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Original article
10 (
1_suppl
); S450-S460
doi:
10.1016/j.arabjc.2012.10.005

Simultaneous determination of dissolved inorganic chromium species in wastewater/natural waters by surfactant sensitized catalytic kinetic spectrophotometry

, ,
 University of Cumhuriyet, Faculty of Science, Department of Chemistry, TR–58140, Sivas, Turkey

⁎Corresponding author. Tel.: +90 346 2191010/2136; fax: +90 346 2191186. rgurkan@cumhuriyet.edu.tr (Ramazan Gürkan) rgurkan95@gmail.com (Ramazan Gürkan)

Received: , Accepted: ,
© The Author(s) 2012
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

A simple, rapid, highly accurate and sensitive kinetic method is proposed for determining chromium(VI). The method is based on its catalytic effect on the oxidation of Celestine blue (CB+) by H2O2 in the presence of 2,2′-bipyridyl (Bipyr) and cetylpyridinium chloride (CPC) at pH 6.50. The reaction was monitored spectrophotometrically by measuring the absorbance of indicator dye at 645 nm. The analytical variables, which have influences on the sensitivity, were investigated and the optimum conditions were established. The optimized conditions made it possible to determine and speciate chromium in a linear range of 5–200 μg L−1 with a detection limit of 0.65 μg L−1. The recoveries and relative standard deviations (RSDs) for the determination of 10, 25, 75 and 150 μg L−1 Cr(VI) (n: 5) were in the range of 99.0–99.8% and 0.2–3.5%, respectively. The selectivity was also studied and greatly enhanced by adding a suitable masking mixture. The method was successfully applied to the simultaneous analysis of Cr(III) and Cr(VI) in natural water and waste water samples with a recovery changing in the range of 95–103% for Cr(III) and 100–104% for Cr(VI). Its accuracy was validated by the analysis of certified reference materials with good agreement between certified and found values.

Keywords

Chromium speciation
Catalytic effect
Kinetic spectrophotometry
CB+
Bipyr
CPC
1

1 Introduction

Metal speciation is usually important in view of studying the behavior of metal ions in the environment. In the case of chromium, the common oxidation states in natural water are Cr(III) and Cr(VI). In spite of Cr(III) not being a significant groundwater contaminant, Cr(VI) is considered to be a human harmful agent (American Water Works Association, 1990; Carry 1982; Forstner and Wittmann, 1983; Nriagu and Nieboer, 1988). Unfortunately, high effective techniques, like graphite furnace atomic absorption spectrometry (GF-AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) only yield the total concentration. Thus, pretreatment techniques such as extraction methods have been used for the separation of chromium species in such determinations (Liang and Sang, 2008; Sumida et al., 2006; Sun and Liang, 2008; Liu et al., 2005; Stasinakis et al., 2003; Hosseini and Rzaei-Sarab, 2007). In the last decade, there has been a rapidly increasing demand for fast and reliable analytical methods, which make available the direct determination of chromium species at trace levels in environmental samples. Hence, analytical techniques, including voltammetry (Grabarczyk et al., 2007; Safavi et al., 2006), chromatography (Arancibia et al., 2003; Castillo et al., 2007), spectrophotometry (Scancar et al., 2007; Singer and Aldstadt, 2003; Cui et al., 2006; Kaneko et al., 2002; Ghaedi et al., 2006) and spectrofluorometry (Paleologos, 1998; Paleologos et al., 2001; Hassan et al., 2005) have been successfully developed for determinations in different sample matrices. Among them, the last two techniques utilizing Cr(VI)-selective methods are of particular interest because of their simplicity and fast handling. These determinations are frequently based on the reaction of organic compounds with Cr(VI). Frequently, an ion-associate is formed containing an organic reagent and Cr(VI) ions. These methods have the disadvantage of high blank values. Another important method, which has been reported for the determination of Cr(VI) is based on absorbance decreasing or luminescence quenching (Revanasiddappa and Kiran Kumar, 2001; Wang et al., 2004a,b; Tang et al., 2004). In these methods, a sensitive organic reagent is generally reacted with Cr(VI) in the presence of Cr(III) species. It should be considered that analytical feature of these methods is strongly relevant to the structural characteristics of the organic reagent. Evidently, the applications of each one of these reagents for the determination present some advantages and suffer from several limitations; however, the general performance in these progresses is the development of a series of viewpoints, including high sensitivity and selectivity, more simplicity and fast, better detectability and applicability to environmental samples of interest.

Catalytic kinetic methods are based on chemical reactions where the rate is influenced by the reaction conditions. These methods have the advantages of high sensitivity, extremely low detection limit, good selectivity, rapid analysis rate, and inexpensive instruments. One of the most effective ways of improving the analytical features of catalytic reactions is the use of activators in kinetic analysis (Perez-Bendito and Silva, 1988). The use of an activator in the catalyzed reactions is usually intended to increase sensitivity and hence lower the detection limit for the catalyst, and to improve the selectivity and precision of the determination. Another way of improving the analytical features of kinetic methods is the use of organic microheterogeneous system surfactants. In the last two decades surfactant molecules and their aggregates above or around critical micelle concentration (CMC) have been used increasingly in analytical techniques in order to alter the properties, including the reactivity, of the analytes as well as analysis time and sampling rate, (Carreto et al., 1990; Loreto-Lunar et al., 1997; Morales et al., 1997; Gonzalez et al., 1993; Esteve-Romero et al., 1995; Lopez-Carreto et al., 1996). In this context, the kinetic methods used for simultaneous determination of the species available in a wide range of samples either alone or alongside each other have recently opened a new research field in analytical chemistry. As it does not require expensive devices its use is easy. So it shows a quick development in solving trace analysis and microanalysis problems. Although there are many catalytic-kinetic methods reported in the literature for the determination of Cr(VI) (Li and Liu, 2006; Reis et al., 1998; He and Wang, 1999; Ashraf et al., 2001; Perez-Benito and Arias, 1997; Cesar et al., 1999; Liu and Zhang, 1994; Mohamed et al., 2006; Chen and Huang, 2003; Bi, 2001; Ma and Yang, 1999; He and Wang, 2000; Yu et al., 2005; Wu et al., 1999; Wei et al., 2006); a researcher using CB+ as indicator to determine trace amounts of Cr(VI) does not exist. Therefore, the main purpose of this study is to use a simple, sensitive, rapid and cost effective kinetic method based on CB+ for the speciative determination of chromium species present in natural waters.

In the present work, the Cr(VI)-catalyzed reaction of CB+ with H2O2 in the presence of Bipyr as activator and CPC as sensitivity enhancement at pH 6.50 was investigated at the first step. In the light of the requirements for the search of an eco-friendly, simple, fast and inexpensive technique for simultaneous determination of individual chromium species, Cr(III), Cr(VI) and total Cr, application of the reaction for the speciative determination of the chromium content of natural water samples was then studied, namely, spectrophotometry. The various analytical parameters, which affect the analytical performance, have been evaluated for this purpose.

2

2 Experimental

2.1

2.1 Instrumentation

Absorbance measurements at 645 nm were made on a double beam UV–Visible Spectrophotometer (Shimadzu UV-1800 PC, Kyoto, Japan) equipped with the 1.0-cm quartz cells. An external thermostatic water bath (BM-302 Nüve) was connected to this device at constant temperature. Eppendorf vary-pipettes (10–100 and 200–1000 mL) were used to deliver accurate volumes. A pH meter (pH-2005 model) was used for pH measurements. A stopwatch to record the reaction time was used.

2.2

2.2 Standard solutions and reagents

A stock standard solution of 1000 mg L−1 Cr(VI) was prepared from potassium bichromate (Fluka). Working solutions were prepared daily by appropriate dilution with 0.01 mol L−1 HCl. A stock standard solution of 1000 mg L−1 Cr(III) was prepared from chromium(III) chloride hexahydrate (Merck). Working solutions were prepared daily by appropriate dilution with bidistilled water. A 1.0 × 10−3 mol L−1 aqueous solution of CB+ was also prepared daily (Sigma). A 1.0 mol L−1 aqueous solution of hydrogen peroxide was prepared from 35% hydrogen peroxide (d: 1.15 g mL−1) (Merck). The solution was standardized against the standard KMnO4 solution to check its concentration when necessary. The pH of reaction medium was buffered with a buffer solution of 0.25 mol L−1, prepared by dissolving 4.0 g of hexamethylenetetramine (HMTA) (Merck) in 1 mL of concentrated HCl and brought to a pH of 6.50 using appropriate volumes of 0.2 mol L−1 solutions of NaOH or HCl. DTPA masking solution (Merck) (5.0 × 10−3 mol L−1 containing 0.01 mol L−1 CaCl2 and 0.1 mol L−1 TEA) was prepared by adjusting to a pH of 7.5 with diluted HCl (1:1, v/v) after dissolution of solids. Bipyr solution of 6.4 × 10−3 mol L−1 was prepared by dissolving a suitable amount of solid (Merck) in ethanol and water. All surfactant solutions of 1.0% (w/v or v/v) were prepared by dissolving a suitable portion of solid or liquid reagent (Sigma) in water and thoroughly homogenizing under ultrasonic effect when necessary.

2.3

2.3 The proposed kinetic procedure

The reaction rate was spectrophotometrically monitored by measuring the absorbance change of reaction mixture using the fixed-time approach of the first 0.5–5 min from the initiation of reaction at 645 nm. A specific portion of a sample solution containing Cr(VI) in the range of 5–200 μg L−1 is added to a calibrated flask of 10 mL. Then 0.8 mL buffer solution (0.25 M, pH: 6.5), 0.3 mL of 1.0 × 10−3 M CB+, 0.6 mL of 6.4 × 10−3 M Bipyr, and 0.15 mL of 1.0% (w/v) CPC were added. The mixture was diluted to approximately 9 mL with water. After that, 0.8 mL of 1.0 M H2O2 was added to this mixture and was diluted to the mark with water and thoroughly mixed. Within the first 5 min after the initiation of the reaction, samples were transferred to 1.0-cm quartz cell. The change in absorbance (ΔAs) was spectrophotometrically measured at 645 nm. Time was measured immediately after the addition of H2O2 drops. In order to obtain the absorbance value for the uncatalyzed-reaction (ΔAb), a blank solution was prepared in the absence of Cr(VI). Net reaction rate was calculated from the difference in the absorbance change (Δ(ΔA)): ΔAs − ΔAb) for the fixed time of 5 min. All reagents before the absorbance measurement were taken to preheat in a thermostatic water bath at 20 ± 0.2 °C for 30 min. The Cr(VI) and total Cr contents of the unknown sample were determined and speciated through calibration and standard addition curves before and after a pre-oxidation of the binary mixture containing Cr(VI) and Cr(III) ions with the standard of H2O2 solution.

2.4

2.4 The oxidation of Cr(III) to Cr(VI) and determination of total Cr

Oxidation of Cr(III) to Cr(VI) has been performed by using the procedure given in the literature (Demirata 2001; Andersen 1998; Narin et al. 2002). After adjustment of the pH of the solution to pH 10, 10 mL of 3% (w/w) H2O2 was added. The amounts of Cr(III) in this solution were kept constant at any value in the linear range of 5–200 μg L−1. The solution was heated at 80 °C for 40 min. Then the solution was boiled for 10 min in order to remove any excess of H2O2. Then, the above mentioned kinetic procedure was applied to this solution. Chromium was spectrophotometrically determined by using the present method at 645 nm. After oxidation of Cr(III) to Cr(VI) by using H2O2 in alkaline media, the method was applied to the determination of the total Cr. The level of Cr(III) is calculated by the difference of total Cr and Cr(VI) concentrations.

3

3 Results and discussion

3.1

3.1 Absorption spectra and possible catalytic reaction mechanism

The absorption spectra of the catalytic and noncatalytic systems against water in the range of 400–750 nm were recorded. Fig. 1 shows the absorption spectra of solutions according to the general procedure. The results suggest that the absorbance values of different systems reach their maximum at 645 nm while a weak absorption peak is observed at 530 nm in the presence of CPC and Bipyr without catalyst. The decolorizing oxidation of CB+ with H2O2 at pH 6.5 is very slow. Fig. 1 indicates that the oxidation of CB+ by H2O2 is clearly catalyzed in the presence of trace amounts of Cr(VI). The acceleration effect was greater when Bipyr was present in the system. The net absorbance difference, Δ(ΔA) has a linear relationship with Cr(VI) concentration within a prescribed range. Therefore, trace amounts of Cr(VI) can selectively be determined at 645 nm by using spectrophotometry throughout our study.

Absorption spectra: 1—pH 6.5 HMTA buffer – CB+–CPC (against water), 2—pH 6.5 HMTA buffer – CB+–CPC–H2O2 (against water), 3—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–H2O2 (against water), 4—pH 6.5 HMTA buffer – CB+–CPC–Bipyr-–0 μg L−1 Cr(VI)–H2O2 (against water), 5—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–100 μg L−1 Cr(VI)–H2O2 (against water), 6—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–150 μg L−1 Cr(VI)–H2O2 (against water) and 7—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–200 μg L−1 Cr(VI)–H2O2 (against water) under the selected experimental conditions: 0.8 mL pH 6.5 HMTA buffer, 0.3 mL of 1.0 × 10−3M CB+, 0.5 mL of 1.0% (w/v) CPC, 0.6 mL of 6.4 × 10−3 M Bipyr and 0.7 mL of 1.0 M H2O2 for the fixed-time method of 5 min and 20 °C in a final volume of 10.0 mL.
Figure 1
Absorption spectra: 1—pH 6.5 HMTA buffer – CB+–CPC (against water), 2—pH 6.5 HMTA buffer – CB+–CPC–H2O2 (against water), 3—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–H2O2 (against water), 4—pH 6.5 HMTA buffer – CB+–CPC–Bipyr-–0 μg L−1 Cr(VI)–H2O2 (against water), 5—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–100 μg L−1 Cr(VI)–H2O2 (against water), 6—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–150 μg L−1 Cr(VI)–H2O2 (against water) and 7—pH 6.5 HMTA buffer – CB+–CPC–Bipyr–200 μg L−1 Cr(VI)–H2O2 (against water) under the selected experimental conditions: 0.8 mL pH 6.5 HMTA buffer, 0.3 mL of 1.0 × 10−3M CB+, 0.5 mL of 1.0% (w/v) CPC, 0.6 mL of 6.4 × 10−3 M Bipyr and 0.7 mL of 1.0 M H2O2 for the fixed-time method of 5 min and 20 °C in a final volume of 10.0 mL.

In this study, CB+ was used as chromogenic agent with reducing character, especially due to its oxazine group containing hetero-nitrogen and oxygen atoms and metal binding conjugate hydroxyl groups, H2O2 as the oxidant, Cr(VI) as the catalyst with oxidizing character. CB+ was selected as an indicator dye containing a reducing group, oxazine and phenolic –OH group that can participate in pH-dependent electron transfer reactions at pH 6.50. It can be seen from Scheme 1 that the aromatic ring in CB+ carries –OH, –N(CH3)2 and an amidic group, which can catalytically be hydrolyzed in acidic and basic media, and has the potential of being a chelating agent. The reagent not only has a strong complex ability and forms various water soluble complexes with metal ions, but also the oxazine group itself can produce color. When the ligand is oxidized or reduced, the oxazine group is destroyed, which results in the slow loss of solution color, or even colorless. It is found in this study that at pH 6.50, trace levels of Cr(VI) catalyzes the decolorizing reaction of CB+ oxidized by H2O2 and based on this principle a novel kinetic method for the determination of trace amounts of chromium was developed. The developed method is characterized by high sensitivity, operation simplicity, and low analytical cost. Based on the principle of catalytic reaction (Bontchev, 1970) authors propose the catalytic reaction mechanism as follows:

(1)
2 H 2 In + + H 2 O 2 2 H 2 ⟶ Slow,pH : 6.50 2 H 2 InO + H 2 O + 2 H +
(2)
2 H 2 In + + H 2 O 2 2 H 2 → Fast,pH : 6.50 ,Cr ( VI ) 2 H 2 InO + H 2 O + 2 H +
(3)
2 H 2 In + + H 2 O 2 2 H 2 ⟶ Very Fast,pH : 6.50 ,Cr ( VI ) in presence of 2 , 2 ′ - bipyridyl and CPC InO + H 2 O + 2 H +
(4)
[ CB + - Cr ( VI ) - H 2 O 2 ] → degradation products + Cr ( V ) or [ CB + - Cr ( VI ) - 2 , 2 ′ - bipyridyl - CPC + ] → degradation products + Cr ( V )
(5)
2 Cr ( V ) + H 2 O 2 → [ unstableCr ( V ) - peroxocomplex formation ] → 2 Cr ( VI ) + 2 OH -
(6)
2 OH - + 2 H + → 2 H 2 O
(7)
or Cr ( VI ) + Cr ( IV ) ⟶ Fast 2 Cr ( V )
(8)
2 H 2 In + + Cr ( V ) + H 2 O ⟶ Fast 2 H 2 InO + Cr ( III ) + 2 H +
(9)
2 [ Cr ( OH ) 6 ] 3 - + 3 H 2 O 2 → 2 CrO 4 2 - + 2 OH - + 8 H 2 O
The open molecular structure of Celestine Blue (CB+ or H2In+Cl−).
Scheme 1
The open molecular structure of Celestine Blue (CB+ or H2In+Cl−).

The rate enhancing effect of chromium can be explained by the formation of much faster reacting stable intermediate complex (CB+–Cr(VI)–H2O2) than H2O2, or a stable ion-associate complex formation such as CB+–Cr(VI)–Bipyr–CPC–H2O2 based on activation and micellar effect in the presence of Bipyr and CPC, respectively. The Cr(V) produced after catalytic oxidation reaction reacts with the excess H2O2. So the catalytic cycle is completed. In general, it can be assumed that the second- and third-reactions proceed more quickly and much more quickly than the first one, respectively. The reduced Cr(IV) or Cr(V) is oxidized to Cr(VI) by H2O2, and then the Cr(VI) is allowed to react with CB+ again. Probably for this reason, a large excess of H2O2 with respect to the reagent (∼2667 fold) is needed. The oxidation product was not identified with any spectroscopic detection method. However, CB+ could be oxidized by a one-electron mechanism to produce a resonance-stabilized product like Cr(V)–peroxo complex, and a quinonoid species seems probable.

3.2

3.2 Optimization of analytical variables

Ideally, for kinetic measurements, except the analyte, it should be in a way that the concentration of each component will give the smallest RSD and the reaction rate will be zero-order according to its optimized species. The conditions that the small fluctuations in concentration did not have any effect on the initial rate, are requested. These conditions should also be chosen in a way of that the initial rate will be pseudo-first order according to the analyte (that is Δ(ΔA): ks′[Cr(VI)]). In this context, kinetic optimization data and calibration curves were repeated at least three times.

3.2.1

3.2.1 The effect of pH on sensitivity

In order to investigate the effect of pH on sensitivity, preliminary experiments were conducted by using two different universal pH buffer systems in a wide pH range. The first buffer system, which is known as a Britton–Robinson buffer consists of a mixture of 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH that has been adjusted to the desired pH with 0.2 M NaOH in the range of 2–12, and the second buffer system consists of 0.0286 M citric acid, 0.0286 M KH2PO4 and 0.0286 M H3BO3 that has been adjusted to the desired pH with 0.2 M NaOH in the range of 2.5–9.2. It has been observed that the best sensitivity is obtained at a pH range near to neutral in the range of pH 4.0–7.5 while other analytical variables held constant at 20 °C under optimized conditions. In order to enhance the selectivity and sensitivity of the buffer, the binary buffer systems such as H3BO3–citric acid, NH3/NH4Cl, sodium tetraborate–HCl and HMTA–HCl were also used, and among these buffer systems HMTA–HCl (pH 6.50) was chosen as the best suitable buffer system due to the maximum signal. For catalytic kinetic determination of Cr(VI) pH was changed from 4.0 to 7.5 with an increase of 0.5 pH unit. The highest absorbance value was obtained at pH 6.5. At pH values lower and higher than 6.50 the sensitivity decreased with an increasing slope. The slow rate at lower pH range may be attributed to the existence of protonated forms of indicator and catalytically active chromium species such as H 2 CrO 4 , HCrO 4 - , which are comparatively less reactive. Another reason may be chromate–bichromate conversion as a result of pH dependence in equilibrium. The slow rate at pHs higher than 6.50 may be attributed to the existence of protonated forms of indicator and catalytically active chromium species such as HCr 2 O 7 - , Cr 2 O 7 2 - , H 2 CrO 4 , HCrO 4 - , which are comparatively less reactive. At pHs higher than 6.50, Cr(III) ions taking place in the catalytic cycle may be their precipitation as Cr(OH)3 and the catalytic activity is lost as a result of conversion to a series of consecutive anionic hydroxy complex formation reactions with increasing pH. Therefore, a pH of 6.50 was chosen as the optimum value for further studies. Also in the range of 0.05–0.04 M, effect of buffer concentration on the analytical signal was investigated and the maximum signal was obtained in a buffer concentration of 0.02 M (Fig. 2).

The effect of pH 6.5 HMTA buffer concentration on sensitivity.
Figure 2
The effect of pH 6.5 HMTA buffer concentration on sensitivity.

3.2.2

3.2.2 The effect of indicator concentration on sensitivity

The effect of CB+ concentration on the sensitivity was investigated in the range of (1.0–8.0)×10−5 M under optimized conditions. The results show that a high sensitivity was obtained when the concentration of CB+ is lower than 3.0 × 10−5 M, but the linear detection range of Cr(VI) as a catalyst is narrow. When the concentration of CB+ is higher than 3.0 × 10−5 M, a great analytical signal error has easily been caused and the repeatability has been bad. In order to guarantee high sensitivity and proper linear detection range, an indicator concentration of 3.0 × 10−5 M was considered as an optimal value for further studies (Fig. 3).

The effect of Celestine blue concentration on sensitivity.
Figure 3
The effect of Celestine blue concentration on sensitivity.

3.2.3

3.2.3 The effect of H2O2 concentration on sensitivity

Effect of H2O2 concentration on analytical sensitivity was examined in the range of 0.02–0.16 M under optimized conditions. The results showed that the analytical signal, Δ(ΔA) increased with the increase in the concentration of H2O2 up to 0.08 M in the beginning and then quickly reduced. When the concentration of H2O2 is 0.08 M, a maximum sensitivity, Δ(ΔA) has been obtained as a measure of difference between the catalyzed- and uncatalyzed reaction rates. Therefore, a H2O2 concentration of 0.08 M was considered as the optimal value for further studies (Fig. 4).

The effect of H2O2 concentration on sensitivity.
Figure 4
The effect of H2O2 concentration on sensitivity.

3.2.4

3.2.4 The effect of activator concentration on sensitivity

Although catalytic reactions offer good sensitivity, addition of suitable complexing agents (activators) permits the improvement in their sensitivity and selectivity. Several attempts have been made to enhance the detection limit of Cr(VI) by using activator ligands such as boric acid (BA), salicylic acid (SA), sulfosalicylic acid (SSA), 1,10-phenanthroline (Phen), pyridine (Pyr) as well as (Bipyr) at equimolar concentrations of 6.4 × 10−3 M. Bipyr showed the highest activation effect followed by SSA, Pyr, BA, SA and Phen. So, bipyr acting as a basic ligand with two dentates of pKb: 4.45 (pHbuffered media: 6.50 < pKa: 9.55) was considered the best suitable activator for indicator system (Fig. 5(a)). The role of the activator on the catalytic reaction is the acceleration of the reaction through facilitation of the charge transfer from the catalyst (usually metal ion) to the dye, also the redox potential of some metal cations with multiple oxidation steps such as chromium, iron vanadium and copper, changed via the formation of such complexes with activators, which is reflected on the increasing of the reaction rate. It was observed that the conditional redox potential of redox systems such as Fe(III)/Fe(II) redox couple increases in the presence of activators, because of the difference in the stability constants of Fe(II) and Fe(III) with similar ligands, (the formation constant, log β3, of the Fe(III) with Phen is 14.1, Bipyr is 17.6, Oxalate is 18.46 and TEA is 41.2) (Sillen, 1964). Then, the effect of bipyr concentration as activator to analytical sensitivity was examined in the range of 6.4 × 10−5–6.4 × 10−4 M under optimized conditions. The rate of the catalyzed reaction increased with increasing bipyr concentration up to a definite concentration of 3.84 × 10−4 M and then decreased gradually at the higher concentration range. Therefore, a bipyr concentration of 3.84 × 10−4 M was chosen as the optimal value for further studies (Fig. 5b. Such dependence is a characteristic feature for the types of activated reactions when the activator is concerned with the formation of ternary complexes of the activator–metal substrate (Bontchev, 1972). The coordination sphere of the catalyst is fully occupied by the activator and any complexation with substrate does not occur at higher concentrations of activator.

The effect of activator type at isomolar concentrations on sensitivity.
Figure 5a
The effect of activator type at isomolar concentrations on sensitivity.
The effect of 2,2′-bipyridyl concentration on sensitivity.
Figure 5b
The effect of 2,2′-bipyridyl concentration on sensitivity.

3.2.5

3.2.5 The effect of surfactant type and concentration

It is known for a long time that micellar media changed the reaction rate (Bunton et al., 1991). The fact that micelles are able to accelerate the reactions, has kindled increasing interest in the last few years in order to improve the analytical properties of both the catalytic- (Sicilia et al., 1992) and non-catalytic-kinetic methods (Athanasiou-Malaki and Koupparis, 1989; Sicilia et al., 1993) such as sensitivity and selectivity. Further to improve analytical sensitivity and selectivity different surfactants with dilute cationic (CPC, CTAB and HDTAB), anionic (SDS) and nonionic (Tween 80, Triton X-100 and Triton X-114) character having the catalysis properties in a concentration range of 0.01–0.07% (w/v or v/v) in the premicellar region were investigated in Figs. 6a and 6b), and premicellar catalysis was observed to change in the sequence of the CPC, CTAB, HDTAB, Triton X-100, Triton X-114, Tween 80 and SDS. In the presence of SDS only, a red shift of 6 nm in the visible region was observed in the measurement wavelength of indicator systems. From the results, it is clear that the best suitable surfactant due to give a maximum sensitivity of 0.226 for a concentration of 0.015% (w/v), is cationic surfactant, CPC for indicator system. At lower and higher concentrations than 0.015% (w/v), a significant decrease in analytical sensitivity was observed in which other variables are held constant throughout. Therefore, a CPC concentration of 0.015% (w/v) was preferred as optimum surfactant type and concentration for further studies.

The effect of cationic surfactant concentration on sensitivity.
Figure 6a
The effect of cationic surfactant concentration on sensitivity.
The effect of anionic or nonionic surfactant concentration on sensitivity.
Figure 6b
The effect of anionic or nonionic surfactant concentration on sensitivity.

3.2.6

3.2.6 The effect of ionic strength on sensitivity

Under optimal conditions selected, the effect of ionic strength to the sensitivity, Δ(ΔA) was examined in the concentration range of 0.005–0.2 M NaNO3 under optimized conditions. Results showed that the sensitivity has changed very little with increasing concentration up to 0.05 M, after this concentration exhibited a negative change with increasing inclination. This case predicated that the catalyzed-reaction would give the right response for catalyst in real life samples with a low matrix. It can be expressed that inert salt effect should be checked at matrix systems with the high salt content such as sea water.

3.2.7

3.2.7 The effect of temperature and time on sensitivity

The effect of reaction temperature on sensitivity, Δ(ΔA) was examined in the range of 20–55 °C under optimized conditions. Results show that the reaction rate increases with temperature rising to 40 °C. The analytical sensitivity remains constant at temperatures higher than 40 °C and no longer changes. This situation is a clear indication of the fact that catalyzed-reaction completed and reached the thermal equilibrium. Although this temperature gives maximum sensitivity, due to ease of work and better signal reproducibility for different catalyst concentrations it has been decided that a temperature of 20 °C is sufficient for the catalytic reaction.

The time to measure changes in absorbance was also optimized. The effect of time on the catalyzed- and uncatalyzed-reaction rates was examined in a time interval of 0.5–12 min under optimized conditions without adding an inert salt like KNO3 to the reaction media. It was found that the analytical sensitivity, Δ(ΔA): ΔAS − ΔA0 occurred and completed within the first 5–6 min after the initiation of the catalytic reaction. The best correlation coefficient between sensitivity and the catalyst concentration was obtained for a fixed-time of 5 min. For this reason, the fixed time measurement of 5-min was adopted as the most suitable reaction time.

3.3

3.3 Calibration curve, detection limit and precision

Different Cr(VI) standard calibration solutions were sampled to measure absorbance change, Δ(ΔA) (that is, ΔAS or ΔAb) of catalyzed- and uncatalyzed-systems under optimized conditions: [H2O2]: 0.8 mL of 1.0 M, [CB+]: 0.3 mL of 1.0 × 10−3 M, [Bipyr]: 0.6 mL of 6.4 × 10−3 M, [CPC]: 0.15 mL of 1.0% (w/v) at 20 °C and pH 6.50 in a final volume of 10 mL for a fixed-time of 5 min at 645 nm. The results showed that Δ(ΔA) versus concentration of chromate ion, Cr(VI) in the range of 5–200 μg L−1 obeyed a good linear relation. The regression equation was Δ(ΔA): 0.0045[Cr(VI), μg L−1] − 0.062 with a good correlation coefficient of 0.9954. The limits of detection and quantification of the developed kinetic method, LOD and LOQ respectively, were 0.65 and 2.28 μg L−1 Cr(VI), which were obtained by dividing the slope of calibration curve with 3.3 and 10 multiples of standard deviation of ten replicate measurements for a blank solution without catalyst. The surfactant medium allowed the determination of Cr(VI) in a working range of 40-fold with a detection limit, which is about 5 times lower than those of the methods implemented in aqueous media. The recoveries and RSDs of the method were found to change in the range of 99.0–99.8% and 0.2–3.5% for determinations of 10, 25, 75 and 150 μg L−1, respectively (n: 5) (Table 1). It can be concluded that the results are quantitative and highly reproducible from the analytical point of view.

Table 1 The accuracy and precision of the kinetic method based on catalytic effect of Cr(VI).
Added Cr(VI), μg L−1 ⁎Found Cr(VI), μg L−1 aREs% aRSDs% Recovery%
10 9.90 ± 0.35 −1.0 3.5 99.0
25 24.85 ± 0.32 −0.6 1.3 99.4
75 74.65 ± 0.30 −0.5 0.4 99.5
150 149.70 ± 0.26 −0.2 0.2 99.8
⁎ The average values plus their standard deviation of five replicate measurements at 95% confidence level.
a The relative errors and relative standard deviations of five replicate measurements of each concentration level by means of the kinetic method.

3.4

3.4 Interference studies

In order to determine the selectivity of the method, the effect of different anionic and cationic interfering species to the catalytic reaction rate was studied by using a fixed chromium concentration of 5 μg L−1 (Table 2). Results showed that when determining 5 μg L−1 of chromium by the method in a final volume of 10 mL under optimized conditions and the relative error is below ± 5%, the following concurrent ions had no interference on kinetic measurements. The allowable amounts of Cr3+, Fe3+ and Al3+ ions as interfering species were lower than the other investigated interfering species. Special attention was paid to the interference of Cr(III) on the Cr(VI) kinetic signal. On the basis of literature data an increase in the temperature of the kinetic measurements led to an improvement of the selectivity against Cr(III) and obtain reproducible and stable analytical signals so interfering removal studies for Cr(III) were carried out at 40 °C prior to its detection. As reported elsewhere (He and Wang 1999), at this temperature the time for conversion of the active protonated complex Cr(III)–H2DTPA from the Cr(III) present in the sample to the most stable inert deprotonated form, Cr(III)–DTPA, was shortened from 30 to approximately 10 min as a result of accelerating stepwise complex formation with an increase in temperature. After removal of Cr3+ ions with masking agent, the effect of this interfering ion as well as Zn2+, Cu2+, Cd2+, Pb2+, Fe2+ and Fe3+ could be suppressed with the tolerance levels changing from 125 to 350 with increments of 5 °C in the range of 20–40 °C. Also, the interfering effect of Cr(III) including interfering cations showing catalytic effect in the indicator reaction could efficiently be eliminated by using a cation-exchange resin such as Amberlite IR-120 plus. The usage of a masking agent such as NH4F for Al3+ and Fe3+ ions interfering in the present method as well as using NH3 and (NH4)2CO3 solutions for suppressing the effect of foreign ions and improving the selectivity can be advised. However, the tolerance ratio for Fe3+ ion could be increased 35 times when 2 mL of 0.25 M NH4F solution is also added when necessary in addition to removal of the interference of Fe3+ ions as the insoluble compounds of iron hydroxide with aqueous NH3 solution. The major interferents were some strong oxidants such as BrO 3 - , IO 3 - , MnO 4 - and NO 2 - , which oxidize the indicator to degradation products. The interfering effects of NO 2 - , W(VI) and Mo(VI) were successfully removed in the presence of 2 mL of 0.01 M urea or 2 mL of 0.012 M citric acid, respectively. The interfering effect of MnO 4 - could be removed by dropwise addition of dilute sodium nitrite and urea solution into the sample solutions (Liu et al., 1999). By drop decolorizing the pink solution, Mn2+ and NO 3 - are produced, which could be tolerated up to very high concentrations. The excess amount of NO 2 - was finally decomposed by urea:

(1)
MnO 4 - + 5 NO 2 - + 6 H + → 5 Mn 2 + + 5 NO 3 - + 3 H 2 O
(2)
2 NO 2 - + 2 H + + CO ( NH 2 ) 2 → CO 2 + 3 H 2 O + 2 N 2
Table 2 Tolerance levels of interfering ions in the determination of 5 μg L−1 Cr(VI).a
Foreign species Tolerance ratio, [Cinterfering ion/CCr(VI)]
Bicarbonate, acetate, citrate, oxalate, tartrate, salicylic acid, sulphosalicylic acid, borate, NO 3 - , F - , Cl - , SO 4 2 - , S 2 O 3 2 - , Na + , K + , NH 4 + ,Ca ( II ) ,Mg ( II ) and Sr(II) 750–1500
EDTA, CyDTA, Ba(II), Al(III), Zn(II)b, Fe(III)b, As(III), As(V) and Sn(IV) 350–750
I - , Br - , SCN - , HSO 3 - ,Be ( II ) ,Sr ( II ) and Zn(II) 250–400
NO 2 - c, S 2 O 3 2 - , Ag + , Hg 2 2 + ,Ba ( II ) ,Cd ( II ) b, Cu(II)b, Hg(II), Ni(II), Pb(II)b, La(III) and Zr(IV) 125–350
Co(II)b, Mn(II)b, Pd(II), V(IV) and W(VI) 50–125
Ag+, Fe(II), Fe(III), V(V), Mo(VI) c and W(VI)c 15–45
NO 2 - ,Sn ( II ) ,W ( VI ) ,Mo ( VI ) and Mn(VII) 1–10
a Reaction conditions are as given in the proposed kinetic procedure without masking in the presence of masking mixture containing DTPA, TEA and CaCl2.
b After removal of especially Cr(III) ion as well as Cu(II), Zn(II), Cd(II), Pb(II), Co(II), Mn(II), Fe(II) and Fe(III) in the presence of masking mixture containing DTPA, TEA and CaCl2.
c After addition of 2 mL of 0.01 M urea solution or 0.012 M citric acid solution into the reaction media.

Additionally, the interfering effect of Br− and SCN− ions could also be controlled and improved in the range of 250–400 by precipitating as insoluble silver salts and removing from reaction media via filtration.

3.5

3.5 Analytical applications

3.5.1

3.5.1 Recovery of Cr(VI) and Cr(III) in binary mixtures

The present method was applied to the analysis of binary mixtures containing Cr(VI) and Cr(III) ions at different molar ratios in order to check the accuracy of the method under the selected experimental conditions. Table 3 shows the analysis results obtained for each molar concentration ratio. The recoveries for both species, Cr(VI) and Cr(III) were in the range of 99.0–100.6%, indicating that the accuracy can be acceptable in all situations considered.

Table 3 Recovery studies for speciative determination of total Cr, Cr(VI) and Cr(III) in binary mixtures.
Cr(III)/Cr(VI) ratio Cr(III) Cr(VI)
Added, μg L−1 Found, μg L−1 ⁎Total Cr, μg L−1 Recovery% Added, μg L−1 ⁎Found, μg L−1 Recovery%
0.25 20 19.8 100.3 ± 3.6 99.0 80 80.5 ± 3.0 100.6
0.50 30 29.8 89.3 ± 3.4 99.3 60 59.5 ± 2.3 99.2
0.50 20 19.8 59.5 ± 2.3 99.0 40 39.7 ± 1.3 99.3
2.00 40 39.7 59.5 ± 2.4 99.3 20 19.8 ± 0.6 99.0
2.00 80 80.5 120.2 ± 3.8 100.6 40 39.7 ± 1.3 99.3
2.00 60 60.3 90.0 ± 3.4 100.5 30 29.7 ± 1.0 99.0
3.00 60 59.8 79.6 ± 3.0 99.7 20 19.8 ± 0.7 99.0
4.00 80 79.8 99.6 ± 3.51 99.8 20 19.8 ± 0.7 99.0
⁎ The average values plus their standard deviation of six replicate measurements at 95% confidence level.

3.5.2

3.5.2 The applicability of the method to the analyses of environmental water samples and certified water samples

Industrial effluent, tap, drinking, lake, hot- and cold-spring water samples used for the development of the method were collected in polytetrafluoroethylene containers, filtered using a 0.45 μm pore size membrane filter to remove suspended particulate matter, and stored in the dark at 4 °C. Tap and drinking water samples were obtained from the analytical research laboratory. Hot- and cold-spring water samples were collected from the pools used for health purposes, which are 10 and 35 km away in the center of Sivas (Turkey), respectively. Lake water was taken from the surface of Hafik Lake, Sivas (Turkey). Industrial waste water sample was also taken from the sewage system of the industrial zone in Sivas (Turkey). The measurements also were done according to total and inorganic chromium contents of certified water samples. Validation of the method was performed using certified water samples such as ERM-CA010a, NIST-1643e and BCR 544, which are commercially marketed. The results obtained by analysis of certified water samples spiked and unspiked are given in Table 4.

Table 4 The analysis and recovery results for five replicate measurements of dissolved chromium species in certified water samples.
Certified water samples aThe experimental one sided t-value according to Student's t-test Certified value (μg L−1)a Added (μg L−1) Found⁎ (μg L−1) Recovery%
Cr(III) Cr(VI) Cr(III) Cr(VI) Total Crc Cr(VI)b Cr(III) Cr(III) Cr(VI)
ERM-CA010a hard drinking water–metals 0.298 48.00 – – – 47.60 ± 3.0 – 47.60 – –
10 57.70 ± 3.1 – 10.1 101 –
25 72.85 ± 3.3 – 25.25 101 –
50 97.85 ± 3.4 – 50.25 100.5 –
NIST-1643e Simulated fresh water–trace elements 1.26 20.40 – – – 19.95 ± 0.8 – 19.95 – –
10 29.90 ± 1.1 – 9.95 99.5 –
25 44.85 ± 1.5 – 24.90 99.6 –
50 70.30 ± 2.2 – 50.35 100.7 –
BCR 544 lyophilized water 0.559, 0.489 26.80 22.80 – – 50.30 ± 2.8 23.15 ± 1.6 27.15 – –
10 30 90.32 ± 3.1 53.20 ± 2.4 37.12 99.7 100.2
20 20 90.35 ± 3.2 43.05 ± 2.2 47.30 100.8 99.5
30 10 90.25 ± 3.1 33.15 ± 1.8 57.10 99.8 100.0
a The critical t-value at 95% confidence level and degrees of freedom of 4 is 2.78.
b The amount of Cr(VI) directly found by the present kinetic method before oxidation with H2O2 in alkaline media.
c The amount of total Cr found after oxidation with H2O2 in alkaline media.
⁎ The average values plus their standard deviation of five replicate measurements at 95% confidence level.

The agreement between the measured and certified values of the chromium species demonstrated that the method was quantitatively accurate for trace analysis of such species in the complex matrices. Any significant difference between the measured values and certified values was not statistically observed for five replicate measurements at 95% confidence level. The method was then applied to the speciative determination of Cr(III) and Cr(VI) in waste water and natural water samples including tap water, drinking water, lake water, hot- and cold-spring waters. In the treatment with the certified reference materials and natural water samples, at the first step, aliquots of the water samples (1 L) were initially treated with 5 mL of 65% (w/w) HNO3 and boiled for approximately 30 min to remove chromium binding organic substances such as humic and fulvic acid and convert the Fe(II) content to Fe(III). The pH of the residual was then adjusted to 9.0 with ammonia and warmed again for 30 min to precipitate Fe(III) and Cr(III) contents as Fe(OH)3 and Cr(OH)3 and remove by filtration. After filtration of the water samples and adjusting the volume to the initial value by distilled water, the Cr(VI) content was determined according to the proposed procedure. For determination of the total chromium content, 2 mL of 30% (w/w) H2O2 was added to the samples before adjusting the pH to 9.0. The process was then continued according to the above discussion. In such conditions, the total Cr contents were determined by using masking agents to remove the possible interfering ions when necessary. The Cr(III) content was obtained by subtracting Cr(VI) from the total Cr content. The results are shown in Table 5. As it can be seen in Table 5, the recoveries obtained for the spiked amounts are in the range of 99.0–101.6% for Cr(III) and 99.0–101.5% for Cr(VI). This observation confirms the analytical applicability of the method, and indicates that it is free from interference when applied to the analysis of natural water samples.

Table 5 Speciative determination of total Cr and dissolved inorganic chromium species (Cr(VI), Cr(III)) in some environmental water samples.
Water sample The amount of standard spiked into 100 mL of sample (μg L−1) Found by using kinetic method (μg L−1)⁎ Recovery%
Cr(III) Cr(VI) Total Cr Cr(III) Cr(VI) Cr(III) Cr(VI)
Tap water – – 22 ± 0.7 13.35 8.65 ± 0.4 – –
20 10 52.05 ± 2.5 33.45 18.60 ± 0.7 100.5 99.5
10 20 52.25 ± 2.6 23.45 28.80 ± 1.0 101.0 100.8
Drinking water – – 1.28 ± 0.07 – 1.25 ± 0.08a – –
20 5 26.50 ± 0.9 20.30 6.20 ± 0.3 101.5 99.0
10 10 21.3 ± 0.8 9.95 11.35 ± 0.5 99.5 101.0
5 20 26.34 ± 0.8 5.09 21.25 ± 0.6 101.8 100.0
Hot-spring water – – 15.2 ± 0.5 6.75 8.45 ± 0.3 – –
20 10 45.30 ± 1.2 26.75 18.55 ± 0.6 100.0 101.0
10 20 45.48 ± 1.3 16.43 28.85 ± 0.8 98.8 102.0
Cold-spring water – – 17.05 ± 0.6 5.70 11.35 ± 0.4 – –
15 25 57.25 ± 2.1 20.55 36.70 ± 1.2 99.0 101.4
25 15 57.35 ± 2.2 30.50 26.75 ± 0.8 99.2 102.7
Lake water – – 43.00 ± 1.2 17.35 25.65 ± 0.31 – –
10 30 83.55 ± 3.1 27.45 56.10 ± 0.28 101.0 101.5
30 10 83.25 ± 3.0 47.55 35.70 ± 0.30 100.7 100.5
Industrial effluent – – 15.85 ± 0.33 13.20 2.65 ± 0.12a – –
10 30 55.90 ± 2.0 27.45 32.72 ± 1.0 101.0 101.5
30 10 56.15 ± 2.0 43.45 12.70 ± 0.4 100.8 100.5
a The results found by means of spiked calibration curve method around detection limit (n: 10).
⁎ The average values plus their standard deviation of five replicate measurements at 95% confidence level.

4

4 Conclusions

An attempt was made in the present study to develop analytical applications of CB+. In this study, the determination of Cr(VI) with CB+ in the presence of bipyr and CPC at pH 6.50 enables the speciation of chromium without a prior to the separation step, since the reaction is selective and sensitive to Cr(VI) in the presence of Cr(III). The method was successfully validated using certified reference materials, CRMs and applied to the determination of chromium species in natural water samples. The results obtained from real samples were highly quantitative in terms of recoveries of spiked samples. In comparison with the high cost techniques requiring hard conditions and expertise in his/her field, it has advantages in aspects of (i) high selectivity (selective determination of Cr(VI) without the separation of Cr(III), (ii) a short reaction time (5 min), (iii) obtaining of accurate, stable and reproducible analytical signals in premicellar media, (ıv) a simple instrument (available in almost every analytical research laboratory) and simple procedures except for preoxidation and interference removal. The method proposed, which has been successfully applied to the simultaneous determination of dissolved Cr(III), Cr(VI) and total Cr in natural waters, has great value in environmental analysis and monitoring.

Acknowledgements

The authors are grateful to Cumhuriyet University Scientific Research Projects Council (CÜBAP) (with F–260 code) for financial support of the present work. Thanks are also due to the many scientists who generously shared their results before publication. The present study was also presented as a poster in the 5th National Analytical Chemistry Congress, June 21st–25th 2010, held in the Faculty of Pharmacy, Erzurum, Turkey.

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