Introduction

The literature concerning the analysis of copper-containing materials is vast. This is due to the widespread interest in copper content in several branches of science such as medicine, biology, industrial chemistry and geology. Copper is widely distributed in nature and various analytical techniques like volumetry [1], potentiometry [2], atomic absorption spectrometry [3], adsorption stripping voltammetry [4] and the use of selective electrodes [5] have been reported for the determination of trace amounts of copper(II). Some catalytic methods for the determination of trace amounts of copper(II) have been reported by several workers using various indicator reactions which have been reviewed [6, 7, 8, 9, 10, 11]. A very recent method for the determination of copper is based on the catalytic redox reaction between methylene blue and ascorbic acid [12]. Most of the methods reported have shortcomings like product instability, the need for extraction, heating and long times for the development of colour, or the addition of too many reagents, which also include oxidizing agents. Another disadvantage is that some of the methods are inapplicable to environment samples.

In view of the above, here we report a sensitive, simple and rapid method for the determination of traces of copper(II). The reaction is based on the catalytic oxidation of 3-hydroxyacetanilide by copper(II), followed by coupling with 3-methyl-2-benzothiazolinone hydrazone in the presence of ammonia and hydrochloric acid to give a pink-coloured product, which is stable for 48 h. The proposed method has been successfully employed for the determination of copper(II) in water and soil samples.

Experimental

Instrument

A JASCO Model UVIDEC-610 UV-VIS spectrophotometer with 1.0-cm matched cells was used for absorbance measurements.

Reagents and solutions

Copper sulfate pentahydrate (Merck), 3-hydroxyacetanilide (3-HA; Sigma, USA) and 3-methyl-2-benzothiazolinone hydrazone hydrochloride hydrate (MBTH; Sigma, USA) were used without further purification. Ammonia (Merck) and hydrochloric acid (BDH) were used. All other chemicals and solvents used were of analytical reagents grade. Deionized water was used to prepare all solutions and in all experiments.

Stock solution of copper(II) (1 mg mL−1) was prepared by dissolving 0.3929 g of copper(II) salt in water containing 1 mL of conc. sulfuric acid and the solution was diluted to 100 mL. Working solutions of copper(II) were prepared by further dilution of the stock solution. A 0.2% and 0.1% (w/v) solutions of 3-hydroxyacetanilide and 3-methyl-2-benzothiazolinone hydrazone hydrochloride hydrate were prepared by dissolving 200 mg and 100 mg respectively in 100 mL of water. Solutions of 5 mol L−1 ammonia and 5 mol L−1 hydrochloric acid were used for the experiment.

Standard procedure

A solution containing 0.2–4.0 μg (0.008–0.16 μg mL−1) of copper was transferred into a series of 25-mL calibrated flasks to which 2.5 mL of 3-HA, 2.5 mL of MBTH and 0.5 mL of 5 mol L−1 ammonia solution were added; the mixture was shaken for about 20 s for the completion of the reaction. Then, 4 mL of 5 mol L−1 hydrochloric acid was added, this was made up to the mark with water and the solution was mixed well. The absorbance of the resulting pink-coloured coupled product was measured at 540 nm against a reagent blank, and the calibration graph was constructed.

Procedure for soil samples

The soil samples were collected from agricultural land in an airtight polythene bag, stored at room temperature (27 °C) and analysed within 48 h. A 20-g air-dried soil sample was taken in a 500-mL beaker and 250 mL of 1:10 hydrochloric acid was added. The sample was heated at reflux on a boiling water bath for 3 h, shaking frequently. Hot solution was filtered into 1-L calibrated flask, the residue was washed three times with 10-mL portions of 1:10 hydrochloric acid and diluted to the mark with water. The standard procedure was followed for copper determination.

Procedure for water samples

Tap water samples were collected in polythene bottles from different sources. For the analysis, 1-L water sample was evaporated to 75 mL and transferred along with the washings into a platinum dish. Then, 2 mL of 1:1 sulfuric acid was added to neutralize the alkalinity. A known volume of the neutralized sample was analysed by the standard procedure.

Water sampling and storage

Copper ions tend to be adsorbed on the surface of (glass and metal) sample containers. Therefore, samples were analysed immediately after collection. If storage is necessary, use 0.5 mL of 1:1 hydrochloric acid per 100 mL of sample or acidify to pH 2 with nitric acid to prevent this adsorption.

Literature method

For the validation of the proposed method, the authors selected the method proposed by Haworth and Murnroe [13] as the literature method, which was simple for analysis of copper(II) using the reagent, N8-quinolyl-p-toluenesulfonamide. The literature method gives a faster rate of colour development for the complex, and the absorbance of the complex is stable for a longer period of time. Most of the common cations and anions did not interfere in this method.

Results and discussion

Spectral characteristics

The proposed method involves the formation of a pink-coloured coupled product with a λmax of 530 nm. The reagent blank had negligible absorption at this wavelength. The absorption spectra of the product and the reagent blank are shown in Fig. 1.

Fig. 1.
figure 1

Absorption spectra of the reaction product (Cu(II) + 3-HA+MBTH) and reagent blank (3-HA+MBTH). Final concentration of copper(II) is 0.08 μg mL−1

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Effect of reagent concentrations

We found that 0.2% of 3-HA and 0.1% of MBTH in the range of 2.0–3.0 mL were necessary to achieve maximum colour intensity. The colour intensity of the product was decreased when the volume of the reagents added was less than the minimum as mentioned above. Hence, 2.5 mL each of 3-HA and MBTH were used for all the subsequent work.

Effect of ammonia and hydrochloric acid

Various concentrations and volumes of ammonia and hydrochloric acid were tried. However, 0.3–0.7 mL of 5 mol L−1ammonia and 3–5 mL of 5 mol L−1 hydrochloric acid were necessary to achieve maximum colour intensity. Hence, 0.5 mL of 5 mol L−1 ammonia and 4 mL of 5 mol L−1 hydrochloric acid were used.

Effect of time and temperature on the coloured product

The effect of time on the absorbance of pink product was studied as a function of time for solutions containing different amounts of copper and prepared as described in the standard procedure. The results obtained showed that the maximum absorbance was attained almost instantaneously. In order to obtain maximum sensitivity and constant reading, the absorbance should be measured after 20 s. The colour development is independent of temperature in the range of 20–50 °C, and the coloured product was stable for 48 h; the absorbance varied by not more than 2% over a period of 3 days.

Optical parameters

Beer's law was obeyed over the copper(II) concentration range of 0.008–0.16 μg mL−1. Molar absorptivity, Sandell's sensitivity and other parameters are given in Table 1. The precision and accuracy of the method was studied by analysing the solution containing known amounts of cited reagents within the Beer's law limit. The lower values of relative standard deviation (%) and percentage errors indicated the high accuracy of the method.

Table 1. Optical Characteristics and precision data
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Reaction mechanism

The reaction involves catalytic oxidation of 3-hydroxyacetanilide by copper(II) and the oxidized product is then coupled with MBTH in the presence of ammonia solution to give a red precipitate and dilution with hydrochloric acid gives a pink-coloured product (Scheme 1). Under the present experimental conditions, MBTH is not oxidized by copper(II), as the 3-HA has a phenolic group which is prone to oxidation first. Another significant aspect is that ammonia acts as an activator [7] for the catalytic action of copper(II).

Scheme 1.
scheme 1

Reaction sequence for the formation of the coupled product

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Interference studies

The effect of various cations and anions on the determination of copper(II) was investigated. The tolerance limit was taken as the amount that caused ±2% absorbance error in the determination of 0.08 μg mL-1 of copper(II). The results are given in Table 2. Cyanide, EDTA, ferrocyanide and ferricyanide interfere seriously by decolouring the solution. The interference from Fe(III) can be eliminated by the addition of fluoride.

Table 2. Effect of foreign ions on the determination of 0.08 μg mL−1 of copper(II)
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Analytical application

The proposed method is applied to the determination of copper(II) in tap water and soil samples. A comparison of our results with the literature method is given in Table 3. Recovery experiments were performed using the method of addition for synthetic samples only.

Table 3. Determination of copper (II) in various samples
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Conclusions

The present method is simple, rapid and highly sensitive for the determination of copper(II). Extraction or heating is not necessary. This fact makes it easy for the metal ion determination, because a tedious and expensive prior solvent extraction step, which generally uses toxic organic solvents, is not required. A number of associated elements do not interfere in the determination. The method is well suited for the analysis of environmental samples. A comparison of the analytical performances with those of literature methods is given in Table 4.

Table 4. Comparison of spectrophotometric methods for the determination of copper(II)
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