Anal. Chem. 1981, 53,847-851
847
Direct Determination of Chelons at Trace Levels by One-Drop Square-Wave Polarography Zblgnlew StoJek’ and Janet Osteryoung” Department of Chemistry, State Unlversity of New York at Buffalo, Buffalo, New York 14214
The direct anodlc oxldaitlon of mercury In the presence of chelons can be used for determinatlon of the chelons at trace levels. One-drop square-wave polarography proved superior to dlfferential pulse polalrography for thls purpose and gave detection limits of 7, 7, 5, and 20 X IO-* M for EDTA, DCTA, EGTA, and NTA, respecttvely, with callbratlon curves linear to M. At pH 4.8 heirvy metals can be determlned In the presence of Ca2+ by using amperometrlc titratton based on this method.
Chelons such as EDTA (ethylenediaminetetraaceticacid) are used in a wide range of industrial, pharmaceutical, and agricultural applicationa. It has been shown that EDTA is resistant to biodegradation ( I ) . It has also been shown that under the conditions normally obtaining in natural waters EDTA exists as negatively charged complexes with protons or metal ions and is not (absorbedon container walls, humic acid, silica, kaolin, river sediments, or humus solids (1). Gardiner infers from the existing data that complexation of trace metals in natural waters by EDTA 1s more important than complexation by humic acids ( I ) . A simple numerical example illustrates this point. With the data of Buffle and Greter (2), the overall conditional formation constants for Pb(I1) with fulvic acid (molecular weight nominally 1000)at pH 6 are given by log PI = 5.1 and log 02 := 9.7 for the complexes PbL and PbLz, where L represents fulvic acid. At the same pH the conditional iformation constant for PbY is given by log K’ = 13.4, where represents EDTA. Suppose that Cy, the analytic concentration of Y, is 100 pg/L. K’ is sufficiently large that the fraction of lead present as the aquated ion is negligible as long a8 the analytic concentration of lead, Cp,, is less than that of Y. Under these conditions, the fraction of lead complexed with L, is less than 0.1 for concentrations of L less than 7 g/L (7 mF). Similar conclusions are drawn for other cases. Gardiner h a carried out similar calculations which include the effects of competition of Ca(I1) for EDTA and illustrate some limitations of this generalization (1). Undoubtedly EDTA is capable of affecting natural systems to an appreciable extent by mobilizing trace metals. A variety of methods have been employed for the determination of EDTA and related compounds. Gardiner used gas-liquid chromatograplhy of the ethyl derivatives of the sample with DCTA (1,2-cyclohexanediaminetetraacetic acid) added as an internal standard for the determination of EDTA in river water, sewage, arid sewage effluent (3). Detection M (15 pg/L) with recoveries of ca. 99 limits of ca. 5 X f 13% were found with a sample size of 25 mL. Kaiser employed the absorbance of the Co(II1) complex and was able to determine EDTA in mixtures of EDTA and NTA (nitrilotriacetic acid) by selective oxidation of the NTA complex (4). Replicate analyses at the 200 pg/L level of EDTA in mixtures of EDTA and NTA gave precision of 7% (standard Present address: Uniwersiytet Warszawski, Wydzial Chemii, ul. Pasteura 1,02493 Warszawa,, Poland.
deviation). Manahan and co-workers have reported two methods for determining chelons based on stoichiometric reaction of Cu(I1) with the chelon and subsequent determination of Cu(I1) by atomic absorption spectrometry. The first employs the solubilization of “Cu(OH),(s)” at pH 10 by chelons and is appropriate for solutions not containing large amounts of metals which bind strongly with the chelons sought (5). The second employs a mixed Chelex column with alternating sections in the sodium and copper forms; other metals are removed in the Na+ sections, and the chelons freed are converted to the Cu(I1)-bound form in the Cu(I1) sections (6). Methods based on the Cu-PAN titration (71, an electrogeneration of Bi(II1) (8), and interference with the Mn(II)-catalyzedreaction of periodate (9,10) have been developed for determination of EDTA in a variety of applications. Voltammetric methods have been reported for determination of NTA (nitrilotriacetic acid) or EDTA by reduction of a metal chelate formed by the chelon. Afghan et al. determined NTA in natural waters, detergents, and sewage samples by using differential (twin-cell) rapid-scan polarography to reduce the Bi(II1)-NTA complex at pH 2 (11). Detection limits of 10 pg/L (5 X M) were obtained without preconcentration, and precision of 1.3% was achieved at 100pg/L (5 X lo-’ M). This basic principle has also been utilized at the HMDE (hanging mercury drop electrode) using DPP (differential pulse polarography) (12). Haberman has reported similar work employing the In(II1) complex (131, and Stolzberg has employed the Cd(I1) complex (14). The objective of the work cited above has been to determine the total EDTA (or NTA) content of the sample. In part for that reason the methods chosen have been indirect, most usually based on the formation of a complex between the chelon and a heavy metal. Our interest was in developing a method for determining the excess or unsaturated binding capacity of natural water samples. Because we were not interested in determiningchelons bound in chelates and because we were not interested in identification of specific chelons, we adopted the direct approach of examining the electrochemical oxidation of mercury. Although the electrochemical oxidation of mercury in the presence of EDTA and other chelons its well-known (15, 16) and has been used as the reaction for end point detection in amperometric titrations (In,it has not been used previously for the direct determination of chelons. The findings of Gardiner ( I ) suggest that EDTA is not only a convenient and chemically reasonable model compound to use in method development but also a potentially important determinant of heavy metal concentrations and the chemical form of metals in natural waters. We report here on the direct determination of EDTA, EGTA ((ethylene glycol)bis(P-aminoethyl ether)-N,N,N’,N’-tetraacetic acid)), DCTA, and NTA by one-drop square-wave polarography and on the related determination of heavy metals by amperometric titration. EXPERIMENTAL SECTION Normal and differential pulse polarograms were obtained with a PARC Model 174 polarographicanalyzer. One-dropsquare-wave polarograms were obtained by using the one-drop square-wave
0003-2700/81/0353-0847$01.25/00 1981 American Chemical Soclety
848
* ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY
1981
analyzer (ODSWA)constructed in our laboratory. The ODSWA carries out rapid-scan square-wavevoltammetry synchronously with the drop life of a DME and operates at a square-wavefrequency of 30 Hz and square-wave amplitude (peak-to-peak) of 50 mV. At 5 mV step height the scan rate is 150 mV/s arid a scan of 600 mV is completed in 4 s. This corresponds to operation with a 10-s controlled drop time and delay of 6 s before initiation of the scan. Details of the design and operation of the ODSWA can be found elsewhere (18). In each case curves were recorded with a Houston Omnigraphic Model 2000 X-Y recorder. Two kinds of working electrodes were used: a DME with flow rate of 0.81 mg/s and natural drop time of 9.9 s in 0.1 F acetate buffer, pH 4.8, at -0.3 vs, SCE; a rotating thin-film mercury electrode (TFME) using a glassy carbon substrate with area 0.46 cm2. The latter was used with a Pine Instruments PIR analfiical rotor. Mercury films were deposited from solutions F in Hg(I1) and 0.1 F in HCIOl at -0.5 V vs. SCE. Film thicknesses were calculated from the charge consumed during deposition and also from the current on stripping the film into 1 F KCSN. A Pt wire inserted directly in the cell served as the counterelectrode. Potentials were measured vs. a SCE or NaC1-saturatedcalomel electrode. However all potentials are reported vs. SCE. All chemicals were reagent grade and used without further purification in solutions prepared with water purified with a Milli-Q purification system. Solutions were deaerated with argon purified by passing through vanadous sulfate solutions. Experiments were carried out at 25 et 1 O C . RESULTS AND DISCUSSION The oxidation of mercury in the presence of chelons is well-known (15, 16) and may be described by the reaction Wg H,Y(4-.n)-= HgY2- + 2e- nH+ (1) The oxidations in which Y is EGTA or DCTA appear to be nearly reversible while that of EDTA is somewhat less so. Niki and Suzuki have studied the oxidation of mercury in the presence of EDTA in detail and have found that the overall reaction is complicated not only by reactant adsorption but also by product adsorption (19). However, these effects do not complicate direct analysis at the concentration levels we employed (+a. M). A variety of techniques and chemical conditions were investigated in the search for optimum conditions for analytical purposes. The half-wave potential for oxidation of mercury in the presence of the chelon Y, E 1 / 2 (depends ~ ~ , on log K’HgY,where K’H~Yis the conditional formation constant for the chelate. Therefore E112(Hgv is a complicated function of pH and moves to more negative potentials with increasing pH. However, the background wave for oxidation of mercury in the absence of chelon also moves in the negative direction with increasing pH, not only because of complexation of Hg(I1) with components of the buffer used but also because of complexation with hydroxide (20). A variety of supporting electrolytes were examined in the neutral to fairly basic range: ammonia, pH 8.5 -9.8; phosphate, pH 6- 7; carbonate, pH 8.3-10.3; neutral NaN03, NaC104. None of these solutions gave waves as well-defined or as well separated from background as acetate buffers in the pH range 4.8-6.8. All of the work reported here was carried out in 0.1 F NaOAc/HOAc, pH 4.8-6.8. Normal and differential pulse polarography (NPP and DPP), one-drop square-wave polarography (ODSWP), and DPP at the rotating disk TFME were used to investigate the oxidation of Hg in the presence of EDTA, EGTA, DCTA, and NTA in the concentration range 5 X lo4 to loa M. NPP gave extremely large sloping base lines and therefore was not pursued. The TFME appeared promising because of previous experience with it for the oxidation of mercury in the presence of hydroxide (20), for which it responded fully as well as the DME. Experiments were carried out here over a variety of conditions, and the optimum performance was found at film thickness 0.8 pm, rotation rate 1600 min-l, DP pulse amplitude 50 mV, and pH 5.8. EDTA solutions were investigated over
+
LI-1-I-I-1-1-
t0.2
0
-02
E, V vs SCE Flgure 1. OxIdation of mercury at the DME In the presence of chelons using DPP: pulse amplitude, 25 mV; drop time, 2 s; scan rate, 0.5 mVls; pH, 5.8; [Y] = 5 X lo-’ F: EDTA (-), DCTA EGTA (---), NTA (-.-). (..a),
+
to2
0
-02
E, V vs SCE Flgure 2. OxIdation of mercury at the DME in the presence of chelons using ODSWP: delay time, 2 s; step height, 5 mV. Other condltlons are those of Figure 1. the range 0.18-7.0 pF and the sensitivity was found to be ca. 1.2 mA/mM. Under these experimental conditions the theoretical value for a diffusion-controlled system is about 0.5 mA/mM (21). The dependence of peak height on concentration was found to be linear up to a concentration of about 1 pM but decreased markedly in the micromolar range. At 7 pM, the sensitivity was ca. 0.6 mA/mM. In addition, the dependence of peak height on rotation rate proved to be irregular and irreproducible. These effects persisted over the range 0.2-1 pm in film thickness. Other work on the mechanism of oxidation of mercury under these chemical conditions a t the DME (25) suggests that this anomalous behavior can be attributed mainly to the strong role of product adsorption, which does not occm in the case of hydroxide mentioned above (20). The electrode gave nearly ideal behavior for the Pg(II)/Pb(Hg) couple under similar conditions. Therefore we confined further studies to DPP and ODSWP a t the DME. Polarograms for the chelons are shown in Figures 1. and 2. Two features are notable. First, although conditions are similar, the ODSW peak currents are about 5 times the DP peak currents. This occurs in part because of the difference in drop area. In Figure 1the area a t current measurement is that of a 2-s drop. In Figure 2 the delay time is 2 s and the effective scan rate 150 mV/s. The EDTA peak occurs about 300 mV after initiation of the scan and thus current is measured at ca. 4 s. The resulting current should be ca. 1.6 times the DP current due to the greater drop area. The effective
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
measurement time for the ODSWA is 12.5 ms (18) while the effective measurement time in the PARC 174 is 48 ms. This gives rise to an additional enhancement factor of 2.0. Finally, one would expect the square wave difference current to be about 1.3 times as great as the forward current (22)whereas the differential pulse diffierence current is 0.62 times less than the pulse current (23). Thus one would expect the currents of Figure 2 to be about 7 times as great as those of Figure 1. The actual ratios are 5.1 (EDTA), 6.8 (DCTA), 5.1 (EGTA), and 9.7 (NTA). The comparison given above is crude and the reactions are complicated to various extents by irreversibility and reactant and product adsorption. The currents of Figure 2 are thus reasonable. The foregoing does not include a variation of instrumental parameters; however, because of the similarity of these techniques such a variation is not necessary for an adequate comparison. Consider the instrumental parameters in turn. The scan rate used for DPP was 0.5 mV/s. That is the lowest scan rate available on the PARC 174, and for instrumental reasons it gives optimum performance at the price of long time required for the scan. Performance at higher scan rates was significantly inferior. The response in square-wave voltammetry is almost independent of scan rate (i.e., step height at constant frequency) (22) and exhibits a dependence at the DME only because the drop area at the peak depends on scan rate. Thus the choice of scan rates here optimizes the performance of DPP with respect to the ODSWA, yet the time required for the scan in DPP is 17 min while that required for the ODSWA is 5 s. Regarding the parameter of DP or SW pulse amplitude, the amplitudes chosen here are the same. In addition, for a reversible reaction the dependence of the peak current signal on pulse amplitude is almost identical for DP and SW polarography. In each case increase in pulse amplitude from 25 to 50 mV would increase the peak current by a factor of 513. The shorter current measurement time for the ODSWA compared with the PARC 174 gives SW an advantage in sensitivity by a factor of 2. In the SW case synchronization with line frequency discriminates against line noise while for DP the same discriminatilon is provided by a relatively longer pulse and integration of the current over one period of the line frequency. In each case the dependence of the peak current on measurement time is the same for reversible reactions and there is no intrinsic barrier to operation at somewhat shorter times. The delay time used for the ODSWA was 2 s which corresponds to the 2-s drop time in DPP. Again, each response would be enhanced in about the same way by increase of these times by providing a larger electrode area and smaller rate of change of that area. Thus we conclude that rapid-scanning square-wave polarography is in general superior to differential pulse polarography for analytical purposes. It is not so clear why the NTA wave is so much better resolved from background in the square-wave case. The background current measured at +0.15 V with respect to a base line extrapolated from the region -0.1 to -0.3 V is only 3 times as large in Figure 2 as in Figure 1. Calibration curves using the ODSWA were found to be linear up to lo-' F. Howevcsr, in every case there was a nonzero intercept attributable to trace metal contaminationof reagents and glassware. No special ]precautionswere taken to eliminate this phenomenon. Detection limits (dl) were calculated by use of pooled standard deviations, s, and the slopes of Calibration curves, m, from the formula dl = 2s/m. Values for the conditions of Figure 2 are 7, 5, 7, and 20 X lo-* M for EDTA, EGTA, DCTA, and NTA, respectively. These values are illustrative but do not give a comprehensive picture of the dependence of detection limit on the
849
Table I. Detection Limits for Determination of Chelons by Square-Wave Voltammetry -log dla at log K'H~Y r1/2
Kc
m
0.1 0.01
18
15
14
13
12
11
7.3b 6.8 6.8
7.3 4.0 3.2
5.0 3.7 2.7
4.5 3.2
4.0
3.5
Value dl is detection limit (cf. eq 2) in mol/L. limited by instrumental noise. r is the square-wave y ) ~ z , k" is the stanperiod(s) and K = ko/DR c y ~ z D o ( l - c where dard heterogeneous rate constant (cm/s), D R and DO are the diffusion coefficients of the reduced and oxidized forms (cm2/s),and cy is the transfer coefficient.
conditional formation constant of the complex and the kinetics of the electrochemical reaction. For this purpose we have employed calculations of detection limit from theoretical square-wave voltammograms using the definition dl = [i(E, Wi/Z/2)- i(Ep)]/m (2) where i is the current due to reaction 1, E,, is the peak potential, and WIl2is the peak width at half-height. Application of this formula to experimental curves produced results in good agreement with the values of detection limit given above. Values of i and m were calculated by using the scheme developed by O'Dea (24). The results are presented in Table I. They show clearly the pronounced effect of electrode kinetics on the ability to detect chelons based on reaction 1. Slow electron transfer not only decreases the peak amplitude and increases the peak width but also moves the peak toward more positive potentials. The difference between the results for 7% = 0.1 and 0.01 is due almost entirely to the latter effect. The theoretical basis does not now exist for treating the effects of adsorption in the same manner. However, these effects can be expected to be pronounced. For a reversible reaction the peak potential in square-wave voltammetry coincides with the half-wave potential. The peak for EDTA in Figure 2 occurs at -0.075 V although the standard potential under those conditions is +0.104 V. The negative shift in potential is undoubtedly due to product adsorption (19). In fact if the reaction is carried out on an electrode at adsorption equilibrium with product, the thermodynamic value of the half-wave potential is observed experimentally (25). Resolution with the ODSWA is sufficiently good that EDTA can be determined in the presence of EGTA or NTA. Representative examples are shown in Figure 3. It is of particular interest that EDTA and NTA are clearly distinguished, for they are the two synthetic chelons most likely to be present in natural waters. Application of this method to the direct determination of metal binding capacity in natural waters or to the determination of free (or labile) metals in natural waters requires further consideration of the equilibria and rates of the processes involved. This is a complicated subject much discussed in the literature, e.g., ref 26. The aspect which concerns us here is the ability of this approach to discriminate between Ca(I1) and heavy metals such as Cu(II), Zn(II), etc. Equilibrium considerations show that for pH 4.8 Fe3+,Hg2+, Ni2+,Cu2+,Pb2+,Zn2+,Cd2+,Co2+,and A13+are 99.9% converted to EDTA complex for Cy = M, where Cy is the analytic concentration of Y not bound to metal, 99% converted for Cy = lo4 M, and 90% converted for Cy = lo-' M. At pH 6.8, Fez+and Mn2+are included in this list. Thus to a reasonable approximation one would expect these metals and EDTA to react quantitatively even at rather low concentrations. Although Ca2+does not react quantitatively in this pH range, it does complex a significant fraction of EDTA.
+
850
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
I-
-1-1
to1
-01
-03
E,V vs SCE Figure 3. Oxidation of mercury using the ODSWA at the DME in the presence of mixtures of chelons: EDTA alone (.-), mixture (-). Conditions are those of Figure 1 except that [NTA] = 6 X lo-’ F. The fraction of EDTA not bound to Ca2+when Cy + [Cay] > ml the detection limit is approximately proportional to the inverse of the slope, m2. Consequently complexation of EDTA to Cap+,by decreasing the value of the slope, m2,would increase the detection limit for heavy metals. The kinetics of chelate formation and dissociation must also be considered. If the rate of dissociation of a chelate is small in comparison with the rate of the electrochemical process, then the bound chelon will not contribute to the wave for oxidation of mecury in the presence of chelon. According to Michel, the rate of dissociation of metal-EDTA complexes is proportional to hydrogen ion concentration (28). Reilley et al. found that at pH 4 equilibrium was established sufficiently rapidly on the time scale of DC polarography that sequential titration of, e.g., Fe(II1) and Mn(1I) could not be carried out (29). However amperometric titrations of Cd(I1) and Ni(I1) at pH 4.6, Cu(I1) at pH 5.5, and Pb(1I) at pH 4.7 were successful. Thus square-wave polarography with characteristic time of 17 ms should not be influenced by dissociation of these complexes in the diffusion layer. On the other hand Michel found that Mn(II), Co(II), N U ) , Cd(II), and Zn(I1) could be titrated amperometrically with EDTA at pH 6.4 using the DC anodic mercury wave but that the alkaline
+
earth chelates were too labile to permit titration (28). We have examined the effect of Cap+on the square-wave behavior of mercury in the presence of EDTA at pH 4.8 in 0.1 F HOAc/NaOAc; calibration curves in the range 1-3 FM were the same with 0.77 mM Ca2+present and with no Cap+. At higher pH values the slope of the calibration curve is less. Of course at pH 9 the rate of dissociation is sufficiently small that Ca2+can be titrated quantitatively by using this technique (17). Therefore, we conclude that at pH 4.8 the rate of dissociation of Cay2-is so large that for square-wave voltammetry at 30 Hz the current response due to oxidation of mercury to form HgY2- depends on the total EDTA concentration not on the free EDTA concentration. Thus the pessimistic predictions of equilibria are somewhat softened by the relatively fast kinetics of the CaY equilibrium. It might be expected that complexation of Ca2+with EDTA would cause the anodic mercury wave to shift to more positive potentials. This effect is in fact very small. The positive shift in Ellz is -30 log K’cay[Ca2+1,and for the experimental range of pH and [Ca2+] of interest KIaY[Ca2+]is nearly unity. Thus at pH 4.8 it should be possible to determine total free (or labile) chelon or total free (or labile) heavy metal by observing directly the mercury oxidation wave or by amperometric titration with EDTA using the mercury wave to detect the end point. Using standard techniques (30),we have determined total Pb(II), Cd(II), Zn(II), and Cu(I1) in 0.1 M HOAc, pH 4.8 in solutions 0.77 mM in Ca(I1). Results at the micromolar level were in excellent agreement (s -4%) with both the known values and those obtained in the absence of Ca(I1). Since the median value of Ca(I1) concentration in terrestrial waters is ca. 1 mM (32),this procedure should prove widely useful for real samples. We have examined also samples of tap water, effluent from a sewage treatment plant, and a series of 35 samples with high organic carbon and nitrogen content (obtained from the U.S. Geological Survey). Samples with chloride content greater than 55 mg/L could not be analyzed directly because of interference due to anodic oxidation of mercury to Hg,C12(s). Interferences such as these have been discussed in detail by Reilley and Schmid (32). In the case of high chloride concentration it is possible to perform the analysis after a pretitration with silver ion by a procedure similar to that of Karchmer and Walker (33). This aspect of the work was not pursued in sufficient detail to establish a procedure which is generally applicable. Samples described above were diluted 1:l with 0.2 F acetate buffer, pH 4.8, and examined by square-wave polarography. In each case a flat background current similar to that of Figure 2 was obtained, and there was no evidence of free or labile chelon. The conclusion is that in all of the samples examined any chelon present was bound to metals such as Pb(II), Cu(II), and Fe(II1). No attempt was made to determine the amount of nonlabile bound chelon through any of the indirect procedures referred to in the introduction. In each of the samples referred to above, after the initial observation that there was no peak due to the anodic oxidation of mercury in the expected potential range, an amperometric titration was carried out by adding successive aliquots of chelon and running a square-wave polarogram after each addition. Representativeresults for the samples obtained from the U.S.G.S. are presented in Table 11. In each case dissolved Fe(II1) is the predominant titratable metal, and the amount of suspended Fe(II1) is even larger than the amount dissolved. The operational distinction between “dissolved” and “suspended” (filtration through 0.45 pm filter) assigns some portion of colloidal material to the “dissolved” fraction. In each case the value found by titration is somewhat less than the value obtained by summing the values determined by
ANALYTICAL CHEMISTRY, VOL. 53,
sample no. PH Ca (mg/L)
total hardness (mg/L) bicarbonate (mg/L) total alkalinity (mg/Ll) total organic nitrogen (mg/L) total organic nitrogen (mg/L) heavy metals (pM) Fe Zn Mn Pb
1
2
3
8.1 56 190 171
6.7 1.5 7 11
7.6 26 110
2.0 0.11
8.3 0.33
82 8.9 0.75
1.08 0.31 0.18 0.08 0.03 1.7 1.15
5.76 0.08 0.91 0 0 6.8 4.2
851
LITERATURE CITED
1.43 0.61
cu total 2.0 available heavy metals; (pM) 1.05 a Data of U.S.G.S. By amperometric titration with EDTA, this work.
MAY 1981
it may mislead judgments of the impact of metal loadings in natural waters. The measurement of available metal defined operationally by the amperometric titration described here may be important in physicochemical and limnological studies of natural water systems.
Table 11. Metals Available for Binding with EDTA in Natural Water Samples water quality analysisa
NO. 6,
-
elemental analysis. Similar experiments were carried out with tap water samples taken over a period of months from the Ft. Collins, CO, municipal water system. This water has been characterized by repetitive chemical analyses employing standard methods as having the following composition: pH 6.9 f 0.3; total alkalinity (mg/L) 26 f 6; total hardness as CaC03 (mg/L) 27 f 5; chloride (mg/L) 3.4. Iron is the most important trace metal (0.1-0.2 mg/L, ca. 3 pM), and the total contribution of the other trace metals is ca. 0.4 pM. In a typical experiment available metal was determined by amperometric titration with .EDTA at pH 4.8, and a value of 1.6 pM was obtained. To a similar sample was added an exct?ss of EDTA (2.6 pM), and the resulting free EDTA concentration was determined by standard addition to be 0.9 pM. The difference, 1.7 pM, is in good agreement with the value obtained by direct titration. As was observed in Table 11, the available metal concentrationis less than the total heavy metal concentration. The above results are reasonable when one considers that the “total” and available^" metals are quite different quantities. The compositional analyses (“total”’)are designed to give the total amount of each metal present in the dissolved and suspended fractions. The amperometric titration (“available”) measures thle total amount of metal which is available for binding to the chelon on the time scale of the experiment. While the compositional analysis is important,
(1) Gardiner, J. Water Res. 1978, 70, 507-514. (2) Buffle, J.; Greter, F.-L. J . Electroanal. Chem. 1979, 101, 231-251. (3) Gardlner, J. Analyst(London) 1977, 702, 120-123. (4) Kaiser, K. L. E. Wafer Res. 1973, 7, 1465-1473. (5) Kunkei, Robert; Manahan, Stanley E. Anal. Chem. 1973, 45, 1465-1468. (6) Jones, David R., IV; Manahan, Stanley E. Anal. Lett. 1975, 6, 421-434. (7) White, W. W.; Murphy, P. J. Anal. Chem. 1975, 47, 2054-2057. (8) Pouw, Th. J. M.; DenBoef, G.; Hannema, U. Anal. Chlm. Acta 1973, 67,427-436. (9) Hadjiloannou, T. P.; Koupparls, M. A.; Efstathlou, C. E. Anal. Chim. Acta 1977, 68, 281-287. (IO) Nikolelis, D. P.; Hadjiioannou, T. P. Anal. Chim. Acta 1978, 97, 1 1 1-120. (11) Afghan, Badar K.; Goulden, Peter D.; Ryan, James F. Anal. Chem. 1972, 44,354-359. (12) Haring, B. J. A.; V. Delft, W. Anal. Chlm. Acta 1977, 94, 201-203. (13) Haberman, John P. Anal. Chem. 1971, 43, 63-67. (14) Stolzberg, Richard J. Anal. Chim. Acta 1977, 92, 139-148. (15) Rubel, Stanislaw; Wojciechowski, Marek. Anal. Chim. Acta 1979, 104, 215-223. (16)Jackson, L. L.; Osteryoung, Janet; Osteryoung, R. A. Anal. Chem. 1980, 52, 66-70. (17)Jackson, L. L.; Osteryoung, Janet; O’Dea, John; Osteryoung, R. A. Anal. Chem. 1980, 52, 71-75. (18) Yarnitsky, Chaim; Osteryoung, R. A.; Osteryoung, Janet Anal. Chem. 1980, 52, 1174-1178. (19) Niki, K.; Suzuki, K. J. Electroanal. Chem. 1974, 49, 27-39. (20) Kirowa-Eisner, E.; Osteryoung, Janet Anal. Chem. 1978, 50, 1062-1966. (21) Myers, David J.; Osteryoung, R. A.; Osteryoung, Janet Anal. Chem. 1974, 46,2089-2092. (22) Chrlstie, J. H.; Turner, John A.; Osteryoung, . - R. A. Anal. Chem. 1977, 49, 1899-1903. (23) Osteryoung, Janet; Hasebe, Kiyoshi Rev. fo/arogr. 1978, 22, 1-25. (24) O’Dea, John J. Ph.D. Dissertation, Colorado State University, Ft. Collins. -, CO. .- , 1979. . . (25) Stojek, 2.; Osteryoung, Janet, unpubllshed work. (26) Davison, W. J. Electroanal. Chem. 1978, 87, 395-404. (27) Llteanu, C.; Rica, I. “Statistical Theory and Methodology of Trace Analysis”; Ellls Horwood Limited: Chichester, England, 1980;Section 5.10. (28) Michei, G. Anal. Chim. Acta 1954, 10, 87-96. (29) Reilley, Charles N.; Scrlbner, Wiiilam G.; Temple, Carroi Anal. Chem. 1958, 28, 450-454. (30) Myers, David J.; Osteryoung, Janet Anal. Chem. 1974, 46, 356-359. (31) Stumm, Werner; Morgan, James J. “Aquatic Chemistry”; Wiiey-Interscience: New York, 1970;p 384. (32) Reilley, Charles N.; Schmid, R. Anal. Chem. 1958, 39, 947-953. (33) Karchmer, J. H.; Walker, M. T. Anal. Chem. 1955, 27, 37-41.
RECEIVED for review July 7, 1980. Resubmitted January 2, 1981. Accepted February 13,1981. This work was supported in part by a grant from the Environmental protection agency.
