ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2014, том 69, № 11, с. 1183-1188
ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
A NEW SENSITIVE METHOD FOR THE DETERMINATION OF TRACE MERCURY BY DIFFERENTIAL PULSE POLAROGRAPHY: APPLICATION TO RAW SALT SAMPLE
© 2014 G. Somer1, A. C. Call § kan, and O. § endil
Gazi University, Science and Art Faculty, Chemistry Department TR- 06500 Ankara, Turkey 1E-mail: gsomer@gazi.edu.tr Received 24.06.2013; in final form 02.04.2014
A new indirect differential pulse polarographic (DPP) method is established for the trace determination of mercury(II). Because of its toxic effects on human health, trace determination of mercury is very important. An indirect method had to be used since no polarographic peak is observed in its direct determination. According to the standard potentials, the reaction between Sn(II) and Hg(II) was found suitable. The peak of Sn(II) at about —0.40 V is sharp, high and very reproducible, which enables the determination of low concentrations of Hg(II). For this purpose, to a known amount of Sn(II) present in the polarographic cell (acetic acid, HAc, pH 1—2), the unknown Hg(II) sample is added and the quantitative reaction takes place directly in the cell. The Hg(II) concentration is calculated simply from the decrease of the Sn(II) peak. The limit of detection (LOD) was found as 2 x 10-7 M for S/N = 3. Interferences of some common cations, such as Fe, Cd, Cu, Zn and Pb and anions have been investigated. Only Pb had an overlapping peak with Sn(II). This peak overlap was eliminated simply by working at pH 2 (HAc electrolyte), because of the shift of the Pb peak in the Ac complex to —0.7 V This method was successfully applied to synthetic samples and raw salt sample taken from a salt lake in Turkey.
Keywords: mercury determination, differential pulse polarography, reduction by Sn(II).
DOI: 10.7868/S0044450214110139
Trace determination of mercury ion is important because of its toxic effect on human health. It may be present in many materials such as organic or inorganic mercury compounds. Mercury is known as a very complex metal because of its chemical forms and great number of physical states in the environment. Its toxicity and bioavailability depends on its chemical form. Elemental mercury and insoluble HgS are the least toxic. Hg organic compounds, such as methyl mercury (a highly toxic form) may be accumulated in aquatic systems and enter the food chain [1—3]. On the other hand, inorganic mercury species are less toxic than the organic compounds. It is known that some soluble mercury compounds can deposit in organs and cause serious damage to living organisms [4—6].
For the determination of mercury, mostly neutron activation analysis (NAA) [7], inductively coupled plasma mass spectrometry (ICP—MS) [8, 9], inductively coupled plasma optical emission spectrometry (ICP—OES) [10], cold vapor atomic absorption spectrometry (CVAAS) [11, 12], atomic fluorescence spectrometry (AFS) [13] and atomic absorption spectrom-etery (AAS) [14] are used. The mentioned methods have the advantage of being sensitive and reproducible
but are cost ineffective. They also need preconcentra-tion and extraction which are time-consuming procedures. Due to this, the electrochemical techniques emerge with the purpose of eliminating the disadvantages of these high-cost techniques. As main advantages we can mention high sensitivity, high reproducibili-ty, high selectivity, simple sample preparation, trace metal determination and direct speciation. For this purpose mostly voltammetric, polarographic methods and ion selective electrodes are being used. An electrochemical sensor for mercury ions using chelating ion-ophore was prepared; this electrode could be used in 5 x 10-6 — 0.1 M range [15]. For the determination of electropositive elements, a graphite thick film electrode modified with Au(III) was prepared, and mercury and arsenic could be determined [16]. In an electrochemical method using anodic stripping voltammetry with a chitosan modified carbon paste electrode mercury determination in water have been made. The LOD was 6 x 10-7 M [17]. In another work [18] using stripping voltammetry, mercury was determined in 1.0 M H2SO4 with a rotating ring-disk electrode having a platinum ring and a glassy-carbon disk. Satisfactory results were obtained only after a thin film of Au
(two monolayer equivalents) was electroplated on the disk electrode. Stripping voltammetry using screen printed electrodes has been applied to the determination of mercury and lead traces in tap water [19]. Mercury determination in coal was made using iodide ion selective electrode after elimination the interference of iron [20]. With a newly prepared mercury selective electrode, mercury in coal was determined [21].
In our newly established DPP method, mercury can be determined indirectly by a simple and fast procedure, and there is no need of any separation or pre-concentration which is time consuming and also constitutes a danger of losses. The results obtained with DPP are very reproducible since with the use of a dropping mercury electrode (DME) the behavior of the electrode is independent of its history. Not much work has been found using polarography, most probably because no polarographic peak is observed for mercury. Thus, it can only be determined indirectly.
EXPERIMENTAL
Apparatus. A polarographic analyzer (PAR 174 A) equipped with a PAR mercury drop timer was used. The drop time of the electrode was in the range 2—3 s (2.4 mg/s). A Kalousek electrolytic cell with a saturated calomel electrode (SCE), separated by a liquid junction, was used in the three-electrode configuration. The counter electrode was platinum wire. The polarograms were recorded with a Linseis (LY 1600) X—Y recorder under the conditions ofa drop life of1 s, a scan rate of 2—5 mV/s, and pulse amplitude of 50 mV.
Reagents. All chemicals used were of analytical-reagent grade (pro analysis). Triply distilled water was used in the preparation of all solutions. Solutions of 1 x 10-3 M and more dilute ones were prepared before every use in order to avoid the aging process of solution. In order to expel the oxygen present in polarographic cell, 99.999% purity nitrogen was passed through the solution.
Mercury (p. a.) used in the dropping mercury electrode was obtained from Merck (Darmstadt, Germany). Contaminated mercury was cleaned by passing it successively through dilute HNO3 (3.0 M) and water columns in the form of fine droplets by using a platinum sieve. This mercury was then washed in the same way until no acidic reaction. The collected mercury was stored in a closed vessel covered with water. It was dried between sheets of filter paper when it was needed. Thus, no mercury loss was possible and it could be reused. Before use, a DPP polarogram of this mercury was recorded each time to confirm the absence of impurities.
Preparation of reagents. 1.0 M HAc/Ac- buffer solution was prepared by adding 6 g of NaOH, washed with distilled water in order to remove the carbonate formed, to 57 mL of 1.0 M HAc and diluting to 1 L.
The pH was adjusted to the desired value using a pH meter.
Sn(II) solution (0.01 M). To 0.26 g SnCl2 • 2H2O, 5 mL HCl was added, for its dissolution about 30 min was needed. After it was totally dissolved it was diluted to 100 mL with oxygen-free distilled water and kept under nitrogen atmosphere. The solution had to be prepared daily.
Hg(II) solution (0.1 M). To 3.43 g Hg(NO3)2 • H2O, 30 mL of distilled water and 1.0 mL of HNO3 were added and diluted with distilled water to 100 mL in a volumetric flask.
RESULTS AND DISCUSSION
Polarography cannot be used directly for the trace determination of Hg(II). Thus, an indirect method has to be used. For this purpose a substance which will react quantitatively with mercury, is electroactive and has a sharp and high polarographic peak has to be chosen. The reduction potentials of Hg(II) and Sn(IV) are +0.86 and +0.15 V, respectively. According to these potentials there is a high probability that this reaction (as given below) will be quantitative under properly chosen conditions. For this purpose, the reaction conditions, electrolytes and pH has to be determined first. For the trace determination of Hg, the peak height of Sn(II) is also very important, since its decrease after the addition of mercury will be measured. No doubt, the larger this change, the more accurate will be the determination.
Although Sn(II) seems suitable for the determination of mercury, it might be present in various forms depending on pH [22]. At higher pH values it will be in Sn(OH)Cl and Sn(OH)2 forms which are not electro-active. Thus, to follow the change in Sn(II) concentration after the reaction with mercury ions, the pH has to be low, so that the reactant would be only Sn(II) in its ionic state.
Electrochemical behavior of Sn(II). The DP polarograms of Sn(II) were taken in various electrolytes such as HCl, HNO3 and HAc under various pH 2—6. It was found that Sn(II) had two peaks in the pH range of 1—6. With the increase of acidity they were shifting to lower potentials and the peak heights were increasing. While at pH 6 (NaAc), the peaks were at —0.3 and -0.55 V, at pH 2 (HAc) they were at -0.07 and -0.43 V. On the other hand, in HCl medium at pH 2 they were at -0.03 and -0.42 V. The polarogram of 1 x 10-4 M Sn(II) in HAc (pH 2) is given in Fig. 1. As can be seen, the second peak is larger than the first one, and thus the second peak was chosen for the determination of mercury. In all pH values Sn(II) peaks were proportionally increasing by standard additions, which is a good indication for quantitative determination.
Reaction between Sn(II) and Hg(II). The reaction between Sn(II) and Hg(II) can be followed precisely
Fig. 1. The behavior of Sn(II) in HAc (pH 2) medium; a — 10 mL of 1 M HAc, b - a + 0.1 mL of 1 x 10-2 M Sn(II).
from the second Sn(II) peak at pH values in the range of 1—2, since it is sharp and h
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