passing N 2 through the solution, (b) over the solution, and (c) gas exit; (G) Stopcocks, double oblique bore, Teflon plug, 1 X 7 mm; (H) Fritted glass disk, 20 M; (I) Nitrogen inlet: (J) Standard taper inner and outer joints, T 34/ 45: (K) Unpurified electrolyte reservoir flask, 2-1. capacity; (L) Pressure equalizing tube; (M) Lab-crest solv-seal O-ring joints, 25 mrn: (N) Teflon bushing, 25 mm, with two Viton O-rings; (0)Electrolytic column (id,2 crn, I, 14 cm). connected to K with pinch clamp, No. 40; (P) Pt wires (0.03 in dia) fused to 0 through Co-glass bead; (Q) P welded to R and buried in S; (R) Pt gauze, 4 cm2, 52 mesh: (S)Pt sponge bed, 3-4 cm deep, l o 4 cm2 area: (T) Draining tube for unwanted electrolyte; (U) Standard taper inner and outer joints, 5 19/38; (V) To vacuum pump, needed only for viscous electrolytes; (W) Purified electrolyte receiver flask, 2-1. capacity; (X) Stopcock, Teflon plug, 2 X 7 mm: (Y) Drip tip; (2) Glass rod reinforcements to decrease the fragility of 0
Brauer. "Handbook of Preparative Inorganic Chemistry," Vol. 2, 2nd ed. (Engl. trans.),Academic Press, New York, N.Y., 1965, p 1562. (32) D. C. Johnson, Ph.D. Thesis, University of Minnesota, Minneapolis,
The remaining portion of the supporting electrolyte to be purified was poured in the preelectrolysis cell, and deoxygenated using nitrogen. An i-E curve was obtained using the P H R E as the reference electrode, the Pt wire as the indicator electrode, and the platinum sponge bed as the counter electrode. After establishing the hydrogen and oxygen evolution potentials, the Pt sponge bed was made the indicator electrode, its potential was set 0.2-0.3 V more cathodic than oxygen evolution and a solution flow rate of 50 ml/ min established. Previously deposited impurities were oxidized for ten minutes t o regenerate the column, and the solution coming out of the column was discarded. The Pt sponge potential was then set 5-350 mV more anodic than the hydrogen evolution potential, depending on the supporting electrolyte to be purified, and the first 50 ml of the column effluent was discarded. When pretreated, the column was capable of removing the reducible and adsorbable impurities from 1.OM perchloric or sulfuric acid a t flow rates less than 50 ml/min. Gas bubbles (hydrogen or oxygen) were not allowed to evolve a t the Pt sponge, as bubbles decreased the efficiency of the column. If gas evolution from the P t sponge occurred inadvertently, column efficiency could be restored by maintaining the potential (for one hour) a t a value where neither H P nor 0 2 evolution occurs and where Pt oxide is reduced. Data from the purity tests were obtained as above (Methods 14) in the purified solution and compared with those ohtained in the untreated supporting electrolyte. P r e t r e a t m e n t of R P D E . RPDE experiments involved "pretreated electrodes" (35, 35, 36). The pretreatment consisted of a ) lightly polishing with 0 . 3 ~alumina, b) washing with distilled water, anodizing a t 1.8 V us. SCE for ten minutes while rotating in the supporting electrolyte a t the speed to be used, and c) holding the potential of a stationary electrode a t 0.0 V.
Minn.. 1967. (33) J. Giner, J. Nectrochem. SOC.,Ill,367 (1964). (34) S.H. Cadle and S. Bruckenstein. Anal. Chem., 44,2225 (1972). (35) S. Bruckenstein and M. 2. Hassan. Anal. Chem., 43,928 (1971). (36) D. F. Untereker. Ph.D. Thesis, State University of New York at Buffalo, Buffalo, N.Y.. 1973.
RESULTS AND DISCUSSION HzS04 was tested first, as it is generally regarded by us as the purest of all commercially available oxyacids. In addition, prior literature on its purity existed (261, and it was
(31) G.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1963
Figure 3. Effect of t,, on b-6 curves in electrolyzed 1.0MH2S04 Figure 2. Effect of t,, on b - 6 curves in unelectrolyzed 1 .0MH2S04 s = 100 mV sec-’; w = 0 rpm; 6held at 0.0 V, w = 1600 rpm, for (1) 0, (2) 30, (3) 90 seconds. See text for other details
in frequent use in our laboratories. A pretreated RPDE was rotated a t w = 1600 rpm for different times, t H , a t E D = 0.0 V in a freshly prepared deoxygenated 1.OM H:,S04. Next, rotation was stopped, E D was switched to -0.2 V, and the ED was scanned from -0.2 to 1.4 to -0.2 a t 100 mV/sec. A portion of the first scan of some of the i-E curves obtained a t 0 < tu < 270 sec is given in Figure 2. As can be seen in part from the curves in Figure 2, as t~ increases, there is a decrease in hydrogen adsorption and an increase in the platinum oxidation current, 0.6 V < E D < 1.36 V, during the disk electrode anodic potential scan. Both these results indicate that impurities were adsorbed a t 0.0 V on the electrode surface. The impurities block some hydrogen sites, and, a t E D < 0.6 V, are simultaneously oxidized with the Pt electrode, thereby augmenting the anodic charge. The occurrence of electrochemical processes other than electrode oxidation is deduced from the appearance of two distinct disk isopotential points, DIP-I and DIP-11, a t ED = 0.63 m d 0.78 V, respectively. In an earlier study (37) the increased current, 0.6 V < ED < 1.36 V (isopotential points were ignored), was attributed to sulfite ions present 2s an impurity in HzS04. T o test this hypothesis, we fumed concentrated H2S04overnight before dilution to 1.OM H2S04. This treatment should volatilize and/or oxidize all sulfite. The family of i-E curves obtained using fumed 1.OM H:,S04 also contained these IP’s. Also, it should be noted that DIP-I does not occur at the potential where the oxidation of adsorbed sulfite and Pt have recently been reported (30) to yield an IP. Two of the three IP’s ascribed to SO:, in reference 30 are missing in Figure 2. Only our DIP-I1 can be correlated with DIP-I11 of reference 30. In another series of experiments, we recrystallized (3-4 times) concentrated sulfuric acid from a freshly opened bottle using a mixture of Dry Ice and acetone as a coolant. One molar HzS04 prepared from recrystallized acid and triply distilled water also showed DIP-I and DIP-I1 of Figure 2. Another series of experiments was performed in which the recrystallized sulfuric acid was first fumed and then oxidized with H:,O:,. I t was found that the physicochemical treatments of recrystallization, fuming, and oxidation did not remove the impurity(ies) causing the IP’s of Figure 2. (37) D. C. Johnson, D. 1493 (1970).
1964
T.Napp, and S. Bruckenstein, Nectrochim. Acta,
15,
Other conditions same as in Figure 2
TO check whether the water used to prepare the 1.OM solution was the source of the impurity causing IP’s, the triply distilled water used above was recrystallized and distilled twice more in an all-quartz apparatus. The above experiment was repeated in the 1.OM H:,S04 prepared from this “super pure” water and recrystallized, fumed, and oxidized concentrated H:,S04. DIP-I and DIP-I1 were observed even in this solution. Thus, as will be shown, electroactive impurities unremovable by the above treatments are responsible for DIP-I and DIP-11. Finally, we passed the 1.0714 H2S04 solution, prepared from freshly opened concentrated H2S04 and triply distilled water, through the pretreated Pt sponge column as described in the Experimental. Ecolumn was 50 mV us. PRHE in the same solution. We repeated the experiment of Figure 2 in the electrochemically purified 1.OM H2S04. Figure 3 shows the results obtained using the same conditions as in Figure 2. Figure 3 does not have any isopotential points. However, a slight increase (much less compared to that in Figure 2) in iD a t E D > 0.9 was observed a t t~ > 90 seconds even after passing 1.OM HzS04 through the Pt sponge column. This current is apparently caused by a species that could not be removed even by the combination of all of the above physicochemical and electrochemical methods of purifications. There seems to be evidence (38) that the anodic current decrease is due to the desorption and electrooxidation of organics of undetermined origin which adsorb on the Pt electrode a t potentials more cathodic than 0.5 V. One oxidation product has been found to be CO:, using electrochemical mass spectrometry (38). The identity of the adsorbed species has not been ascertained. It is hard to believe that organics in H2O and H2S04would survive the treatments by which we prepared our few special solutions for these purity tests. Probably this species is coming from ambient sources, cell walls, and nitrogen. The nitrogen used, according to its specifications, contained 0.002% impurities, including hydrocarbons. The tests for electrolyte purity based on the oxygen reduction currents, as described above in Method 2, showed that the current decreased by 15-20% in two minutes a t 1600 rpm in nonelectrolyzed 1.OM HzSO4. After the same solution was passed through the electrolysis column, the decrease in the oxygen reduction current was only 1-2%. In another experiment (Figure 4), the hydrogen adsorp(38) J. Comeau, State University of New York at Buffalo, private communication, 1973.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
L--r '
r
7 - 7
eo-
L
-
Figure 4. Comparison of H-adsorption in electrolyzed and unelectrol-
Figure 5. H-Adsorption peaks in electrolyzed and unelectrolyzed 1.OM HC104
yzed solutions
__ unelectrolyzed; - - - electrolyzed; f H
Filled, unelectrolyzed; Unfilled, electrolyzed. 0 = 1.OM NaOH; 0 = 1.OM HCI04; S = 1.OMH2S04. & is the quantity of adsorbed hydrogen at time tw,and &,,,,= & at 41 = 0
= 0 rprn; 5th scan recorded
tion charge was determined during the anodic cycle of the fifth potential scan at a stationary RPDE. (Successive i-E scans superimpose after the fifth cycle.) In nonelectrolyzed 1.OM HzS04, two minutes rotation at 1600 rpm reduces the hydrogen adsorption by 25% as compared to the maximum adsorption charge. After passing the same 1.OM H2S04 through the purification column a t 50 ml/min, the decrease was 10%. The effect on hydrogen adsorption charge of passing 1.OM NaOH, HC104. and H2S04 through the column can be seen in Figure 4, on which corresponding data for unpurified solutions have also been plotted. A remarkable improvement has been achieved in the purity of HC104 and NaOH solutions. These solutions are usually so impure that only one of the hydrogen adsorption current peaks is generally observed in nonelectrolyzed solutions (Figure 5). The inhibition of H-adsorption as measured in the anodic scan from the desorption peaks is a good test for detecting traces of depositable metals. For example, Cu and Ag preferentially inhibit the sites occupied by weakly adsorbed hydrogen to the same extent (39). This test is not reliable when the previously adsorbed species is reduced in the potential region of the two peaks. Organic substances also displace adsorbed hydrogen from Pt surface and affect the H-adsorption (38). Similar inhibition of oxygen reduction, Pt oxide formation/reduction, and H-adsorption was observed in unelectrolyzed and electrolyzed O.1M and 1.OM HC104 and analogous phenomena were observed in 0.1M and concentrated H3P04. The improvement brought about by passing a particular solution through the column was more obvious in concentrated solutions. The immense adsorbing capability of the column was shown by the complete removal of all Son2- present in a liter of 10-'M KzS03 solution in one single passage through the column. The potential of the column used, E, = 0.1 V us. PRHE, was such that no steady-state electrolysis of SO+ occurs. The repeated passage of any particular solution being purified through the column had no significant effect on its purification and it appears that the column may have a very long lifetime. Also, the simultaneous deposition of Pt (39) S. H. Cadle and S. Bruckenstein. Anal Chem.. 43, 1858 (1971)
= 60
sec; s = 100 mV sec-'; w
c
4
d -80
t ?/I I
I
0 2
W
0 Ed
"
- 2
1
Figure 6. Development of isopotential points in H-adsorption peaks during successive scans in electrolyzed 1.0MHC104 s = 100 rnV sec-': o = 0 rpm; arabic numerals indicate number of scans
from Pt(S04)Zsolutions had no influence on the purificational capacity of the column. Hence, no useful purpose would be served by platinizing the sponge repeatedly-Le., more than once a week. Decreasing the flow rate of the electrolyte through the column below 50 ml/min did not improve the results of any of the above experiments in the case of H2S04 and HsPO4. However, in the case of HC104 a flow rate slower than 50 ml/min contaminated the electrolyte being purified with traces of chloride, as shown by the Pt oxide formation/reduction inhibition. If the column potential was set at the start of H-evolution, or cathodic to it in the case of HC104, C1- contamination of the effluent was observed. The optimum conditions again for purifications were found to be a column potential of 0.35 V us. PHRE and a flow rate of 50 ml/min. Under these conditions, no C1- contamination was detected and the electrolyzed solution was much purer than the unelectrolyzed one, e.g., in unelectrolyzed 1.OM HC104 we did not generally observe a well-developed peak for the strongly bonded H-adsorption peak, whereas in the electrolyzed 1.OM HC104, both H-adsorption peaks are well developed, even up to t~ = 4 min, w = 1600 rpm. Figure 5 gives representative curves for t~ = 1 min in unelectrolyzed and electrolyzed HC104. One of the notable features in Figure 6, is the appearance
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1965
of an isopotential point a t E D = 0.03-0.05 V in the purified HClOI, both in the successive cathodic and anodic disk potential scans. These IP's are surprisingly well developed. As has been shown by electrochemical mass spectrometry (38), an unknown organic impurity is adsorbed when the Pt disk electrode is held more cathodic than 0.5 V. This organic species can be desorbed simultaneously with the adsorption of strongly bonded adsorbed H , resulting in the I P in the cathodic scan. Seemingly, this impurity starts adsorbing with the desorption of H in the anodic disk electrode scan, o = 0 rpm, resulting in the I P in the anodic disk scan. In unelectrolyzed solutions, especially a t higher concentrations of HC104, simultaneous reduction/oxidation of other electroactive impurities interferes and IP's are either illdeveloped or disappear. The results of the above experiments showed that the commercially available oxyacids and NaOH we studied
contain impurities which can be deposited on a Pt electrode a t 0.5 < E D < -0.2 V. These impurities prevent Hadsorption, inhibit oxygen reduction, and can be oxidized simultaneously with Pt. However, these impurities can rapidly be removed by passing the solution through a bed of reduced Pt sponge held a t a potential slightly anodic of bulk hydrogen evolution.
ACKNOWLEDGMENT The cooperation of J. Comeau in performing and evaluating the electrochemical mass spectrometric study is acknowledged. RECEIVEDfor review March 12, 1974. Accepted July 29, 1974. This research was supported by the Air Force Office of Scientific Research by AFOSR Grant No. 70-1832 and Grant No. 72-2572.
Effect of Hydrogen Ion Concentration on the Determination of Lead by Solvent Extraction and Atomic Absorption Spectrophotometry R. J. Everson and Department
H. E. Parker
of Biochemistry,
Purdue University, West Lafayette, lnd. 4 7907
The effect of hydrogen ion concentration on the chelating agents ammonium pyrrolidine carbodithioate (APCD) and sodium diethyldithiocarbamate (DDC) on the extraction of lead by these ligands, and on the stability of the lead chelates in the organic phase following extraction was studied. At low pH values, both chelating agents are decomposed, with APCD being the more stable. However, if the extraction of the lead chelate follows quickly after the addition of the chelating agent, then there appears to be little effect of pH on the extraction of lead by APCD and the extraction by DDC is affected only at very low pH values (below 3). Following extraction, the lead chelates appear stable in the organic phase for at least one hour even though the pH of the aqueous phase may vary from 2 to 8.
Lead is often determined in samples a t low concentration by solvent extraction of a lead chelate followed by atomic absorption analysis of the organic phase. Many of the published procedures (1-9) call for a pH adjustment to 2.8 prior to the addition of the chelating agent. Although reported procedures ( I , 2, 4, 5, 7-9) show excellent recovery data with spiked samples, detailed data on the effect of J. B. Willis, Anal. Chem., 34, 614 (1962). W. Slavin and S.Sprague, At. Absorption Newsleft., No. 17, 1 (1964). S. Sprague and W. Slavin, At. Absorption Newsleft., No. 20, 11 (1964). E. Berman, At. Absorption Newslett., 3, 111 (1964). J. D. Pierce and J . Cholak, Arch. Environ. Health, 13, 208 (1966). R. R. Brooks, B. J. Presley. and I. R. Kaplan. Talanta, 14, 809 (1967). J. A . Platte, in "Trace lnorganics in Water," Advan. Chem. Ser., 73, 247 (1968). (8) M. J. Fishman and M. R. Midgett, in "Trace lnorganics in Water." Advan. Chem. Ser., 73, 230 (1968). (9) D. W. Yeager. J. Cholak, and E. W . Hendersen, Environ. Sci. Technol. 5 , 1020 (1971).
(1) (2) (3) '(4) (5) (6) (7)
1966
pH on the extraction were not reported. Recently, Childs and Gaffke (IO)reported on a study of the effect of pH of dilute aqueous metal solutions on the measurement of lead and cadmium diethyldithiocarbamate in methyl isobutyl ketone (MIBK). They found the optimum response range for lead and cadmium to be between pH 5 - 8 5 The length of time between the addition of the chelating agent and the extraction into MIBK was not mentioned. A detailed study was made to determine the effect of hydrogen ion concentration on: chelating agent, the extraction of the lead chelate, and the stability of the lead chelate following extraction. Although pure solutions were used in the study, thus "matrix effects" were not present, knowing how lead is extracted under these conditions will serve as a reference for matrix studies. The chelating agents ammonium 1-pyrrolidine carbodithioate (APCD) and sodium diethyldithiocarbamate (DDC) were used in the study with the solvents 4-methyl2-pentanone (MIBK) and 3-heptanone.
EXPERIMENTAL Apparatus. A Perkin-Elmer Model 214 atomic absorption spectrophotometer with a Westinghouse neon filled lead hollow cathode lamp was used for atomic absorption measurements using the 283.3-nm absorbing line. Flow rates used were 6.5 l./min for air and 0.70 l./min for acetylene while aspirating 4-methyl-2-pentanone and 1.5 l./min while aspirating 3-heptanone. Ultraviolet measurements were made on a Cary 14 recording spectrophotometer. Radioactivity measurements were made with a Packard Tri-Carb Model 5112 well-scintillation counter. Reagents. A 20% ammonium citrate buffer was prepared by dissolving 400 grams of ammonium citrate, dibasic (Baker Analyzed Reagent), in 500 ml of double deionized water and neutralizing (IO) E. A. Childs and J. N. Gaffke, J. Ass. Offic. Anal. Chem., 57, 360
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
(1974).
