[43] S. L. Johnson, M. Bliss, M. Mayersohn, K. A. Conrad, Clin. Chem. 1984, 30 ... [48] A. Albert, E. P. Serjeant, The Determination of Ionization. Constants, 3rd.
Mikrochim. Acta 134, 229±235 (2000)
Catalytic Determination of Vanadium Based on the Bromate Oxidative-Coupling Reaction of Metol with Phloroglucinol Ashraf A. Mohamed , and Khaled F. Fawy Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, 11566 Egypt
Abstract. A selective, sensitive and simple catalytic method is developed for the determination of vanadium in natural and highly polluted waste waters. The method is based on the catalytic effect of VV and/or VIV on the bromate oxidative-coupling reaction of metol with phloroglucinol (PG). The reaction is followed spectrophotometrically by tracing the oxidation product at 464 nm after 10 minutes of mixing the reagents. The optimum reaction conditions are metol (8:0 10ÿ3 M), PG (4:0 10ÿ3 M) and bromate (2 10ÿ2 M) at 35 C and in presence of an activator-buffer mixture of 5 10ÿ2 M of each of citric and monochloroacetic acids (pH 2:40). Following the recommended procedure, vanadium can be determined with a linear calibration graph up to 8.0 ng mLÿ1 and a detection limit, based on the 3sb criterion, of 0.1 ng mLÿ1 . Spectrophotometric determination of as little as 1.0 ng mLÿ1 of VV or VIV in aqueous solutions gave an average recovery of 98% with relative standard deviations of 1.8% (n 5). The proposed method was directly applied to the determination of vanadium in Nile river water and highly polluted industrial wastes. Statistical treatments of analytical results could not detect any systematic error and showed the high accuracy and precision of the developed method. Key words: Catalytic-spectrophotometric determination of vanadium; oxidative coupling reactions; metol; natural and wastewaters.
To whom correspondence should be addressed
Determination of trace levels of vanadium, as a characteristic pollutant, is receiving increased interest in biochemical and environmental studies [1±4]. Major sources for the emission of vanadium in the environment include combustion of fuel oils, dyeing, ceramics, ink, catalyst and steel manufacturing. Discharges from such sources along with hydrological processes can contribute to the presence of vanadium in natural waters and water supplies. Sensitive techniques for vanadium determination in natural and waste waters include neutron activation analysis (NAA), electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma-mass spectrometry (ICP-MS), in addition to high performance liquid chromatography (HPLC) and solid phase spectrophotometry (SPS). Improved detection limits of 0.01±0.10, 0.16±0.56, 0.05±0.30, 0.15±0.94 and 0.10±5.0 ng mLÿ1 were reported, respectively, for NAA [5±7], ETAAS [8±10], ICP-MS [11±13], HPLC [14±16] and SPS [17, 18], only after applying the suitable pre-concentration and/or separation processes. Despite of their sensitivity and their applicability to wide ranges of samples, such techniques have two common disadvantages; namely, the need for suitable separation and preconcentration techniques and the high instrumental and/or operational costs. However, catalytic methods of analysis [1±4, 19±37] offer simple, sensitive and inexpensive alternatives for vanadium determination, of which the Fishman-Skougstad method [19] was adopted as a standard for vanadium determination in natural and waste waters [3]. However, the reliable application of that method [3, 19] or its modi®cations [20±22] to a
230
wide variety of waters, seemed to be doubtful [3, 23]. Therefore, low cost, simple, sensitive and selective methods for trace level determination of vanadium are desirable. On the other hand, metol (4-methylaminophenol sulfate) and phloroglucinol (1,3,5-trihydroxybenzene or PG) were frequently applied to the analysis of some pharmaceutical compounds [38±43]. However, metol and PG were encountered only twice in catalytic analysis; namely for the determination of iron [44] and copper [45] down to 0.8 and 1.0 ng mlÿ1 based on the metol-H2 O2 and PG-H2 O2 reactions, respectively. The present work describes a very simple, sensitive and selective method for trace level determination of vanadium based on its catalytic effect on the bromateoxidative coupling reaction of metol with PG in presence of citric acid as an activator. Moreover, the developed method was conveniently applied to a wide variety of waters without the need for any preconcentration or separation processes. Experimental Apparatus Absorbance measurements were made on a pre-calibrated double beam UV-Visible Spectrophotometer (Shimadzu UV-1601, Kyoto, Japan) equipped with 10 mm matched cells. The cell compartment of the spectrophotometer was thermostatically controlled by circulating water from a PolyScience thermostated water bath with a temperature stability of 0.1 C. Eppendorf vary-pipettes (10± 100 and 200±1000 mL) were used to deliver accurate volumes. pH measurements were made on a calibrated E.D.T. (Dover Kent, UK) pH-mV meter model GP 353 equipped with an E.D.T. combined glass electrode with an accuracy of 0.01. All glass-ware and storage bottles were soaked in 10% nitric acid overnight and thoroughly rinsed with fresh distilled, de-ionized water prior to use. Reagents All chemicals were of ACS and/or analytical-reagent's grade and were used without further puri®cation, unless otherwise stated. Fresh distilled, de-ionized water was used throughout. Metol (Sigma, St. Louis, Missouri, USA) and phloroglucinol (Merck, Darmstadt; Germany) solid reagents were inde®nitely stable when kept at 4 C; however, reagents purchased from different manufacturers gave slightly different calibration data but with essentially the same calibration slopes. A working 8:0 10ÿ2 M solution of metol was prepared by dissolving the reagent in 0.5 mL of 1.0 M phosphoric acid and about 20 mL of water in an ultrasonic bath. The resulting solution is made up to 50 mL in a calibrated ¯ask, wrapped with an aluminum foil and stored at 4 C, when not in use. This solution is stable for at least two days, however, it should be replaced when a red tint becomes apparent. A working 4:0 10ÿ2 M solution of PG was prepared by dissolving the reagent in water in an ultrasonic bath. The resulting solution is made up to 50 mL in a calibrated ¯ask, wrapped with an
A. A. Mohamed and K. F. Fawy aluminum foil and stored at 4 C, when not in use. This reagent is stable for at least one week. A stock standard vanadium solution of 1000 mg mLÿ1 of VV was prepared from NH4 VO3 [3]. Also, a stock standard VIV solution of 1000 mg mLÿ1 was purchased from Fluka (Buchs, Switzerland). Working 50 ng mLÿ1 standard solutions of VV or VIV were prepared freshly from their corresponding stocks. An activator-buffer mixture of pH 2:40 0:02 was prepared by dissolving 0.025 mol of each of citric and monochloroacetic acids in about 90 mL of water, adjusting the pH to the desired value and diluting to the mark in a 100 mL calibrated ¯ask. A working standard 0.25 M solution of sodium bromate was prepared by dissolving the reagent (Aldrich, Milwaukee, Wisconsin; USA) in water. In the study of interference, cations were in the form of nitrate or sulfate; however, use of chlorides was kept to a minimum. Anions were in the form of sodium, potassium or ammonium salts. Recommended Procedure Water samples were collected, ®ltered through 0.45-mm Millipore membrane ®lters of the MF-HA type, acidi®ed with HNO3 to pH of 1:8 0:2, stored at 4 C and analyzed within 48 h of collection. Prior to the analysis step, each sample is brought to a working pH of 2:40 0:2. Pyrex grade A, 20 mL, stoppered glass test tubes were kept at 35 C in the thermostated water bath, whereas the standard working solutions were kept at room temperature. Transfer a portion of the unknown sample or the working standard VV solution to one of the test tubes and dilute with water to 2.60 mL. Add 1.00 mL of the working activator-buffer mixture and 0.50 mL of each of the working PG and metol solutions. Shake and place the test-tube in the thermostated water bath for 15 min to attain the equilibrium temperature. Start the reaction by adding 0.40 mL of the standard working bromate solution, shake well and immediately transfer a portion of the reacting mixture to the thermostated cell of the spectrophotometer to record the absorbance after 10 minutes at 464 nm against water as a reference. The vanadium concentration of the unknown sample is calculated from a calibration graph similarly prepared with the working standard vanadium solution.
Results and Discussion Metol may be oxidized slowly by acidic bromate to give an oxidation product exhibiting one absorption maximum at 360 nm. However, the oxidation of PG was too slow to be detected spectrophotometrically within 20 min of mixing the reagents. On the other hand, oxidation of a mixture of metol and PG rapidly gave an oxidative-coupling product that exhibited two strong absorption maxima at 365 and 464 nm, respectively, Fig. 1. Vanadium exerted a pronounced catalytic activity on that oxidative-coupling reaction leading to the development of a highly sensitive and selective method for vanadium determination down to 0.1 ng mLÿ1 that was conveniently applied to polluted river and waste water samples. Preliminary experiments showed that the positions of the two absorption
Catalytic Determination of Vanadium Based on the Bromate Oxidative-Coupling Reaction
231
procedure to provide a moderate sensitivity and a lower blank reading. Optimization of the Reaction Variables
Fig. 1. Absorption spectra: 1 and 2, the bromate oxidative coupling product of metol with phloroglucinol; 3 and 4; the oxidation product of metol alone. 1 and 3, in presence of 5 ng mLÿ1 of vanadium whereas 2 and 4 are their reagent blanks. Spectra were recorded after 10 min of starting the reaction, in 10 mm cells. Other reaction conditions are as given in the recommended procedure
bands at 365 and 464 nm were essentially stable with time even with changing the reaction conditions; with the latter peak showing a much higher intensity. However, the initial parts of the resulting A-t graphs showed poor linearity. Therefore, measurements at 464 nm with the ®xed time method, after 10 min of starting the reaction, was adopted in the recommended
The absorbances of the catalyzed and uncatalyzed reactions, Ac and Au , and the sensitivity, Ac ÿ Au , decreased almost linearly with pH, Fig. 2a. However, in order to provide a moderate sensitivity and a lower reagent blank, a pH of 2:40 0:02 was adopted in the recommended procedure. An activator-buffer mixture of citric and monochloroacetic acid was used for pH adjustment. Citric acid was selected to act as a buffering medium and as a possible activator and a masking agent for some transition metal cations [46, 47]. However, because of the poor buffering capacity of citric acid at lower pH values [48], monochloroacetic acid was added to provide a reasonable buffering action. The absorbance of the catalyzed reaction sharply increased with citric acid (of pH 2:40 0:02) concentration up to 2:5 10ÿ3 M, after that however, Ac linearly increased with citric acid concentration, Fig 2b. On the other hand, Ac and Au slightly and linearly increased with the concentration of mono-
Fig. 2. Effects of: (a), pH; (b), citric acid concentration; (c), bromate concentration; (d), PG concentration; (e), metol concentration; and (f), temperature. Except for the abscissa variable, reaction conditions were as given in the recommended procedure. Au , un-catalyzed reaction and Ac , reaction catalyzed by 5 ng mLÿ1 of vanadium
232
A. A. Mohamed and K. F. Fawy
Table 1. Effects of different activators on the reaction system Activator Name ± Potassium hydrogen tartrate Citric acid Malonic acid Oxalic acid# Succinic acid Nicotinic acid# Salicylic acid 5-Sulfosalicyclic acid 8-Hydroxy quinoline sulfate# Tiron
Absorbance M10 ± 5 10 10 5 10 5 10 10 5 10
3
Ac 0.313 0.430 0.458 0.414 0.380 0.427 0.442 0.415 0.435 0.399 0.365
Au 0.115 0.119 0.118 0.120 0.110 0.115 0.123 0.128 0.129 0.195 0.124
Except for the activator's type and concentration, other conditions were as given in the recommended procedure. Au , uncatalyzed reaction and Ac , reaction catalyzed by 5 ng mLÿ1 of vanadium. # Higher concentrations gave lower Ac values.
chloroacetic acid (of pH 2:40 0:02). Therefore, 5:0 10ÿ2 M concentration of each of citric and monochloroacetic acid (of pH 2:40 0:02) was adopted in the recommended procedure to provide reasonable activating, masking and buffering actions, and moderate sensitivity and blank readings. The choice of a proper activator may greatly enhance the sensitivity of a catalytic method and increase its selectivity [4, 46]. The use of the activating and masking effects of citric acid in catalytic analysis was ®rst proposed and applied by Bonchev to the vanadium-catalyzed reaction of pphenetidine with chlorate where citric acid enhanced the selectivity towards FeII and CuII and increased the sensitivity by a factor of 15 [46, 47]. Therefore the effects of citric acid and other possible activators were tested in the present study. Table 1 showed that the best activator for the reaction was citric acid that gave the highest sensitivity and the lowest blank value and therefore it was adopted in the recommended procedure. Moreover, the effects of some surfactants of the cationic, anionic and non-ionic types were studied in order to investigate the possible rate enhancement by micellar catalysis due to the formation of organized assemblies that are formed above the critical micellar concentrations (CMC's) of the respective surfactants [25]. Namely, the effects of cetyl trimethylammonium chloride, cetylpyridinium bromide, sodium lauryl sulfate, Triton X-100 and Tween-80 were investigated. However, all of the tested surfactants, unexpectedly, exerted pronounced decelerating rather than accelerating effects.
The absorbance of the catalyzed reaction increased gradually, whereas that of the uncatalyzed reaction increased almost linearly with bromate concentration, Fig. 2c. However, 2:0 10ÿ2 M bromate was adopted in the procedure to provide moderate sensitivity and lower blank readings. The absorbances Ac and Au and the sensitivity, gradually increased with PG concentration up to 2:4 10ÿ3 M and after that they remained almost constant, Fig. 2d. Therefore, a 4:0 10ÿ3 M PG was adopted in the procedure. The absorbance of the catalyzed reaction increased gradually, whereas that of the uncatalyzed reaction increased linearly with metol concentration, Fig. 2e. However, a metol concentration of 8:0 10ÿ3 M was used in the procedure to provide a moderate sensitivity and a low blank reading. The absorbances Ac and Au slightly increased with the concentration of an added salt as sodium nitrate, but without any effect on the sensitivity. For example, the presence of 0.2 and 0.4 M NaNO3 resulted in 5% and 13% increase in Ac values, respectively. Therefore, samples were safely acidi®ed with HNO3 , as a preservative, to a pH value of 1:8 0:2. The absorbances, Ac and Au , gradually increased with temperature as shown in Fig. 2f and a working temperature of 35 C was adopted in the recommended procedure because of its convenience for operation and to provide a moderate sensitivity and reagent blank. The absorbance was a function of the order of mixing the reagents. Table 2 showed that order of addition No. I was the best and therefore was adopted in the procedure. The table shows that addition orders III, IV and VI gave higher blank values 0.188, 0.198, 0.188 compared with the adopted order with a blank value of 0.184. Addition orders I, II and V gave almost the same readings. However for practical considerations, any extra lag time between the addition of reagents, as with order II, was not necessary. Also, the addition of bromate before metol, order V, was not recommended to avoid the possible involvement of PG and bromate in a process other than the oxidative coupling reaction. The effects of potential interferents, which generally accompany vanadium in natural and polluted waste-waters, were studied using 3 ng mLÿ1 of vanadium. The maximum tolerable concentrations of 70-foreign species are shown in Table 3, where the tolerance level was de®ned as the concentration of
Catalytic Determination of Vanadium Based on the Bromate Oxidative-Coupling Reaction Table 2. Effects of the order of addition of reagentsa Order number
Metol
I II III IV V VI
2 (3) 2 2 2 4 4c
PG
3 (2) 3c 4 4c 2 2
BrOÿ 3
4 4 3 3 3 3
Absorbance
b
Ac
Au
0.540 0.541 0.539 0.550d 0.538 0.541
0.184 0.186 0.188 0.198d 0.184 0.188
a
Reaction conditions, except for the order of addition, were as given in Table 1 where reagents were added successively, without undue delay, to the reaction cell containing vanadium, water and the buffer mixture. b Absorbance values were averages of ®ve replicate determinations. c A one-minute standing time was allowed before addition of that reagent. d Data with high initial absorbance values.
foreign species that produced a change in the absorbance of the catalyzed reaction of less than 5%. The reported concentration levels for the common pollutants in natural waters [3] are generally orders of magnitude lower than their tolerance limits shown in Table 3, indicating the high selectivity of the developed method. Moreover, a comparison of Table 3 with the corresponding one in ref. 3 clearly indicates the advantageous selectivity of the present method over the Fishman-Skugstad standard method, espeTable 3. Tolerance levels of foreign ions in the determination of 3 ng mLÿ1 of vanadium(V)
233
cially in the analysis of polluted waters containing II II appreciable concentrations of Iÿ , NOÿ 2 , Co , Cu , II II Mn , and/or Ni ions. However, in some samples the tolerable concentration of FeIII may be exceeded but because of the high sensitivity of the method, such effect is rendered harmless by simple dilution of the sample. Recovery of VIV, Calibration Graph and Detection Limit The determination of 1, 3 and 5 ng mLÿ1 of VIV, following the recommended procedure gave an average recovery of 98%; indicating that the developed method may be equally applied to the determination of both VV and VIV without any pretreatment. A linear calibration graph (r 0.9998) for up to 8.0 ng mLÿ1 vanadium was obtained, following the recommended procedure. The least squares equation for the calibration graph is: A 0:185 0:071[VV ], where [VV ] is the VV concentration expressed in ng mLÿ1 . The detection limit, based on the 3sb -criterion, was 0.10 ng mLÿ1 of V and the relative standard deviations, RSD%, for ®ve replicate determinations of 1, 3 and 5 ng mLÿ1 of VV or VIV were 1.8%. Moreover, the Student's t-test values were 0.8, showing that the t-test could not detect any systematic error in the method. (The tabulated t-value for the 95% con®dence level and n 5, is 2.78 [49]).
Tolerance level, mg mLÿ1
Foreign ions$
Determination of Vanadium in Nile-River Water and Polluted Waste-Waters
>600
acetic acid, tartaric acid, lactic acid, H2 PO4ÿ#, 2ÿ III III II NOÿ 3 , SO4 ,Na , K , NH4 , Al , Bi , Cd , CeIII , NiII , SrII
100
Acetyl acetone, dimethylglyoxime, hydrazine sulfate, Sulfamic acid, AsIII , AsV , BeII , CaII , CoII , LaIII , LiI , MgII , MnII , PbII , ThIV , TlI , WVI, ZnII # II II # Cl ÿ#, Fÿ#, NOÿ 2 , Tiron , Cu , Hg # I III IV IV VII # Nÿ , Ag , Cr , Ce , Hf , Mn , TaV #, 3 TiIV #, UVI CyDTA#, oxine#, phenanthroline#, oxalate#, # # Brÿ#, Iÿ#, SO2ÿ , S2 O2ÿ , AuIII , NbV #, PdII , 3 5 PtIV , SnII EDTA#, DTPA#, thiourea#, SCNÿ #, CrVI, FeII , FeIII , MoVI ZrIV #
Owing to its high sensitivity and selectivity, the developed method was conveniently applied to the determination of vanadium in Nile River water, highly polluted synthetic water and industrial wastes without any separation or pre-concentration processes. A freshly collected sample was ®ltered through 0.45mm Millipore-HA membrane ®lter, acidi®ed with concentrated nitric acid to a pH of 1:8 0:2, kept at 4 C and analyzed within 48 h of collection. (The aim of the acidi®cation step was to prevent bacterial growth and the adsorption of vanadium on the walls of the polyethylene container and on colloidal particles [3]). However, just prior to analysis, each sample was brought to a working pH of 2:4 0:2. Table 4 shows the analytical results for Nile river water and waste water samples, obtained following
40 5 2 0.5 0.1
Reaction conditions were as given in Table 1. $ Ions suspected to have buffering actions were adjusted to pH of 2:4 0:2, before studying their effects. # Ions producing negative interferences.
234
0.480, 0.395, 0.371, 0.478, 0.331, 0.401, 4g
2e 3f
a b
0.478, 0.390, 0.370, 0.477, 0.335, 0.405,
Method A is based on the developed metol-PG-bromate reaction where conditions were as given in the recommended procedure. Method B is based on the modi®ed gallic acid-persulfate standard method [Ref. 20]. The equation of the calibration graph was A 0:025 0:0114[V], where [V] is the vanadium concentration in ng mLÿ1 . c Added to or found in the original sample. d Collected on 24-6-1999 from River Nile, about 1-km from Kaser El-Nile bridge, Cairo; pH 8:14. e A synthetic sample prepared by analyzing sample No. 1 in presence of 10-mg mLÿ1 of each of CuII , CoII and NiII , and 1-mg mLÿ1 of each of Iÿ and Brÿ ions as interfering species. f Wastewater collected on 28-6-1999 from batteries industry at the 10th of Ramadan city, Cairo. g Wastewater collected on 30-6-1999 from the cement industrial area, Helwan city, Cairo.
106 474 ± ± ± ± 102 103 ± 96 ± 98 1.0 2.0 1.4 0.9 1.9 1.1 0.475, 0.393, 0.367, 0.480, 0.333, 0.402, 2.00 2.00 2.00 2.00 1.00 1.00
0.477 0.401 0.374 0.476 0.337 0.403
0.288, 0.286, 0.294, 0.284, 0.287 0.390, 0.385, 0.392, 0.395, 0.389 1.00 2.00 1d
0.473, 0.398, 0.369, 0.482, 0.330, 0.399,
Volume taken (ml) Aa No.
7:18 0:16 7:22 0:13 Av. 7:20 0:21 10:27 0:10 7:41 0:15 6:52 0:09 10:34 0:09 10:43 0:20 15:28 0:16
10:35 0:29 33:90 0:63 ± ± ± ± 3.00 ± ± 4.00 ± 5.00 0.072, 0.073, 0.071, 0.074, 0.071 0.178, 0.183, 0.176, 0.179, 0.182 ± ± ± ±
2.8 1.9 ± ± ± ±
B
± ± ± ±
A B A
± ± ± 0.056, 0.058, 0.059, 0.057, 0.058
2.2 1.8
Added Found S.D. (n 5) A B Bb
± 7:15 0:25
RSD % Vanadium, ng mLÿ1 c Absorbance values Sample
Table 4. Determination of vanadium in Nile River water and industrial wastes
± 3.5
Recovery (%)
A. A. Mohamed and K. F. Fawy
the recommended procedure. The reliability of the proposed method was checked by comparison with the modi®ed gallic acid-persulfate standard method [3, 20] and by recovery experiments, which gave quantitative results (96±103%) with convenient reproducibility (RSD 0.9±2.2%) and thus revealing the high accuracy and precision of the proposed method. Namely, the data obtained for sample No. 1 were in excellent agreement with those obtained using the reference method. However, for sample No. 2 the reference method gave a recovery of 474%, due to its high susceptibility to the added interfering ions, whereas the developed method gave a quantitative recovery of 103%, showing the advantageous selectivity of the developed method. Conclusion In comparison with the high cost instrumental techniques for vanadium determination [5±16], the present work describes a selective, simple, sensitive and precise method that can monitor vanadium concentrations down to 0.1 ng mLÿ1 using a simple spectrophotometer, indicating the advantageous applications of catalytic methods in chemical analysis. References [1] D. Perez-Bendito, M. Silva, Kinetic Methods in Analytical Chemistry. Ellis Horwood, Chichester, 1988. [2] M. J. C. Taylor, J. F. Van Staden, Analyst 1994, 119, 1263. [3] A. E. Greenberg, L. S. Clesceri, A. D. Eaton (Eds.) Standard Methods for the Examination of Water and Waste-water, 18th Edn. APHA-AWWA-WEF: Washington DC, 1992. [4] A. A. Mohamed, M. Iwatsuki, T. Fukasawa, M. F. El-Shahat, Analyst, 1995, 120, 2281. [5] R. R. Rao, D. G. Goski, A. Chatt, J. Radioanal. Nucl. Chem. 1992, 161, 89. [6] J. Kucera, J. Lener, L. Soukal, J. Horakova, J. Trace Micropr. Tech. 1996, 14, 191. [7] Y. Sakai, K. Ohshita, K. Tomura, S. Koshimizu, Bunseki Kagaku 1994, 43, 919. [8] R. Chakraborty, A. K. Das, Fresenius J. Anal. Chem. 1994, 349, 774. [9] P. Bermejo-Barrera, E. Beceiro-Gonzalez, A. Bermejo-Barrera, F. Bermejo-Martinez, Analyst 1990, 115, 545. [10] A. Adachi, K. Ogawa, Y. Tsushi, N. Nagao, T. Kobayashi, Water Res. 1997, 31, 1247. [11] L. C. Alves, L. A. Allen, R. S. Houk, Anal. Chem. 1993, 65, 2468. [12] R. A. Reimer, A. Miyazaki, Anal. Sci. 1993, 9, 157. [13] E. M. Heithmar, T. A. Hinners, J. T. Rowan, J. M. Riviello, Anal. Chem. 1990, 62, 857. [14] S. Oszwaldowski, Analyst 1995, 120, 1751. [15] H. Wang, Y. X. Miao, W. Y. Mou, H. S. Zhang, J. K. Chen, Mikrochim. Acta 1994, 117, 65.
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