Anal. Chem. 1987, 59,209-211
209
Indirect Trace Determination of Nitrilotriacetic Acid in Water by Potentiometric Strlpping Analysis M a n a r Fayyad Analytical Chemistry Division, Chemistry Department, University of Jordan, Amman, Jordan Release of the ligand nitrilotriacetic acid (NTA) into sewage systems and receiving waters resulting from ita use in detergents (1)as a phosphate substitute is causing great concern. Certain methods for its determination have been introduced, but the need for development of reliable and efficient techniques is still essential. The spectroscopic methods reported (2-4) for the determination of NTA in natural waters are rather expensive to carry out, and even then their detection limit is typically above 100 ppb. Obviously they cannot be employed for trace determination of the ligand in water. Polarographic and voltammetric techniques (5-12) can be used for the indirect determination of NTA by using a metal:NTA complex as a polarographic-active species. Detection limits of 50 ppb were reached without preconcentration. Recently (13)an indirect trace determination of NTA in natural waters, using BtNTA complex, by differential pulse anodic stripping voltammetry has been reported. However, this method is affected by the nature of the electrolyte present in solution and its concentration. We now report the development of a simple, rapid, and reliable method for indirect trace determination of NTA by potentiometric stripping analysis (PSA). The adaptation of PSA for the determination of NTA in water involves adjustment of the p H value to 2.0-2.3, optimization of the potentiometric electrolysis step, and addition of Bi(II1) in concentrations that exceed 1.5-2 times those of NTA present prior to the recording of the stripping time of free Bi(II1). EXPERIMENTAL S E C T I O N Apparatus and Reagents. Measurements were performed on 15-mL solutions in polyethylene disposable cups used aa cells. The working electrode was a mercury film deposited on a highly polished carbon disk mounted on a Teflon rod. A saturated calomel reference electrode and a platinum wire ”counter electrode” completed the three-electrode system. The voltagestripping time data were recorded with a Tecator Striptec System Model equipped with a strip-chart recorder. All solutions were prepared from deionized water, “Aristar” mineral acids, AAS standard solutions, and analytical reagent grade chemicals. Stock solutions of NTA were prepared by dissolving the precisely weighed amount in water with stoichiometric amounts of sodium hydroxide. Stock solutions of bismuth(II1) nitrate were prepared by dissolving metallic Bi in concentrated nitric acid. A 100 mg/L mercury plating solution was prepared by dissolving mercury(I1) nitrate in 0.1 M HC1. The stock reagent solutions were stored in polyethylene bottles and diluted as required for “standard addition” purposes. The pH of the samples was measured with a Model WTW 530 digital pH meter using a glass electrode. Aliquots of the standard solutions of NTA and Bi(II1) were added using Hamilton microliter syringes. Preservation of Water Samples. The samples were acidified to pH 2.0-2.3 with HC1 to prevent further biodegradation of NTA. Procedure. The glassy carbon electrode was polished with the Striptec “polishing kit”, first with ethanol and then with acetone, by “wetting”the polishing cloth with these solvents and rubbing the electrode in a smooth circular motion over the polishing cloth. The polished electrode was finally rinsed with deionized water. The working electrode was “preplated” by immersing the electrode system in the mercury(I1) solution and applying a potential of -0.50 V at the working electrode for 60 s with continuous stirring. This was followed by applying gradually increasing
potentials of -0.60 V, -0.70 V, and -0.80 V for 60-8duration each. A potential of -0.90 V was finally applied four successive times for periods of 60 s each. This procedure gave a stable mercury film on the glassy carbon electrode. The electrode was then rinsed with deionized water, and with this “conditioning”,the electrode was ready for use in the measurement cycles. Standard amounb of NTA ranging between 6 and 255 ppb were spiked into the cups containing 15 mL of deionized water. The pH was adjusted to 2.0-2.3 using WC1. A solution of a known concentration of Bi(II1) was added to give a final concentration which exceeded 1.5-2 times that of NTA in thee sample. Finally for 5.8 X mol of free Bi(III), 4.2 X mol of K2Cr207was added. K2Cr207together with the dissolved oxygen in the sample acted as an oxidant. The solution was stirred for 10 min to attain complex equilibrium. The electrode system was then immersed in the solution, and a deposition potential of -0.30 V was applied for 60 s while stirring. Eight seconds before the end of the electrolyzingtime, stirring was stopped and the voltage stripping time curve was recorded after equilibrium had been reached (by recording the “second stripping time”) (Figure 1). Subsequently the same procedure was repeated twice, each time adding 50 p L of a solution of 10 ppm Bi(III). The method of standard addition technique was applied to evaluate the concentration of uncomplexed Bi(II1) in the sample. The concentration of NTA was determined from the difference between the concentration of added Bi(II1) and that of free Bi(II1) determined by PSA. The mercury film was removed after each run with the polishing cloth as previously described. The same procedure was applied to determine standard amounts of NTA ranging between 6 and 255 ppb in tap-water samples. Similarly, NTA was determined in three authentic samples collected from the wastewaters of a detergent factory in 3ordan located near the Zerka River. The first was from the wastewater before entering the sewage treatment plant, the second from water coming out of the treatment plant, and the third from the Zerka River 1km away from the factory. The results reported in Table IV were average results of three determinations each. Measurements were done after dilution to give a measurable signal. R E S U L T S AND DISCUSSION Choice of Metal Ion. Various polyvalent metal ions form complexes with NTA. Bismuth(II1) was chosen since it forms a very stable 1:l complex with NTA with a conditional stability constant of 3.4 X 1017at p H 2 (14).It has Seen found that NTA can be quantitatively recovered from different metal complexes at pH 2 using Bi(II1) (7). T o ascertain formation of the 1:l complex, Bi(II1) was added in concentrations that exceeded 1.5-2 times those of NTA. This 1:l complex and a p H of about 2.0 were taken as a basis of the indirect PSA determination of NTA. The complex equilibrium in the water sample was attained by stirring the solution for about 10 min prior to the PSA determination of residual uncomplexed bismuth. Deposition Potential. Free Bi(1II) was deposited onto the mercury layer a t the glassy carbon electrode by applying a decomposition potential of -0.30 V vs. SCE. Reduction of Bi(1II) commenced at -0.12 V vs. SCE and at potentials below -0.20 V vs. SCE gave a constant stripping signal for bismuth (15). Reduction of the BtNTA complex starts a t a potential of -0.35 V vs. SCE (13). The applied d e p i t i o n potential must therefore be kept more positive than -0.35 V vs. SCE to prevent reduction of the Bi:NTA complex. The standard addition method has been applied for the determination of
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
dE
Table 11. Measurement of Added NTA Samples
vsSCE
NTA added, ppb NTA found, ppb 6 19 38 64 128 166 191 255
I
lime
to
Tap-Water recovery, %
7 20 39 63 124 157 185 251
117 105 103 98 97 95 97 98
Table 111. Precision and Accuracy of the Indirect PSA Determination of NTA
(5)
Figure 1. Typical potentiometric stripping curve, dEldt vs. t , registered after 60 s of electrodeposition at -0.30 V vs. SCE for a solution that is lo-* M Bi(1II) and 0.5 X lo-’ M NTA at pH 2.3.
Table I. Measurement of Added NTA to Deionized Water Samples NTA added, ppb NTA found, ppb 6 19 38 64 77 128 160 191 223 255
7 20 40 67 79 123 154 201 223 256
recovery, % 117 105 105 105 103 96 96 105 100 100
Bi(II1) since the use of the calibration curve method may lead to erroneus results due to suppression of the Bi(II1) signal by surfactants present in the water samples. Oxidizing Agent. Mercury(I1) together with dissolved oxygen acted as an oxidant in the determination of Bi(II1) in seawater (15, 16). Mercury(I1) cannot be used in the PSA determination of NTA since a stable Hg:NTA complex is formed at around pH 2. The rate of oxidation using dissolved oxygen alone as an oxidant gave broad peaks. Sharper peaks were obtained when potassium dichromate was added to the water sample. The nonelectrolytic oxidation process was thus achieved by using potassium dichromate together with dissolved oxygen as the oxidant. Excess KzCrz07was used so that the stripping time of free Bi(II1) was not affected by the consumption of K2Cr207before eight electrolyzing cycles (two for free Bi(II1) and six for three standard additions). Interferences. Other heavy metals which also form stable complexes with NTA and could be present in water, like Sb, As, Cd, Cr(III), Cr(VI), Co, Cu,Fe(II), In, Ni, V, and Zn, cannot interfere since they have low conditional stability constants at pH (7). Only Fe(II1) forms a stable complex with NTA at pH 2 and thus can interfere in the PSA determination of NTA. However, it has been found that stirring the sample containing Fe(II1) up to a concentration of 6 ppm for 10 min prior to PSA determination eliminates its interference and NTA was quantitatively recovered. If the Fe(II1) concentration is very high it can be reduced to Fe(I1) using John’s reductor prior the PSA determination. Cu(I1) produces a response in the same potential range as Bi(II1). It has been found that concentrations up to 600 ppb did not interfere. However, for higher concentrations Cu(I1) must be removed from the sample by preelectrolysis before running the PSA measurement. Precision and Accuracy. The method was evaluated with respect to precision and accuracy. Use of samples of deionized water with added NTA standards gave a high correlation
95% confidence
interval
NTA added, ppb
av, ppb
re1 std dev, %
(ts/W”?
128 64 6
123 66 7
1.3 1.5 6.3
1.8 0.93 0.38
Table IV. Measurement of NTA in Waste- and Natural-Water Samples during Spring 1986 NTA samp1ea (1)wastewater before entering into the sewage
treatment plant (2)water coming out of the sewage treatment plant (3)Zerka River (1 km away from the sewage treatment plant of the factory)
found, ppb
1.6 x 105 700 400
Samples were diluted as follows: sample 1, 107-fold;samples 2 and 3, 3-fold. Results are averages of three measurements. coefficient (0.9989) between the values for NTA as added and found. An equally high correlation coefficient (0.9999) was found when the experiments were repeated with tap-water samples. Tables I and I1 show that the recovery of NTA is excellent in both cases. Precision of the indirect PSA determination of NTA is shown in Table 111. Precision in the range of 127, 64, and 6 ppb with a relative standard deviation of 1.3, 1.5, and 6.3%, respectively, is very good. The applicability of the method for NTA determination in natural waters has been demonstrated in tap-water samples in the concentration range 6-255 ppb. The recovery of NTA added shown in Table I1 is very high. In Table IV the reliable application of the PSA technique to the determination of NTA in three wastewater samples collected from a detergent factory at the Zerka River in Jordan has been established. Appropriate dilution must be made to obtain a measurable signal. The measured levels confirm the discharge of significant amounts of NTA into the Zerka River. Registry No. NTA, 139-13-9;HzO, 7732-18-5.
LITERATURE CITED (1) Pollard, R. P. Pet. Chem. Develop. 1968, 4 5 , 197. (2) Thompson, J. E.; Duthie, J. J. Water Pollut. Control Fed. 1968, 4 0 , 306. (3) Swisher, R. D.; Crutch-field, M. M.; Gladwell D. W. Envlron. Sci. Techno/. 1987, 1, 820. (4) Games, L. Envlron. Sei. Techno/. 1981, 15, 1488. (5) Afgan, B. K.; Goulden, P. D. Environ. Sci. Technol. 1971, 5 , 601. (6) Asplund, J.; Wanningen, E. Anal. Lett. 1971, 4(5), 267. (7) Afgan, B. K.; Goulden, P. D.; Ryan, J. F. Anal. Chem. 1972, 4 4 , 354. (8) Haberman, J. P. Anal. Chem. 1971, 4 3 , 63. (9) Harlng, B. J. A,; Delft, W. V. Anal. Chim. Acta 1977, 9 4 , 201. (10) Hoover, T. B. EPA 1973, ISS EPA-R2-73-154.
Anal. Chem. 1987, 59, 211-212 (11) Stolzberg, R. J. Anal. Chlm. Acta 1977, 92(1), 139. (12) Taylor, J. K.; Ziellnskl, W. L., Jr.; Maienthul, E. J.; Durst, R. A.; Burke, R. W. Net/. Bur. Stand. 1972, 21903513. (13) VOUlQarOPOUloS. A.; Vdenb, p.; Nurnberg. H. w. Ff8S8nlUS' 2.Anal. Ctmm. 1984.317,367. (14) Karadakov, B. P.; Venkova, D. I. Talanta 1970. 17, 878.
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(15) Eskllsson, H.; Jagner, D. Anal. Chim. Act8 1982, 138, 27. (16) Jagner, D.; Aren, K. Anal. Chlm. Acta 1982, 737, 201.
R~~~~~~ for review~
~12, 1986. ~ i ~ ~l ~ ~ b~~l~ ~ 25, i t t ~ d 1986. Accepted August 15, 1986.
Luminol Chemiluminescence for Determination of Iron( I I ) in Ferrioxalate Chemical Actlnometry M a r k A. Nussbaum, Howard
L. Nekimken,' a n d Timothy A. Nieman*
Department of Chemistry, University of Illinois, 1209 West California Street, Urbana, Illinois 61801 Actinometry is a means of measuring light fluxes from an ultraviolet or visible light source. Chemical actinometry requires a solution that undergoes a chemical change (such as a redox reaction) of known quantum yield upon photoexcitation. Species used in actinometer solutions have included ferrioxalate (I,2))cobaltioxalate (3), uranyl oxalate (4), and Reineckate ion ([Cr(NH,),(NCS),]-) (5). Ferrioxalate is the most widely used actinometer solution. Its reaction sequence is shown below (1,2):
HaO*
[Fe(C20&3l3-
[Fe(C204)l++ 2(C2O4l2-
+ (C204)2 C 0 2 + Fe2+ + (C204)2-
[Fe(C204)]+-kFe2+ (C204)-
-
+ [Fe(C204)]+
Note that a single photon can potentially produce two Fe2+ ions. The Fe2+produced in this reaction has traditionally been detected by measuring the absorbance after complexation with 1,lO-phenanthroline (Arn= = 510 nm) (1). Cobaltioxalate undergoes a similar photochemical reaction in which Co2+is produced instead of Fe2+,at a somewhat lower quantum yield (3). Luminol chemiluminescence (CL) has been shown to be a very sensitive method for the determination of several metal ions, including Fe2+and Co2+(6-8). In the presence of H202, M using luminol CL Co2+can be quantitated to below (8). Because of this low detection limit for Co2+ and the observations that Co3+ (9) and Co(II1) complexes (IO) are inactive as luminol CL catalysts, it would appear that luminol CL shows promise as a means of detecting Co2+produced in cobaltioxalate actinometer solutions. We have found, however, that although luminol CL can be used to detect photogenerated Co2+, the thermal instability of cobaltioxalate (11) results in a high CL background which cannot be reduced sufficiently (via extraction of the cobaltioxalate with dithizone) to allow sensitive determinations. Luminol CL can be used to quantitate Fe2+in the absence of H202,as long as molecular oxygen is present. Seitz (6) reported an Fe2+detection limit of approximately 5 X M using such a system. The use of oxygen as the primary oxidant in the luminol CL reaction allows the selective quantitation of Fez+in the presence of Fe3+, since Fe3+ is not a catalyst for this system in the absence of H202. The sensitivity and selkctivity of Fez+detection by luminol CL indicate that such a detection scheme may be of use in ferrioxalate actinometry. The reported detection limit for Fe2+by luminol CL is 2 or 3 orders of magnitude lower than that obtained from M vs. the iron-phenanthroline absorbance method (5 X Present address: Los Alamos National Laboratory, M a i l Stop
G740, Los Alamos, NM 87545.
0003-2700/87/0359-0211$01.50/0
lo-' M) (1,6). Improved detection limits would lead to the ability to detect lower light levels. In addition, the CL method offers a potential advantage in that no complexation step is required; the ferrioxalate actinometer solution could be analyzed directly for Fe2+immediately after irradiation. Finally, the use of a technique that does not depend on complexation by phenanthroline would obviously circumvent the errors reported to arise from photodegradation of competitive complexation of phenanthroline (12). The purpose of the work reported here was to investigate the potential advantages offered by luminol CL for quantitation of Fe2+ generated photochemically from solutions of ferrioxalate. EXPERIMENTAL SECTION Reagents. Solutions containing 6 mM ferrioxalate (K3Fe(C204)3-3Hz0; Pfaltz and Bauer) in 0.05 M H 8 0 4were prepared on the day of use from a 60 mM stock solution. Standard solutions of Fez+,also in 0.05 M HzS04,were prepared from FeS04.7H20 (Mallinckrodt). Iron-phenanthroline solutions were prepared by using 1.0 mL of 0.1% 1,lO-phenanthroline monohydrate (GFS), 3.0 mL of buffer (pH 4.4) containing 0.18 M HzS04and 0.6 M sodium acetate (Mallinckrodt), and 5.0 mL of the appropriate iron solution, diluted to 10.0 mL with water. The Fez+ CL measurements made use of 5 mM luminol in 0.4 M borate buffer (pH 10.8). Carbonate or ammonia would provide better buffering capacity at the pH used, but each has been found to adversely affect luminol CL measurements (13). The luminol concentration and pH were selected based on the optimum values found previously for determination of Fez+(6) All water used was purified by a Continential/Millipore Milli-Q system. Apparatus. Iron-phenanthroline absorbance measurements were made at 510 nm with a Hewlett-Packard 8450 diode array spectrophotometer and 1-cm cuvettes. A few of the CL measurements were made with a home-built stopped-flow instrument previously described by Stieg and Nieman (14). For the remaining CL measurements, a home-built flow-injectionanalyzer was used (Figure 1). The instrument made use of a Rainin Rabbit peristaltic pump, Rheodyne Model 5020 injection valve (70-pL sample loop), and Teflon tubing (0.8 mm i.d.). One channel contained 0.05 M HzSO4, and the other contained the buffered luminol solution. The observation cell volume was approximately 180 pL. The large cell volume was used so that changes in reaction rate with Fe2+concentration would not cause the maximum CL intensity to occur outside of the cell. Flow rates were 0.7-0.8 mL/min in each channel. Chemiluminescence wm measured with a 1P28 photomultiplier tube and Pacific Precision Model 126 photometer linked to a strip-chart recorder. A tungsten light source was used to irradiate the actinometer solutions. The optics of the source were such that the lamp image was brought to a focus 2 cm beyond the source housing. Procedure. Except for weighing the solid starting material, which was done under dim room light, all solutions containing ferrioxalate were prepared in a darkroom under safelights. Flasks containing stock ferrioxalate solutions were wrapped in aluminum foil and stored in a darkroom refrigerator. 0 1986 American Chemlcal Society
