UDC 614.777:546.815/.819]-074

L. V. Dracheva, T. A. Belyavskaya, A. F. Radchenko, V. V. Bezverkhaya

Determination of Lead in Waters Using a Lead-Selective Electrode

The potentiometric method for determining lead in aqueous solutions [2] is promising because it is rapid, offers simple detection and reliable operation, and can be automated. Its disadvantages are relatively low sensitivity and low electrode selectivity.

The lead-selective electrode is known to be used primarily in indirect methods for determining various elements by potentiometric titration [3]. The aim of this study was to investigate the feasibility of using a lead-selective electrode for the direct potentiometric determination of lead in natural and process waters.

An experimental solid-state lead-selective electrode was used in the study. The electrode membrane was made by pressing lead and silver chalcogenides at a molar ratio of 1:1 in an inert-gas atmosphere for 2 h under a pressure of 4–5 t/cm². Ionic conduction in the membrane is due to silver ions. Contact with the inner surface of the membrane in the electrode was made using silver solder. To adjust the ionic strength of the aqueous media analyzed, a 2.5 M sodium nitrate solution was added to the test solution in a ratio of 1:10. The pH of the solutions was measured using an ESL-47-03 glass laboratory electrode. A saturated EVL-1M3 silver–silver chloride electrode served as the reference electrode. Potential was measured with an F-30 ammeter-voltmeter. The accuracy of potential measurement was ±0.2 mV. Samples of the aqueous media were monitored in a thermostated flow-through electrochemical cell made of fluoroplastic. The cell has four ports: for the indicator and glass electrodes, the reference electrode, and a thermometer. Continuous measurement of potential helps produce more reproducible analytical results. For discrete potential measurements, a magnetic stirrer is required. Analytical data were recorded using a printer.

The stock standard solution (\(C = 1\) mg Pb²⁺/cm³) was prepared from an accurately weighed amount of analytical-reagent-grade Pb(NO₃)₂. Working standard solutions were obtained by successive tenfold dilutions of the stock solution and used on the day of preparation. Volumetric flasks, pipettes, and vessels made of polyethylene were used.

Before use, the lead-selective electrode was conditioned in a \(10^{-3}\) M Pb²⁺ solution for 24 h. It was then rinsed with deionized water until a constant potential of 180–200 mV was obtained. Between measurements, the electrode was stored in a \(10^{-4}\) M Pb²⁺ solution. Deionized water with a specific electrical conductivity of 0.1 µS/cm, obtained from a system with continuous water circulation, was used to prepare the required solutions and at all stages of the analysis.

Figure 1 shows the calibration plot of \(E=f(\mathrm{pPb})\) for determining lead concentration in aqueous solutions. The parameters of the calibration line \(Y=A+BX\) were calculated by regression analysis on an SM-4 minicomputer. The calculated results were: \(A=-29.3\); \(B=27.4\); \(S_{\text{ад}}^{2}=6.2\); \(S_A=3.7\); \(S_B=1.0\), where \(A\) and \(B\) are the calibration coefficients—

Fig. 1. Calibration plot for determining lead concentration in aqueous solutions.

Horizontal axis: logarithm of lead concentration, pPb; vertical axis: potential \(E\), mV.

Fig. 2. Dependence of the potential on the ratio of iron (\(C_{\mathrm{Fe}}\)) and lead (\(C_{\mathrm{Pb}}\)) concentrations.

The abscissa shows the ratio of iron to lead concentrations; the ordinate shows the potential \(E\), mV. 1—mean potential in the absence of iron; 2, 3—lower and upper bounds of the confidence interval for the potential in the absence of iron.

…of the calibration plot; \(S_{\mathrm{ad}}^{2}\) is the variance of adequacy; and \(S_A\) and \(S_B\) are the standard deviations of coefficients \(A\) and \(B\). The calculated correlation coefficient (\(r=0.9964\)) confirms the linearity of the calibration relationship. The electrode response is linear over the concentration range \(10^{-6}\)–\(10^{-1}\) M Pb\(^{2+}\). The slope of the electrode response is 27 mV/pPb. The relative standard deviation \(S_r\) does not exceed 0.05 (\(n=10\)). The equilibrium potential is reached in 1 min, and the analysis of one sample without preconcentration takes 2–3 min.

The indicator electrode can also be used to monitor lead content in hot samples (up to 70°C), which helps accelerate the analysis. However, prolonged use of the electrode at elevated temperatures reduces its service life to 2–3 months.

The electrochemical properties of the lead-selective electrode also depend on the pH of the solutions analyzed. For solutions containing 10 and 1000 mg Pb\(^{2+}\)/dm³, the change in potential was studied over the pH range 1.0–10.0, and an operating range (pH 3.5–6.5) was established for potentiometric measurements. At lower pH values, lead sulfate begins to precipitate; at higher pH values, lead hydroxide precipitates. In both cases, this reduces the concentration of free lead ions and, consequently, lowers the potential.

Table 1

Results of the assessment of accuracy and reproducibility of lead determinations by the potentiometric method (\(n=6\), \(P=0.95\))

Table 2

Results of lead determination by direct potentiometry in natural waters (Moskva River basin) and process waters (\(n=5\), \(P=0.95\))

Natural and process waters generally have multicomponent compositions. To investigate the selectivity of the electrode by the mixed-solution method, its behavior was studied in the presence of iron and copper, major components of natural waters. Figure 2 shows the effect of iron on the potential of the lead-selective electrode. The quantitative measure of selectivity was the limiting ratio \(R_{\max}(C_{\mathrm{Fe}}/C_{\mathrm{Pb}})\), where \(C_{\mathrm{Fe}}\) and \(C_{\mathrm{Pb}}\) are the concentrations of iron and lead in solution (in mol/dm³), at and above which the potential at a given lead concentration differs from the mean potential in the absence of iron by more than a specified threshold. The threshold used was \(t_{P,f}\cdot S/\sqrt{n}\), where \(S\) is the standard deviation of the potential measurements (\(C=100\) mg Pb\(^{2+}\)/dm³) in the absence of iron; \(t_{P,f}\) is Student’s coefficient (\(P=0.95\)); and \(n\) is the number of replicate measurements [5]. To estimate \(R_{\max}\), the experimental data were used to plot \(E=f(C_{\mathrm{Fe}}/C_{\mathrm{Pb}})\) at \(C_{\mathrm{Pb}}=\mathrm{const}\), and the intersection of this curve with one of the lines \(E=\overline{E}\pm t_{P,f}\cdot S/\sqrt{n}\), which bound the confidence interval for the potential in the absence of iron, was determined. The data obtained gave \(R_{\max}(C_{\mathrm{Fe}}/C_{\mathrm{Pb}})=7\cdot10^{-2}\).

The selectivity coefficient for copper was not determined, because potential drift was observed even at a copper concentration of \(10^{-7}\) M. Thus, the presence of iron and especially copper has a substantial effect on potential measurements. These elements must therefore be removed from the solution or masked (sodium fluoride is used to mask iron, and thiourea to mask copper).

When determining microgram quantities of lead in water, preconcentration is necessary. Vacuum distillation [1] was used for this purpose. The method permits rapid processing with minimal losses…

and to concentrate the water sample being analyzed without introducing contamination from external sources. A Rotadest type 2044 rotary evaporator manufactured by Metrimpex (Hungarian People’s Republic) was used for concentration.

On the basis of this work, a procedure was developed for the direct potentiometric determination of lead in natural and process waters. A 100–200 cm³ water sample is placed in a 250 cm³ evaporation flask and concentrated using a rotary evaporator. The concentrate is transferred to an electrochemical cell, the pH is checked, and 1 cm³ of 2.5 M NaNO₃ solution is added. The potential is measured (\(n=3\)), and the lead concentration in the water sample is determined from its mean value using a calibration curve or the equation of its linear portion.

The accuracy and reproducibility of the potentiometric analysis were established by the added–found method (Table 1).

The potentiometric method can be used to determine the relationship between the total lead content and its dissolved form in the water sample. To determine the total lead concentration, acid treatment is carried out during the concentration step: 1 cm³ of high-purity HNO₃ obtained by isopiestic distillation [4] is added to a 100–200 cm³ sample. This helps convert all forms of lead present into a dissolved state. Comparing the results of analyses performed with and without acid treatment makes it possible to draw conclusions about the predominant form of lead in the water sample.

The lead content of natural waters from the Moskva River basin and process waters used in the manufacture of electronic equipment was determined by direct potentiometry (Table 2).

References

1. Dracheva, L. V., and Petrosyan, I. V. Zavodskaya Laboratoriya, 1987, no. 3, p. 19.
2. Kammann, K. Working with Ion-Selective Electrodes. Translated from English. Moscow, 1980.
3. Midgley, D., and Torrance, K. Potentiometric Water Analysis. Translated from English. Moscow, 1980.
4. Guide to the Chemical Analysis of Surface Waters on Land. Leningrad, 1977.
5. Charykov, A. K. Vestnik LGU, 1984, no. 4, pp. 64–71.

Received January 14, 1988.