DEVELOPMENT AND APPLICATION OF POTENTIOMETRIC STRIPPING ANALYSIS

This paper focuses on the voltammetric determination of lead, cadmium and zinc in water. Two ways of determining were investigated: individually and all three metals simultaneously. The experiments were performed using the Potentiometric Stripping Analysis (PSA). Determination of metals in real samples was preceded by preliminary tests. Preliminary investigations were performed in order to determine the optimal conditions of measurement. It was concluded that the process of determining was for most part influenced by: pH, time of metals extraction, stirring rate of the solution and the thickness of the mercury layer on the working electrode. The s mallest concentrations of metals which can be deter mined using this method are: for lead 22.48 μg dm -3 , for cadmium 16.23 μg dm -3 and for zinc 18.75 μg dm -3 . The obtained results deviated from the actual 1.12% for lead, 1.91% for cadmium and 1.81% for zinc. All tests (individually and simultaneously) were conducted from model solution with concentration as follows: 44.96 μg dm -3 for lead, 32.47 μg dm -3 for cadmium and 37.50 μg dm -3 for zinc. The results of individual measurements deviated by 1.02% lead, 1.90% for cadmium and 1.89% for zinc. Simultaneously the contents were lower than real for: −4.58% for lead, cadmium for −1.91% and −1.89% for zinc. For the conditions determined, except for lead, deviations did not exceed ±2% . This indicates that Potentiometric Stripping Analysis is a good way of individual and simultaneous determination of lead, cadmium and zinc and for determination of their concentrations in water (river and groundwater).


INTRODUCTION
Heavy  metals are natural constituents of soil whence are due into waterways and via of plants and in the food chain (Kastori, 1997;Seku lić et al., 2003;Kastori et al., 2006).They are characterized by toxic effects which are manifested in traces.Testing area is the northern part of Kosovo and Metohija.This part is directly affected by 100 million tons of existing flotation landfills of Trepča, which occupy an area of about 350 ha.A voltammetric technique used to determine traces of heavy metals in the surrounding surface water (river) and groundwater (natural spring and borehole) was Potentiometric St ripping Analysis (Wang, 1985;Jin et al., 1997).Determination of metals, by a three-electrode electrolytic system, was preceded by reduction and oxidation processes After reduction of metal ions on the working electrode, at a g ive potential, ext racted metal ions are oxidized with oxygen fro m the solution and in terms of diffusion mass transfer they return to solution while monitoring the dependence of the potentials from o xidation time (Jagner, 1979;Marjanović et al., 1987;Riso et al, 1999;).Th is dependence is proportional to material concentrations.The potential of the working electrode does not change until the entire concentrations of the separate elements are oxidized.After co mplete o xidation of one, the working electrode potential increases to the * Corresponding author: ljiljana.babincev@pr.ac.rs characteristic potentials at which oxidation of the following element takes place (Suturović, 1985;Suturović, 1992;Stanković et al., 2007;Suturović, 2003).
The aim of this study was: I) To establish conditions for the simu ltaneous determination of lead, cad miu m and zinc using Potentiometric Stripping Analysis ; II) Samp ling and sample preparation as well as the reduction to a single sample is suitable for Potentiometric Stripping Analysis ; III) Application of simu ltaneous Potentiometric Stripping Analysis for the determination of lead, cad miu m and zinc in water samples .

Apparatus
The tests in this paper were performed on the device for Potentiometric St ripping Analysis , Striping analy zer M 1 (Faculty of Technology in Novi Sad, Symmetry in Leskovac, Serbia).Basis of this system's functioning is a three -electrode electro lytic cell consisting of: disk working electrode of glassy carbon, the total area of 7.07 mm 2 , which is used as an inert carrier for the mercury layer; a reference silver−silver ch loride electrode (Ag/AgCl/KCl/ 3.5 mo l d m -3 ) and a platinum au xiliary electrode.Metal content using Potentiometric St ripping Analysis were determined by standard addition method (Bab incev, 2004).

Reagents
All the solutions for the execution of th is experiment were prepared fro m h igh purity chemicals (supprapur, Merck).The basic solutions were prepared by standard lead, cadmiu m, zinc and mercury (1.000 g dm -3 ) while working solutions were prepared fro m the basic standards in the concentrations as follows: for the lead 90 mg d m -3 3, zinc 75 mg d m -3 , for cad miu m 65 mg d m -3 .In addition to standard solutions, the following ones were also used: hydrochloric acid (HCl, 30%), nit ric -acid (HNO3, 65%), di-methyl ketone (CH3COCH3, 99.5%), coppersulfate (CuSO4) and galliu m-ch loride (GaCl 3 ).The solutions were stored in polyethylene bottles (Babincev, 2004).

Sample preparation
Sample preparation for Potentiometric Stripping Analysis was done so that the certain amount of filtered water was evaporated after adding 5 ml of concentrated nitrate.The process was repeated three times in order to transform metals into the shape of ions.Following separation and evaporation, the dry residue was dissolved with 5 ml chlorine acid, and adding the ionized water, it has been transferred into the measuring pot of 100 ml.A liquot of 20 ml has been transferred into the glass for electrolytic determination by Potentiometric Stripping Analysis (Bab incev et al., 2011;Bab incev, 2012a;Babincev, 2012b).

Procedure
Metal determination was preceded by the formation of the working electrode on the surface of glassy carbon from acidic solution of mercury(II)-ion concentration 10 mg d m -3 .Mercury film format ion at glassy carbon electrode was performed at a constant current of −48.90 µA fo r the time of 240 s.In order to define optimal experimental conditions for determination of metals, the series of solutions (model solutions) of 20.0 ml of deionized water and 0.5−200 μl working standard solutions of lead, cadmiu m and zinc were prepared (Babincev, 2012c).Working standards were added to a micropipettes with variable volume of 0.10 (±0.05) to 200 (±1) μl.Extract ion of lead, cadmiu m and zinc fro m prepared solutions was carried out at potentials of: −0.999 V fo r lead, cadmiu m to −1.106 V and −1.035 V for zinc.Simu ltaneous determination of all three metals was carried out at a negative potential (−1.400 V) compared to the potential of mercu ry that is positive (Babincev, 2012).The simu ltaneous determination of metals was performed after adjusting the pH and potential of separation (reduction) because in the analysis of strongly acidic solutions, for the reduction potential that is more negative than −1 V, it co mes to hydrogen evolution at the working electrode.For these reasons, the value for the hydrogen evolution potential is increased for the overvoltage of hydrogen on the metals tested.Simultaneous determination of metals was carried out with prior addition of Ga(III)-ions, in order to prevent the format ion of intermetallic compounds of zinc and possibly the present copper.Intermetallic compound in the o xidation process leads to a decrease in signal for zinc, wh ich depends on the ratio of copper and zinc.Studies were performed in model solutions with zinc concentrations of: 37.5, 187.5, and 375.0 µg d m -3 , and copper concentrations of 10.0, 20.0, 30.0 and 35.0 µg d m -3 .It was found that interference in the determination of zinc can come fro m the copper concentrations less than 10.0 µg dm -3 with an error of up to 30%.By adding galliu m of 40.0 µg d m -3 in solutions containing copper from 35.0 µg dm -3 , for wider range of zinc concentration, the creation of their co mplex is prevented because more stable complex of galliu m and copper is created.
In order to define the conditions of determination, the impact of the following was particularly examined: pH; stirring rate of solution, time of metals separation and time of working electrode formation.
After the conditions were established, the determination of lead, cad miu m and zinc in real samples (river and groundwater) was performed.

RES ULTS AND DISCUSS ION
The influence of pH solution on the efficiency of determination was examined in the value range from 1.3 −4.5 for metal concentrations of 224.30 µg d m -3 lead, 162.01 µg d m -3 cadmiu m and 375.00 µg d m -3 zinc.It was found that the results obtained deviate by ±2% when the determination were performed fro m solutions pH 1.3, 1.6 and 2.0.The ratio of oxidation time and content, represented as a constant of Potentiometric Stripp ing Analysis (K/s dm 3 mol -1 ), has the highest value for lead when determination was performed fro m solution pH 1.6, for cad miu m fro m solution pH 1.6 and 2.0, and for zinc fro m the solution pH 2.1−3.5.Simu ltaneous determination was performed at pH 2.1, because for a given value of pH constant of pH potentiometric stripping analyses for all three metals have the closest values (Fig. 1).

Figure 1. Dependence the constants Potentiometric Stripping Analysis for simu ltaneous determination of lead, cadmiu m and zinc by pH solutions
Effect of stirring rate on the determination of metals was -1.The most efficient determinations have a stirring rate of 4000 min-1, which can be explained by the optimal thickness of diffusion layer of ions in solution.Metal extraction was tested during periods of: 180, 240, 300, 360 and 420 s.The results of the determination showed that the most efficient metal ext raction is achiev ed for the period of 300 s.Contents extracted for the time deviate fro m the actual by: 1.05% for lead, 1.90% for cad miu m and 1.94% for zinc.The least time necessary for metal extract ion is 240 s, metal contents extracted for that time deviate from real by: 3.34% for lead, 3.00% for cadmiu m and 3.76% for zinc.The impact of the mercury layer thickness on metal determination was analyzed as the time required fo r the formation of the working electrode.The working electrode formation was done by separating the mercury on glassy carbon within : 120, 180, 240, 300 and 360 s.A good reproducibility was obtained for all three ions when the mercury layer was formed within 240 s.The mercury layer fo rmed in 240 s, based on reproducibility, provides a good homogenization of ext racted metal ions, easier and faster dissolution process and better separation.In order to determine detection limits of potentiometric analysis for lead, cad miu m and zinc, solutions of 20.0 ml of deionized water and different working standard volumes fro m 0.5 to 600 µl were prepared.Metal ext raction was performed within 300 s; solution stirring rate of 4000 min-1 and the pH values were 1.6 fo r lead and cadmiu m and 2.1 fo r zinc.The results for the determination are presented in Tables 1-3.Based on results presented, the minimu m contents of lead, cadmiu m and zinc that were determined with an error of ±2% are: 22.48 µg d m -3 of lead (determined with an error of 1.12%), 16.23 µg d m -3 cadmiu m (determined with an error of 1.91%) and 18.75 µg d m -3 zinc (determined with an error of 1.81%).In accordance with the standard deviations and the reproducibility, these contents can be considered the limits of determination.Standard deviations were 2.52 µg for lead, 1.56 µg of cad miu m and 1.99 µg of zinc.The detection limit of Potentiometric Stripping Analysis for lead, cadmiu m and zinc is about 20 µg dm -3 .The highest concentrations that are determined by PSA with an erro r ±2% were: 2194.05 µg d m -3 for lead; 960.12 µg dm -3 for cadmiu m and 1312.50 µg dm -3 for zinc.These contents were determined with the error of: −1,98% for lead, −1.93% for cadmiu m, and −1.95% for zink and standard deviations of: 169.24 µg for lead, 68.73 µg for cad miu m and 111.70 µg for zinc.

Simultaneous determinations
The determination of elements was studied simultaneously in 20.0 ml of deionized water and 10 µl working standard solutions of lead, cadmiu m and zinc, for metal concentrations of: 44.96 µg dm -3 lead, 32.47 µg dm -3 cadmiu m and 37.50 µg dm -3 zinc.Results of this study have shown that potential of the working electrode for the simu ltaneous determination of lead, cadmiu m and zinc is 490510 mV mo re negative than the potential of zinc dissolution (an element with the lowest redo x potential).It was determined that all three elements can be determined at a negative potential (1.400 V) in relation to the potential of mercury that is positive.For that reason, values for extraction potentials are increased by hydrogen overvoltage on mercury, lead, cad miu m and zinc.The simultaneous determination of lead and cadmiu m has shown optimal values for pH 1.6.For pH 2.1, cad miu m and zinc were simu ltaneously determined.Simu ltaneous determination of lead and zinc was examined for pH 1.3, 1.6, 2.0, the most effective determinations were for p H 2.0.Zinc is most accurately determined for p H value of 2.33.5, which may part ly be exp lained by the qualitative properties of zinc.The simu ltaneous determination of all three elements was performed (with the mentioned separation potential) for pH 2.1, because the constants of Potentiometric Stripping Analysis for this value were the closest.
With prior pH ad justment, metal content was determined by standard addition method.During the determination, metals are firstly reduced (separated) on working electrode and then (after electrolysis) they are oxid ized (returned) to the solution.After returning to the solution, oxidation potential is registered, which is changed until the entire separated contents of one metal is returned to the solution.After returning one element to the solution, working electrode potential increases to characteristic potential at which o xidation of the following takes place.Results of simultaneous determination of lead, cadmiu m and zinc are given in Table 4.The obtained results suggest that there is a difference in the determinate contents when we take individual and simultaneous determinations into consideration.The results for the individual determinations deviated from the actual values by 1.02% for lead; 1.90% for cad miu m and 1.89% for zinc.During the simu ltaneous determinations, the deviations were as follows: −4.58% for lead; −1.91% for cadmiu m and −1.89% for zinc.Results obtained for lead were less accurate.As it is about micro quantities, having in mind s mall standard deviations of simu ltaneous determination in relation to indiv idual, simu ltaneous determination of these elements is considered possible.Potentiometric Stripping Analysis is applied in determination of heavy metals in samples of different nature (Kaličan in et al., 2001a;Kaličanin et al., 2001b;Kaličan in et al., 2001c;Suturović et al., 2001: Kaličanin et al., 2002).In this work, lead, cadmiu m and zinc in samp les of waters are determined.The results for determination of the concentrations of lead, cadmiu m and zinc in river water (sampled successively every three months starting from april 2015 to february 2016) are shown respectively in Tables 5−8.3.00 3000.00Sample 1. Ibar River, the entrance to KM (pontoon bridge), a few kilometers away from the river flows into minutiae, Trepčanska rivers and river peeler; Sample 2. The river Ibar, output from the KM, after casting sundries and peeler, below the city waste disposal Upper Field at Dudinog rubble; Sample 3. The river Ibar, rudarački bridge downstream from the waste disposal,, Trepča,, near the Trepča RM HK in Zvecan; Sample 4. River Ibar bridge Grabovac-Žitkovac, exit behind the city waste disposal Žitkovac; Sample 5. Ibar River, the village Kutnje, entrance to Leposavić, in addition to the city waste disposal Bostaniste in Leposavić; Sample 6. Ibar River, the village of Gornji Krnjin, exit from Leposavić, in addition tailing impoundments Gornji Krnjin in Leposavić; MCL maximum contaminant.
Results of testing in a small nu mber of cases show exceeding the MCL.Lead concentrations above the allowed MCL in the spring season are registered only in the samples 4 and 6, which refer to water of Ibar.These samples are taken fro m the bridge Grabovac-Žitkovac behind the place of the waste disposal Žitkovac and in the village Upper Krn jin, the output fro m Leposavić, in addition to the landfill tailing upper Krnjin or Bostaniste in Leposavić The increase in lead content in these samples there was probably washing and filtrat ion with a delay of those cities, and cities delays that are located upstream or on the bank of the Ibar.Near the MCL and the concentration of lead in the samples 3 and 5, which were taken fro m the measuring points are also close to the waste disposal.Lead is present in the water at the measuring point 1, which was chosen as the site of male pollut ion.Ho w is this place located a few kilo meters fro m the flows of the river Sitnica Trepčanska river and river Lushta into the river Ibar, and of all places of disposal of industrial was te, it can be assumed that the lead content in the sample contribute to exhaust fumes of motor vehicles on the road that passes by the banks of the Ibar River, as well as washing with asphalt street.
The minimu m content of lead in the Ibar river water was found in the sample 2, which can be exp lained by the fact that the sampling is done with the left bank of the Ibar river, opposite the city delay the Top Field.This measuring point is the furthest fro m the city traffic as a possible source of lead contamination.Increased concentrations of cadmiu m in the river water Ibar at the measuring points 3, 4, 5 and 6 above MCL is probably fro m the same causes as the increased concentration of lead.Sample 1 contains little cad miu m.In samp le 2 also has a low conten t of cadmiu m, but more than in a samp le1.This measuring point is the first in a series in which the mixed water of the river Ibar, Sitnica, Trepčanska river and Lushta, which are before the casting of the Ibar already polluted.The content of zinc, compared with MCL is significantly lower compared to lead and cadmiu m.As his low concentrations it can be said that the existing pollutants in the area tested not significantly affect the water pollution zinc.The results of the content of heavy metals in water samples taken at the same measuring points and in different seasons of the year have shown that there are certain differences, but it is noted the same trend of the contamination, that is determined by the measuring points are more polluted than the other.
The highest concentrations of lead in the samples fro m the measuring points 3.4, 5, and 6 and the highest concentration of cadmiu m also at locations 3, 4 and 5.The results of determining the content of heavy metal in the spring and of W ell water are shown in Tables 9−16.Significantly, based on the result of determin ing that the concentration of lead in all the samples wells and water sources is less than the MCL for drinking water.Variations may occur in the lead content at the measuring point 10 (in the drilled wells in the village Grabovac), wherein the concentration i Increased content of lead probably arose due to the reduced quantity of water (drought) and the pumping raises the sediment and stirred with water, which was noticeable by the color of the sample.nthe summer and autumn rather above the MCL of the drinking water (Slu žbeni glasnik Republike Srbije, 1994).In the colder period of the year (autumn and winter) lead content is generally lower.Concentrations of cadmiu m and zinc in well and spring waters are also less than the Maximu m allo wable concentrations at all meas uring points and in different seasons.

CONCLUS ION
The objective of this study is simu ltaneous determination of lead, cadmiu m and zinc by stripping analysis, as well as application of the same in water analysis, establishment of determination conditions, extraction potential, ext raction time, optimal solution stirring rate, pH, time of working electrode formation, as well as manner of samples preparation, using the approach of comparing the results of simultaneous with the results of individual determination.It is obvious that there is a difference in results obtained.The efficiency of the indiv idual determination was greater due to the better synchronization of the solution pH value and the extraction potential.Results accomplished in case of simultaneous determination have shown somewhat lo wer values which can be exp lained by the occurrence of hydrogen extraction in the analysis of acid ic solutions, when extraction potential is more negative than −1 V.The maximu m efficiency for all determinations was obtained at metal extraction time of 300 s, on electrode that is formed in 240 s and at solution stirring rate of 4000 min -1 .At higher stirring rates, the determination efficiency is reduced due to the decreased diffusion layer thickness.The lowest concentrations which can efficiently be determined by Potentiometric Stripping Analysis are: 22.48 µg d m -3 for lead, 16.23 µg dm -3 for cadmiu m and 18.75 µg d m -3 for zinc.Good efficiency is up to 2194.05 µg dm -3 for lead, 960.12 µg dm -3 for cadmiu m and 1312.50 µg dm -3 for zinc, which can be attributed to identical mechanism of mass transfer during extraction for the one during metal dissolution.For the simu ltaneous determination of all three metals, determination error was in range of 2-5%.The reproducibility of these measurements for lead, cad miu m and zinc in the water samples ranged fro m: 10-13% for lead, 10-14% for cad miu m and 8-9% fo r zinc.The obtained results indicate that Potentiometric Stripping Analysis is efficient in determination X s -concentration of the elements in the model solution, m X i -measured concentration, b X -average measured concentration, number of measurements=5, n -time of oxidation, K p -constant stripping analysis, c S-standard deviation, d K v -coefficient of variation, e E r -error determination 1. Žitkovac Village, 200 m of the Ibar, 2 m from the main road, the age of 5 years; 2. Village Grabovc, 100 m of the Ibar, the main road, 30 years old; 3. Village Žitkovac, 50 m from the landfill Žitkovac, 200 m of the Ibar; 4. The village Rudare, 50 m from Ibra, 30 years; 5. Village Srbovac, 20 m from Ibra, 40 years; 6. Villa ge Srbovac, 20 m from Ibro, 40 years; 7. The upper Krnjin Village, 100 m of the Ibar, 10 y; Drilled wells: 8 village Rudare, 80 m from Ibra, 25 years; 9. Village Grabovac, 30 m from Ibra, 25 years; 10.Village G rabovac, 30 m from Ibra, 25 years; *The planned samples 5 and 6 were not taken due to heavy rainfall

Table 1 .
The results of the determination of lead concentration in model solutions a -concentration of elements in the standard solution, X b -average measured concentration, number of measurements=5, S c standard deviation, K v d -coefficient of variation, E r e -error determination

Table 2 .
The results of the determination of cad miu m concentration in model solutions

Table 3 .
The results of the determination of zinc concentration in model solutions a -concentration of elements in the standard solution, X b -average measured concentration, number of measurements=5, S c standard deviation, K v d -coefficient of variation, E r e -error determination

Table 4 .
The results of the simultaneously determination of lead, cad miu m and zinc in model solution

Table 5 .
The results of concentrations of lead, cadmiu m and zinc (µg d m -3 ) in water of the river Ibar, april 2015

Table 6 .
The results of concentrations of lead, cadmiu m, zinc (µg d m -3 ) in water of the Ibar, august 2015

Table 7 .
The results of concentrations of lead, cadmiu m, zinc (µg d m -3 ) in water of the river Ibar, october 2015

Table 9 .
The results of concentrations of lead, cadmiu m and zinc (µg d m -3 ) in natural spring water, april 2015

Table 13 .
The results of concentrations of lead, cad miu m and zinc (µg d m -3 ) in g roundwater, april 2015

Table 14 .
The results of concentrations of lead, cad miu m and zinc (µg d m -3 ) in groundwater, august 2015

Table 15 .
The results of concentrations of lead, cad miu m and zinc (µg d m -3 ) in groundwater, october 2015

Table 16 .
The results of concentrations of lead, cad miu m and zinc (µg d m -3 ) in groundwater, february 2016