Research Article
General Science
Environmental Assessment

Spectrophotometric determination of non-transition elements (Zn Hg) in environmental sample by the metal substitution after fractional extraction

Mohammad Nasir Uddin1, Nur Mostaq Shah1, Md. Mujahidul Islam1, Mohammad Amzad Hossain1

Abstract

Diethyldithiocarbamate (DDTC), the common reagent used for spectrophotometric determination, forms M(DDTC)2 complexes (M = Cu, Zn, Hg) that are soluble in organic solvents like CHCl3 or CCl4. Though DDTC complex of Cu(II) shows maximum absorption at 435 nm but that of Hg(II) and Zn(II), being non-transition elements with d10 electronic system, shows no absorption in the same region. Again the stability of DDTC complexes is varied as the metal sequence of Hg> Cu> Zn. Hence from Zn(DDTC)2 zinc is substituted with the addition of excess copper, and from Cu(DDTC)2 copper is substituted with the addition of mercury. The subsequent increase or decrease in absorption is equivalent to the amount of zinc or mercury, respectively. Therefore, based on this substitution spectrophotometric method has been proposed for the determination of Zn(II), Hg(II) by the measurement of Cu(DDTC)2. Proposed method has been optimized, validated and applied to the environmental samples for their simultaneous determination. The LOD and LOQ were found to be 0.029 μg mL-1 and 0.098 μg mL-1 respectively for both zinc and mercury determination.

Keywords Metal substitution, spectrophotometric determination, nontranstion elements, environmental sample, fractional extraction

Author and Article Information

Author info
1) Department of chemistry, University of Chittagong, Bangladesh

RecievedMar 31 2014  AcceptedMay 3 2014  PublishedMay 21 2014

CitationUddin MN, Shah NM, Islam M, Hossain MA (2014) Spectrophotometric determination of non-transition elements (Zn Hg) in environmental sample by the metal substitution after fractional extraction. Science Postprint 1(1): e00021. doi: 10.14340/spp.2014.05A0003

Copyright©2014 The Authors. Science Postprint is published by General Healthcare Inc. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs 2.1 Japan (CC BY-NC-ND 2.1 JP) License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

FundingThis research is partially funded by University Grant Commission (UGC, Bangladesh) under University Revenue Budget Scheme. Ref. 371/P&D/7-28/2012(part 3) dated 20/05/2012

Competing interestNo conflict of interest.

Donation massageAny support would be highly appreciated for our further research.

Corresponding authorMohammad Nasir Uddin
AddressDepartment of chemistry, University of Chittagong, Bangladesh
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Introduction

A variety of metals could enter industrial wastewaters as a result of anthropogenic activity. Due to corrosion and geological factors some metal ions in complex form being environmental contaminants could also be present in drinking waters and natural water bodies and thereby to the soil. By the biological cycle, some of them through the food chain pass into plants, animals, and man, thus affecting them negatively.
Copper is one of the important and essential nutrients for human health as well as the growth of animals and plants. Copper is required for normal metabolic processes 1, 2. Zinc is most commonly used as an anti-corrosion agent, galvanization as an anode material for batteries as a substitute for the traditional lead/tin alloy in pipes, in the automotive, electrical, and hardware industries 3, 4. Zinc ensures a healthy immune system and takes part in DNA synthesis as well 5. Zinc dithiocarbamate complexes are used as agricultural fungicides 6. Although both zinc and copper are the essential micronutrients and are required by the body in very small amounts, excess in the human body can cause stomach and intestinal distress such as nausea, vomiting, diarrhea, and stomach cramps. Mercury and its compounds are used in medicine. Mercury is used in dental amalgam. It is used as a preservative in laboratory reagents and related chemicals. Mercury is a heavy metal which is much toxic for human health. It causes many harmful disease when intake by human body 1, 2. Mercury can be inhaled and absorbed through the skin and mucous membranes. The chief sources of mercury pollution are chlor-alkali plants, paper, pulp, cellulose and plastic industries, electrical, paint, pharmaceutical industries, etc. Uses of mercury as fungicides, pesticides, etc., also add mercury to the environment 1, 2, 7. Due to the tremendous applications their exposure to environment and thereby in food cycle is obvious. Therefore, trace and ultra-trace determination of copper, zinc and mercury is of great importance, as some of them have nutritional significance, whilst others are toxic 4, 8, 9.
Atomic absorption spectroscopy (AAS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) have been the method of choice for their analysis because of their utility, sensitivity and reliability. But these techniques suffer from the limitations of being rather costly (considering instrument acquisition and maintenance), time-consuming (with respect to sample preparation), and not always readily available. Contrary, in laboratories of developing countries like UV-visible spectroscopy is a well-established analytical technique with mature methods and equipment. It is commonly used in both research and science as well as in industry in inorganic trace analysis. In these regarding a number of spectrophotometric reagents as chloro(phenyl)glyoxime 4, 3-methoxy-4-hydroxy benzaldehyde-4-bromophenylhydrazone 10, thiosemicarbazone 11, and so on are reported for copper determination. Both zinc and mercury having d10 electronic systems don’t show electronic transition, and hence don’t absorb radiation in visible region. So, spectrophotometric methods for their determination are quite limited. Use of thiosemicarbazones, dithiozone or hydrazones as a sensitive colourful reagent has been reported for the zinc determination 12-14, whereas diacetylmonoxime isonicotinoylhydrazone, diphenylthiocarbazone are reported for the mercury determination 15, 16. However, one of the most common reagents, diethyldithiocarbamate (DDTC) is used for spectrophotometric determination of transition elements Cu(II), Ni(II), Mn(II), Pb(II) and V(V) 17. Copper forms yellow Cu(DDTC)2 complex which is insoluble in water and is extracted in organic solvent for spectrophotometric measurement 18. Hg(II) and Zn(II) give white insoluble complexes with DDTC which are soluble in CHCl3/CCl4. Sequence of stability of DDTC complex is as Hg> Cu> Zn. Therefore, an alternative method for their has been proposed based on the metal substitution followed by the measurement of Cu(DDTC)2 spectrophotometrically. Finally, present method has been successfully applied for their determination in a number of environmental water and soil samples.

Materials and Methods

Chemicals and reagents

High-purity carbon tetrachloride, various acids, salts and reagent grade Na-DDTC (Merck) were used. The standard stock solutions (1000 g mL-1) were prepared by dissolving appropriate amount of each salt in water. Solutions of a large number of inorganic ions and complexing agents were prepared from their analytical grade, or equivalent grade, water soluble salts. Stock solutions and environmental water samples (1000 mL each) were kept in polypropylene bottles containing 1 mL of concentrated HNO3.

Instrumentation

A Shimadzu UV Visible spectrophotometer (model UV-1800) with suitable settings quipped with 1-cm quartz cells was used for measuring the absorbance. The spectral band length was 1 nm, the wavelength accuracy was 0.5 nm with automatic wavelength correction, and the recorder was a computer-controlled in the wavelength range 190–1100 nm. A Jenway (England, U.K) (Model-30100) pH meter were used for the measurement of pH. A Thermo (Model-iCE 3000 C093300131v1.30) AAS spectrophotometer was used for comparing the results.

Preparation of standards

A 0.1% (5.84×10-3M) stock solution of Na-DDTC was prepared by dissolving 0.1 g sodium diethyldithiocarbamate reagent in approximately 80 mL water heated at 60°C. Afterwards, the volume was made up to 100 mL by adding distilled water in volumetric flask and filtered. A stock solution (100 µg mL-1) of copper, zinc or mercury was prepared by dissolving appropriate amount of copper sulphate pentahydrate, zinc sulphate heptahydrate or mercuric chloride (Merck, Germany) in 250 mL of doubly distilled deionized water. The working standards were prepared by suitable dilutions of this stock solution. The buffer solutions were prepared by mixing 1M HCl and 1M sodium acetate (pH 1–3), 0.2M acetic acid and 0.2M sodium acetate (pH 3.2–6.0), 1M sodium acetate and 0.2M acetic acid (pH 7.0) and 2M ammonium hydroxide and 2M ammonium chloride (pH 8.0–12.0). Suitable portions of these solutions were mixed to get the desired pH. A 100 mL stock solution of tartrate (0.25M) or ammonium thiocyanate (0.4M) was prepared by dissolving 7.055 g of A.C.S grade (99%) potassium sodium tartrate tetrahydrate or 3.0428 g of solid ammonium thiocyanate in deionized water. A 100 mL solution of dilute ammonium hydroxide was prepared by diluting 10 mL concentrated NH4OH (28–30% A.C.S grade) to 100 mL with deionized water. A 100 mL stock solution of EDTA (4000 µg mL-1) was prepared by dissolving 0.4 g of A.C.S grade (>90%) dehydrated disodium salt of ethylenediaminetetraacetic acid in 100 mL deionized water. Stock solutions were stored at 4°C, protected from light and were used within three months. Bimetallic complexes, [Zn(en)3][HgI4], [Zn(NH3)6][HgI4] and [Zn(An)6][Hg(SCN)4] used as certified materials, were prepared by the same procedure described by Uddin et al. 19.

Procedure for the simultaneous determination of copper, zinc and mercury

To determine copper, zinc and mercury simultaneously, a mixture of (various volume) 2×10-4 M Cu(II), Zn(II) and Hg(II) solution was placed in a 25-mL separating flask along with 0.3 mL 0.05 M H2SO4 and 5 mL of the reagent DDTC (5.84×10-3 M) solution. Its volume was made up 15 mL with deionized water. The mixture was stirred at 10 min. The solid product of M(DDTC)2 (Cu, Zn, Hg) complexes so formed was extracted carefully with the addition of 15 mL (5×3) CCl4, and organic phase was separated out. pH value of one portion was adjusted to 5 using acetate buffer. A portion of 5 mL extract was separated out and its absorbance, which is equivalent to copper, was measured at 435 nm against a blank. Excess Cu(II) solution of 2×10-4 M was added to remaining portion (10 mL) of extract when additional Cu(DDTC)2 was formed by the replacement of zinc immediately. Organic phase (10 mL) was further separated after vigorous stirring for 10 min. Another portion of 5 mL extract was separated out and the absorbance was measured at 435 nm against a blank. Additional absorbance is equivalent to Zn(II) present in sample. Then known amount of Hg(II) solution was added to third portion (5 mL) of the extract when it’s pH value was adjusted to 10 using ammonium (basic) buffer. The intensity of yellow colour of Cu(DDTC)2 was reduced immediately and organic phase was further separated after vigorous stirring for 10 min. The absorbance was measured at 435 nm against a blank. Reduced absorbance will be equivalent to mercury present in solution. Procedural lay out for the simultaneous determination of copper, zinc and mercury is given in Figure 1.

Figure 1Procedural lay out for the simultaneous determination of copper, zinc and mercury

Results and discussion

Method optimization

Method was primarily optimized for the copper determination. Zinc and mercury determination was based on the measurement of increase and reduction of absorbance of Cu(DDTC)2.

Absorption spectra

The absorption spectra of the reagent and the complex are recorded in the wavelength range 300–650 nm at pH 5.0 against CCl4. The typical superimposed UV-vis spectra of Cu(DDTC)2, Zn(DDTC)2 or Hg(DDTC)2 in CCl4 and reagent blank are presented in Figure 2. The spectra show that Cu(DDTC)2 complex solution has an absorption maximum at 435 nm, where as Zn(DDTC)2, Hg(DDTC)2 or the reagent does not have appreciable absorbance at this wavelength. Therefore, UV-vis spectrophotometric measurements were carried out at a wavelength of 435 nm for subsequent studies.

Figure 2The typical superimposed UV-vis spectra of Cu(DDTC)2 in CCl4

Black curve after Zn substitution, red curve for Cu(II) in mixture and blue curve after Hg(II) addition. Bottom green curve is an overlapped one for the Zn(DDTC)2 or Hg(DDTC)2.

Optimization of reaction conditions

Effect of acid

Acid effect was primarily tested for nitric, sulfuric, hydrochloric and phosphoric acids while sulfuric acid was supposed to be suitable for Cu(DDTC)2 complex formation. The influence of sulfuric acid concentration on the reaction was investigated by varying acid values. At room temperature (25 ± 5)°C a constant absorbance was produced for (0.1–0.8)×10-2 M, H2SO4.
Therefore, all measurements were peformed at 0.1×10-2 M H2SO4.

Effect of reagent concentration

The absorbance of the complex solution was measured for different molar excesses (1:1–1:40) of sodium diethyldithiocarbamate leaving copper(II) concentration constant (1.0 µg mL-1). The reagent molar ratios of 1:10–1:40 produced a constant absorbance. Therefore, 15 fold molar excess of DDTC was used in all the subsequent experiments and greater molar excess of the reagent was not studied.

Effect of temperature and time

The effect of temperature on the reaction was not studied due to the lake of instrumental facilities and experiments were carried out at room temperature (25 ± 2°C). Reaction between Cu and DDTC was allowed to proceed at room temperature for varying period of time when reaction goes to almost completion within 1 min. However, for higher precision the reaction was allowed to proceed for quite longer time of 10 min.

Extraction parameters for the complex, Cu(DDTC)2

Effect of the aqueous phase volume

The volume of the aqueous phase is an important factor for the extraction of metal ions as Cu(II)-DDTC is sparingly soluble in aqueous phase 20. The aqueous phase volume was changed from 5 mL to 30 mL for 1 μg mL-1 of Cu(II). The absorbance was found to be decreased with the increase of aqueous phase volume. Hence total aqueous volume was always confined to maximum volume 15 mL throughout all experiments.

Optimum extraction period

The efficiency of Cu(DDTC)2 extraction in organic phase depends on the extraction period.
Optimum extraction period was checked for various period of time containing for 1 μg mL-1 of Cu(II). Constant absorbance was obtained after 10 min. and it was remained constant up to studied period of 30 min.

Effect of pH on the extraction of the complex exchange reaction

The influence of pH on the exchange reaction, replacement of zinc by copper or copper by mercury and subsequent extraction in organic phase, was investigated by carrying out the reaction in buffer solution of varying pH values.
The effect of pH on the color intensity is studied in the pH range 1–12. The optimum pH value for the replacement of zinc by the copper from Zn(DDTC)2 is attained at pH 4 and remains constant up to 8 (Figure 3). Hence, pH 5.0 was chosen for further studies as convenient. In case of copper replacement by mercury forming Hg(DDTC)2, optimum pH value for the highest recovery of mercury was obtained at 10 as shown in Figure 4. It is assumed that the reaction to form this complex could have competed against hydroxide precipitation above pH 10.0 and at acidic pH, as the sulfur atom in the chelating site of DDTC has more affinity power with proton at a higher concentration of protons.
Summary for the optimization of variables of the proposed spectrophotometric method is presented in Table 1.

Figure 3Effect of pH on the extraction of the Cu(II)-DDTC complex in organic phase

Figure 4Effect of pH on the extraction of the Hg(II)-DDTC complex in organic phase

Table 1Summary for the optimization of variables of the proposed spectrophotometric method

Composition of the complex

Job’s method of continuous variation and the molar-ratio method 21 were applied to ascertain the stoichiometric composition of the complex, Cu(DDTC)2 and the stoichiometry was found to be 1:2 (Copper: Ligand).

Method validation

Validation parameters

Method was validated in terms of ICH analytical performance parameters 22; precision, accuracy, specificity, limit of detection, limit of quantitation, linearity and range, suitability and robustness. To assess the method validation its accuracy and precision were checked analyzing synthetic mixture and certified reference materials. The result of Zn and Hg estimation was compared to that of calculated values. The metal content in the biological samples were determined by AAS and results were compared to the developed method.

Preparation of calibration curves

The calibration curve was constructed with ten standard solutions containing 0.01–16.0 µg mL-1 of copper, zinc or mercury according to the general procedure. Triplicate measurement was taken for each concentration levels. The curve was constructed by plotting absorbance against corresponding concentrations. The linearity range, regression equation and coefficient of determination (r2) were obtained by the method of least squares. The calibration curves are shown in Figure 5 for zinc and copper, respectively. The straight line obeyed the equation y = 0.264x + 0.087 and y = 0.209x + 0.070 for zinc and copper, respectively. The liner plot, between the absorbance obtained after mercury addition and the amount of mercury added, is drawn and the straight line obeyed the equation y = -0.065x + 1.140.
Alternatively, it was drawn plotting reduced absorbance against amount of mercury added when the straight line obeyed the equation y = 0.065x + 0.174. Figure 6 shows the calibration curve constructed by plotting absorbance (-ve slope) and reduced absorbance (+ve slope) against corresponding concentrations of mercury (µg mL-1). For all equations coefficient of determination (r2) was better than 0.998. According to the Beer’s law linearity range of 0.02–14 µg mL-1 was obtained for zinc and that for Cu(II) alone was 0.02–12 µg mL-1. Linearity range for mercury was obtained 0.02–15 µg mL-1. The analytical sensitivity, the calibration sensitivity, which is the slope of the analytical curve, the limit of detection, and the limit of quantitation as well as other analytical characteristics are calculated from the data obtained for calibration curve. Molar absorption coefficients and Sandal’s sensitivity were calculated using linearity equation. Table 2 shows the parameters of the performance for the proposed spectrophotometric method for determination of mercury and zinc content.

Figure 5Calibration curve constructed by plotting absorbance against corresponding concentrations (µg mL-1) of zinc (red) and copper (blue)

Figure 6Calibration curve constructed by plotting absorbance (blue) and reduced absorbance (red) against corresponding concentrations of mercury (µg mL-1)

Table 2Parameters for the performance of the proposed spectrophotometric method for determination of mercury and zinc content

Accuracy and precision

For accuracy check of the method standard mixtures having 1, 5, 8 µg mL-1 of zinc and mercury were analyzed. The recovery was 94.0–102.0% (Table 3) for both intra- and inter day analyses of standards. The results indicate that found values are very concordant indicating the good accuracy of the proposed method. The precision of the method was expressed in terms of percentage relative standard deviations (RSD) for five replicate measurements in the same day within-day repeatability (RSD). The procedure was repeated at same concentration levels on five consecutive days to determine between-day repeatability. RSD did not exceed 4.43% (Table 3) proving the high precision of the proposed method.

Table 3Determination of zinc and mercury in standard mixture for the intra and inter-day accuracy and precision study

Sensitivity

The sensitivity calculation of the method is based on the standard deviation of the response (Sxy) and the slope of the calibration curve (a). The limit of detection were calculated from calibration graph by the formula; LOD = 3•Sxy/a, and the limit of quantification; LOQ = 10•Sxy/a. The results are presented in Table 2. The lower detection limit and quantification limit were found to be 0.029 μg mL-1 and 0.098 μg mL-1 respectively for both zinc and mercury determination.

Robustness

The influence of small variation in the method variables; (DDTC) concentration, buffer pH, reaction and extraction period on its analytical performance shows the robustness of the method. Small variation in the method variables did not significantly affect the procedures; recovery values were (99.87–101.41) ± (0.26–0.66)% for zinc and (99.07–103.11) ± (0.24–1.40)% for mercury.

Ruggedness

A pre-validated AAS method was used as a reference method for determination of zinc and mercury in whole blood and urine samples as to get inter method variation as the test of ruggedness. Whole blood and urine samples were digested according to the previously reported procedure 18. Inter method variations in percentage of error in measurements of zinc and mercury by the proposed method and AAS has been shown in Figure 7. No significant differences were found between the calculated and theoretical values of t- and F-tests at 95% confidence level proving similar accuracy and precision in the determination of metals by both methods.

Figure 7Inter method variations in percentage of error in measurements of proposed method and AAS

Suitability

For at five different concentration levels of standard within 1.0–10.0 μg mL-1 maximum wavelength (435.52 ± 0.33) of absorption was checked. Their relative standard deviation was found to be 0.077%. Molar absorptivity calculated for the four different concentrations gave straight line parallel to concentration axis (x-axis) when plotted.

Effect of diverse ions

The determination of Zn(II) or Hg(II) was carried out in presence of various anions and cations using the above described general procedure. The tolerance limit of a foreign ion is taken as the amount that caused an error in the absorbance value of ≤10%. Large amounts of commonly associated cations and anions do not interfere in the present method. Among the various ions studied, all the anions and the cations Pb(II), Te(IV), U(VI), Na(I), K(I), Li(I), Th(IV), W(VI), Ce(VI), Ti(IV), Al(III) do not interfere even when present in more than 100 fold excess. Copper(II), nickel(II), cobalt(II), mercury(II), cadmium(II), iron(III) and iron(II) ions interfered seriously at all proportions. Interference from Co, Ni, Cr, Mn, Bi, Pb, Cd can be eliminated using EDTA as masking agent up to 50 fold excess. Selective extraction of zinc was performed from thiocyanate solution in 0.5 M HCl medium followed by its back extraction with an ammoniacal solution. If excess iron is present in the sample, it is necessary to add 1300 μg mL-1 NaF/NaI before their extraction. EDTA has been selected to be suitable masking agent for the determination of copper and zinc, and ascorbic acid for the determination of mercury.

Application

The present method was successfully applied to the determination of zinc and mercury in series of synthetic mixtures of various compositions and a number of certified materials as the validation check. Copper, zinc and mercury were determined in environmental water and soil samples. Samples were prepared for the metal analysis according to the procedure mentioned below.

Determination of zinc and mercury in synthetic mixtures

Several synthetic mixtures of varying compositions containing zinc or mercury and diverse ions of known concentrations were prepared and metal content was determined by the present method using a suitable masking agent and the results, both accuracy and precision, were calculated. The results are shown in Table 4. The accurate recovery was achieved in all solutions with maximum RSD value of 1.36% for zinc and 1.70% for mercury of their triplicate measurements. It is indicated that zinc and mercury can be measured eliminating possible interferences from diverse ions.

Table 4Determination of zinc and mercury (n = 3) in synthetic mixture by the proposed spectrophotometric method

Determination of zinc and mercury in reference materials

20 mg of each synthetic complex or 0.1 g of an alloy and amalgam was accurately weighed and acid decomposition 18 was performed in a 50 mL Erlenmeyer flask. The solution was carefully evaporated to dense white fumes to drive off the oxides of nitrogen and then cooled to room temperature (25 ± 5)°C. After suitable dilution with deionized water, solution was neutralized with a dilute NH4OH solution. The resulting solution was filtered if necessary. An aliquot of 2 mL decomposed solution was taken into a calibrated flask and the metal content was determined under general procedure using suitable masking agent. The average percentage recovery and RSD values for five replicate analyses were in good agreement with the certified values (Table 5) indicating good accuracy and precision of the measurements.

Table 5Analysis of certified substances for the determination of zinc and mercury

Determination of zinc and mercury in water and soil samples

250 mL of industrial (KY steel, PHP steel, berger paint, ship breaking Chittagong urea fertilizer limited, battery waste water, ystern refinary) or battery water was mixed with 10 mL of conc. HNO3, 2 mL of conc. H2SO4 and 2 drops HClO4 acid in a distillation flask. The sample was digested until a paste was formed. Other water (250 mL) samples (Tap water, Karnafuli river water and Bay of Bangle water) were pre-concentrated by simple evaporation. Ammonia buffer solution was added to precipitate iron as hydroxide. The resulting solution was then filtered and quantitatively transferred into a 50 mL calibrated flask and made up to the mark with deionized water. 2 g of soil sample was placed into a 250 mL flask. 0.2 mL of sulfuric acid, 1 mL of nitric acid and 1 mL of perchloric acid were added. The mixture was heated to 180°C for 3 hrs on a hotplate. After cooling, 1 g of ammonium chloride and 20 mL of 0.5 N HCl were added, evaporated to approximately 10 mL and filtered into 50 mL plastic bottles through an ashless 5B filter paper. Ammonia buffer solution was added (not exceeding pH 8) to this solution to precipitate iron as hydroxide. An aliquot (2 mL) of pre-concentrated environmental water or acid digested soil sample was pipetted into a calibrated flask and the copper, zinc and mercury content was determined under the general procedure using suitable masking agent. Triplicate measurements possess good precision (RSD not greater than 6%) (Table 6).

Table 6Determination of copper, zinc and mercury in environmental samples by the proposed method

Discussion

Zn(II) and Hg(II) complex formed with DDTC have no absorption whereas copper gives yellow insoluble complexes which has maximum absorption at wavelength 435 nm. Method is primarily optimized for copper determination using DDTC. Cu(DDTC)2 is more stable than Zn(DDTC)2. Zinc can be determined by a quantitative displacement Cu(II) followed by the extraction of Cu(DDTC)2 with a carbon tetrachloride and measuring the increased absorbance. Again, Hg(DDTC)2 is more stable than Cu(DDTC)2. Addition of He replaces the copper form Cu(DDTC)2 and thereby absorbance is reduced. Mercury determination is based on the measurement of reduced absorbance. Difference between two measurements gives the zinc or mercury contents present in mixture. Therefore, copper, zinc and mercury have been simultaneous determined after fractional extraction followed by their spectrophotometric measurement.
Robustness, ruggedness and suitability of the method indicated the reliability of the proposed method during its routine application for the analysis of zinc and mercury. The absorbance of the Cu(DDTC)2 complex remains stable for at least 72 h. This allowed the process of large batches of samples and their comfortable measurements convenient. This gives the high throughput property to the proposed method when applied for analysis of large number of samples in quality control/analytical laboratories. The results of samples analyses by the spectrophotometric method showed the satisfactory accuracy and precision within analytical agreement. The pH value should not be lower than 4 due to the fast decomposition of dithiocarbamates at these experimental conditions. On the other hand, pH value over 10 would accelerate the Cu(OH)2 precipitation. The results of biological analyses by spectrophotometric method were found to be in excellent agreement with those obtained by AAS.
In deep-well water zinc was 0.01μg mL-1 but in Chittagong WASA water it was present 5.07 μg mL-1 which is too much larger than deep well water. The large amount of the zinc and copper was also obtained in steel industry area water samples. For example the amount of Zn in PHP Steel area was 337.16 μg mL-1 but in KY Steel area it was 10.43 μg mL-1. Higher values for both zinc and copper concentration were detected for the water and soil samples collected from ship breaking yard. In case of industrial sample battery waste water contain higher amount of copper and Yestern Refinery water sample contain higher value of mercury. Therefore, respective responsible authorities in Bangladesh should have an attention to control environmental pollution due to zinc exposure.

Conclusions

Among the techniques suitable for the quantification of metal ions, ICP-MS, ICP-AES and atomic absorption or emission spectroscopy are likely to be the most widely employed. The choice of any analytical methods depends on the availability of reagents, cost effectiveness of instruments and the time required for analysis as well as safety and ease of operation. Thus, simple spectrophotometric techniques, which tend to be less costly and labor-intensive, are viable alternative to those methods requiring more sophisticated instrumentation. A spectrophotometric procedure for the simultaneous determination of zinc(II) and mercury using diethyldithiocarbamate (DDTC) as ligand is described. In the presence of a suitable masking agent very good selectivity was achieved for the simultaneous determination of zinc and mercury. Dithiocarbamates are more suitable than other complexing reagents because of their selectivity towards Hg(II) and Zn(II). Great advantage that is interesting also of this method is its selective application to copper, zinc or mercury determination individually or their simultaneous determination as required. Therefore, the method described herein has many advantages: it is simple and rapid; it has high accuracy and sensitivity, use of inexpensive reagents available in any analytical laboratory. The method is practical and valuable for its wide application.

Acknowledgements

Authors feel pleased to express their gratitude toward authority of University of Chittagong, Bangladesh and University Grant Commission (UGC), Bangladesh for financial support to conduct the project. Principal author also acknowledges to the MS students who carried out the research and Department of Chemistry for giving laboratory facilities.

Author Contributions

Uddin MN: Project director, wrote the article and designed the discussion.
Shah NM, Islam MM, Hossain MA: conducted experiments and analyzed the data.

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