Highly sensitive catalytic spectrophotometric determination of ruthenium

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Spectrochimica Acta Part A 69 (2008) 193–197

Highly sensitive catalytic spectrophotometric determination of ruthenium Radhey M. Naik a,∗ , Abhishek Srivastava a , Surendra Prasad b,∗∗ a

b

Department of Chemistry, Lucknow University, Lucknow 226007, UP, India Department of Chemistry, Faculty of Science and Technology, The University of the South Pacific, Suva, Fiji Received 12 February 2007; received in revised form 17 March 2007; accepted 19 March 2007

Abstract A new and highly sensitive catalytic kinetic method (CKM) for the determination of ruthenium(III) has been established based on its catalytic effect on the oxidation of l-phenylalanine (l-Pheala) by KMnO4 in highly alkaline medium. The reaction has been followed spectrophotometrically by measuring the decrease in the absorbance at 526 nm. The proposed CKM is based on the fixed time procedure under optimum reaction conditions. It relies on the linear relationship where the change in the absorbance (At ) versus added Ru(III) amounts in the range of 0.101–2.526 ng ml−1 is plotted. Under the optimum conditions, the sensitivity of the proposed method, i.e. the limit of detection corresponding to 5 min is 0.08 ng ml−1 , and decreases with increased time of analysis. The method is featured with good accuracy and reproducibility for ruthenium(III) determination. The ruthenium(III) has also been determined in presence of several interfering and non-interfering cations, anions and polyaminocarboxylates. No foreign ions interfered in the determination ruthenium(III) up to 20-fold higher concentration of foreign ions. In addition to standard solutions analysis, this method was successfully applied for the quantitative determination of ruthenium(III) in drinking water samples. The method is highly sensitive, selective and very stable. A review of recently published catalytic spectrophotometric methods for the determination of ruthenium(III) has also been presented for comparison. © 2007 Elsevier B.V. All rights reserved. Keywords: Kinetic determination; Catalytic determination; Spectrophotometric determination of ruthenium(III); Ruthenium(III) determination; Ruthenium(III); Catalytic spectrophotometry

1. Introduction Ruthenium is a member of platinum group metals. Resistance to chemical attack, highly catalytic activity and stable electrical properties decide the wide application of ruthenium [1]. Ruthenium and its alloys are of commercial importance as they have widespread application in jewellery. Its major uses are in electronics, electrical and electrochemical industries [1,2]. The chemiluminescent activities of the ruthenium complexes form the basis of their uses for the development of sensitive and selective methods for determination of a number of analytes [2]. Very recently ruthenium complexes have been used in the detection of proteins [3], chlorpheniramine [4] and development of electrochemiluminescence flow injection procedures for the determination of cysteine [5], thyroxine [6], the antibiotics chlo-

∗

Corresponding author. Corresponding author. Tel.: +679 3232416; fax: +679 3231512. E-mail addresses: naik [email protected] (R.M. Naik), prasad [email protected] (S. Prasad). ∗∗

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.03.030

ramphenicol [7], cefprozil [8] in commercial pharmaceuticals and free hydroxyproline and proline in serum [9]. Luminescent ruthenium complexes are also used in sequencing of amino acids residue in peptides or proteins [10,11]. In addition, ruthenium complexes have been investigated as anti-cancer drugs [2]. Above all, ruthenium and its complexes have long been recognized as an efficient catalyst for large number of reactions of commercial and environmental importance [12–14] and exhibit classical catalyst characteristics in acidic as well as in alkaline media [15]. Highly selective ruthenium catalyzed oxidations of organic compounds have been carried out using suitable oxidants [16,17]. Thus, the versatile use of ruthenium in different fields justifies special attention to develop low cost, selective, sensitive and precise methods for its determination at trace levels. The techniques for the determination of ruthenium includes spectrophotometry, atomic absorption spectrophotometry, spectrofluorimetry, voltammetry, high performance liquid chromatography, mass spectrometry, inductively coupled plasma (ICP) mass spectrometry, ICP-atomic emission spectrometry, X-ray fluorescence and neutron activation analysis and have been reviewed [2]. However, the phenomenon of catalysis

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Table 1 A comparison of the proposed method for the determination of ruthenium(III) with the published catalytic kinetic methods along with the indicator reaction system, dynamic range of detection (DRD), detection limit (χd ) and other general characteristics of the methods Indicator reactions

D.R.D. (χd ) ng ml−1

Methods, comments, conditions

Type of samples

Ref. no.

Thymol blue + potassium bromate

1–25

Synthetic samples

[18]

Safranin O + potassium metaperiodate

0.80–100.0 (0.25)

Synthetic water samples

[19]

Benzylamine + [Fe(CN)6 ]3−

10–121 (6.90)

Synthetic samples

[21]

Hematoxylin + H2 O2

5–120

Synthetic mixtures

[25]

Phenosafranine + NaIO4 Macrocylic nickel(II) complex + iodate Ce(IV) + As(III)

Nanogram range – 0.45–9.00 (0.08)

Synthetic water samples Ruthenium in solutions Synthetic samples

[26] [27] [28]

Methyl green + potassium bromate Ce(IV) + formic acid or ethanol Bromocresol green + potassium bromate

0.00–0.80 (0.006) – 0–8

Ores and metallurgical products Synthetic samples –

[29] [30] [31]

Malekite green + potassium bromate

7.6–100

Chlorinated slug

[32]

l-Phealanine + KMnO4

0.101–2.526 (0.08)

CKM, SPM λmax , 544 nm; temp., 35 ◦ C CKM, SPM λmax , 521 nm; temp., 35 ◦ C; time, 5 min CKM, SPM λmax , 420 nm; temp., 35 ◦ C; time, 5 min CKM, SPM λmax , 440 nm; temp., 35 ◦ C CKM, SPM λmax , 520 nm CKM, SPM CKM, SPM λmax , 625 nm; PC-ANN used CKM, SPM λmax , 625 nm CKS, SPM or titration in H2 SO4 CKM, SPM 2,2 -bipyridyl activator used CKM, SPM ascorbic acid used as quencher CKM, SPM λmax , 526 nm; temp., 45 ◦ C; time, 5 min

Tap water, synthetic mixtures

Present work

CKM, catalytic kinetic method; SPM, spectrophotometric monitoring; PC-ANN, principal component artificial neural network.

has widely been used for the development of kinetic methods for the determination of trace amounts of various elements and compounds [18–24]. Thus, the CKMs utilizing spectrophotometric monitoring (SPM) remain popular method for achieving trace level determination of ruthenium(III) [18–21]. A review of the published catalytic kinetic methods involving spectrophotometric monitoring (CKM, SPM) technique along with the indicator reaction system, dynamic range of detection (DRD) and other general characteristics of these methods are provided in Table 1 [18,19,21,25–32]. However, many of these methods [18,19,25–27,29,31,32] require the use of expensive as well as rarely available chemicals as substrates while some procedures use additional chemicals as activator [31] and quencher [32]. In continuation of our efforts to develop CKMs for ruthenium(III) determination [21], the present investigation reports a new, highly sensitive and selective method for the determination of Ru(III) based on its catalytic activity on the oxidation of l-Pheala by KMnO4 in highly alkaline medium. The method permits the determination of ruthenium(III) concentration down to 0.1 ng ml−1 with very good accuracy and reproducibility. 2. Experimental 2.1. Reagents Distilled de-ionized water was used throughout the preparation of all the solutions. All chemicals used were of analytical grade. Standard solution of potassium permanganate (Ranbaxy, India) was prepared by titrating it against oxalic acid and working solution of lower concentrations were prepared immediately before the start of the experiment. Stock solution of l-phenylalanine (Merck) was prepared by dissolving its desired amount in water containing few drops of dilute hydrochloric

acid. This was done due to the limited solubility of phenylalanine in water. Stock solution of ruthenium(III) chloride (100 ␮g ml−1 ) was prepared by dissolving 0.0128 g of ruthenium trichloride (Alfa) in 0.5 M hydrochloric acid and diluting it to 100 ml. The possibility of oxidation of ruthenium(III) in hydrochloric acid was checked by potassium iodide solution. NaCl was used to maintain the ionic strength (I) at 0.5 M while sodium hydroxide and perchloric acid were used to maintain the pH of the solutions at 10.50 ± 0.02 or any desired value. Standard BDH buffers were used for standardization of pH meter. 2.2. Apparatus The spectrophotometric determination of ruthenium was carried out on a Shimadzu double beam UV–vis spectrophotometer, model UV190 equipped with thermostated cell compartment. Elico LI-120 digital pH meter was used to maintain pH of all the working solutions. Standard BDH buffers were used to standardize the pH meter at regular intervals. The glassware used were cleaned scrupulously with detergent solution, rinsed with 10−2 M EDTA, soaked in dilute HNO3 (10%, v/v) and finally rinsed with distilled de-ionized water. Cuvettes of spectrophotometer were cleaned by dipping them in HNO3 (15%, v/v) for 15 min to remove traces of ruthenium adsorbed on the walls. 2.3. Recommended procedure Except the catalyst concentration, which was varied, a set of concentrations of the other reagents was judiciously chosen for analytical procedure from the detailed spectrophotometric kinetic study of the indicator reaction that was verified by us. The concentration of the reactants and other conditions were selected

R.M. Naik et al. / Spectrochimica Acta Part A 69 (2008) 193–197

under which the catalytic effect of ruthenium(III) showed maximum sensitivity All the working solutions were thermostated at 25.0 ± 0.1 ◦ C for 30 min to allow thermal equilibrium. The thermally equilibrated reactants solutions of desired concentration; [KMnO4 ] 1.0 × 10−4 M, [l-Pheala] 2.0 × 10−3 M, [NaOH] 1.25 × 10−2 M and [Ru(III)] 1–25 × 10−9 M were mixed in a 50 ml glass stoppered Erlenmeyer flask in a sequence NaOH, l-Pheala, Ru(III) and the oxidizing agent, KMnO4 , was added in last. The properly shaken reaction mixture was transferred to a 10 mm path length spectrophotometric cuvette kept in the temperature controlled cell compartment. The progress of the oxidation of l-Pheala by KMnO4 in highly alkaline medium was monitored using the “fixed time procedure” by measuring the change in absorbance at 526 nm. The data obtained were used to plot the calibration curves between change in absorbance after a fixed time (At ) versus ruthenium(III) concentration. The “fixed time procedure” as a measure of the initial rate was followed to avoid the complications, which may arise due to interference by the products or other reagents present in the system. 3. Results and discussion

195

(4)

(5) After resorting some valid approximation the rate of reaction consistent with the proposed mechanism is given by Eq. (6) Rate kK1 K2 [Pheala][Ru(III)][OH− ] − = kobs = 1 + K [OH− ] + K [Pheala] [MnO4 ] 1 2 + K1 K2 [OH− ][Pheala]

(6)

3.1. Mechanistic investigation of indicator reaction Potassium permanganate is the most commonly used oxidizing agent in synthetic as well as analytical chemistry. It oxidizes many organic and inorganic compounds in acidic, alkaline and neutral media [33,34]. A detail kinetic and mechanistic study of ruthenium(III) catalyzed oxidation of l-Pheala by heptavalent manganese has already been carried out [33] which is a necessary pre-requisite to develop a kinetic method [23,24]. However, preliminary experiments were performed to confirm the effects of reaction variables and to determine the suitable conditions of concentrations of the reactants and other reaction variables (cf. 3.2). The oxidation of l-Pheala by KMnO4 under pseudo first order condition {[l-Pheala]:[KMnO4 ] ≥10:1 and ionic strength, I = 0.5 M (NaCl)} is found to be first order with respect to [KMnO4 ] and [Ru(III)] while the fractional order less than unity in [l-Pheala] and [OH− ]. The mechanism of the oxidation of l-Pheala by KMnO4 is reproduced below [33].

(1)

(2)

(3)

In the catalytic reaction solution, the catalytic and uncatalytic reaction occurs simultaneously. Thus, the second term in Eq. (6) is indicative of rate due to uncatalyzed reaction. With the other variables held constant, Eq. (6) shows that the rate of the indicator reaction is proportional to the total concentration of ruthenium(III). 3.2. Optimization of reaction variables The reaction variables were optimized in order to maximize the sensitivity and precision of the proposed kinetic method. The effects of the concentrations of KMnO4 , l-Pheala and OH− and ionic strength on the reaction rate were studied, where each variable was changed in turn keeping all other constant. The effect of [KMnO4 ] was studied in the range of 0.5–5 × 10−5 M where the reaction rate was found to increase with increasing [MnO4 − ]. The order of the reaction with respect to [MnO4 − ] was calculated as one in concentration range studied. The influence of [l-Pheala] on the initial rate of the reaction was studied over a range of 1–10 × 10−3 M. The initial rate showed an increase in the reaction rate with increasing [lPheala] but the order of the reaction was less than unity. The effect of [NaOH] on the reaction rate was also tested in the range 0.01–0.1 M. The rate of reaction was found to increase with this variable up to 0.02 M above which it remains virtually constant. The effect of ionic strength was also studied by varying concentration of [NaCl] from 0.05 to 0.5 M. The initial rate of the reaction increased with increase in [NaCl]. In all the dependence studies [Ru(III)] was kept constant at 1 × 10−7 M. Under the conditions studied, the rate of the uncatalyzed reaction was negligible where the catalyzed reaction showed maximum sensitivity. Thus, the concentrations 1 × 10−4 M KMnO4 , 2 × 10−3 M l-Pheala and 1.25 × 10−2 M

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Table 2 Analytical figures of merit for the determination of ruthenium(III) at [KMnO4 ] = 1.0 × 10−4 M, [l-Pheala] = 2.0 × 10−3 M, [NaOH] = 1.25 × 10−2 M, I = 0.5 M (NaCl), pH 10.50 ± 0.02, temp. = 25.0 ± 0.1 ◦ C Regression equations

Linear range (M)

Detection limit (ng ml−1 )a

Correlation coefficient (r2 )

Average %error in linear rangeb

A5 = 2.2695[Ru(III)] ng ml−1 + 0.0674 A10 = 1.4439[Ru(III)] ng ml−1 + 0.1821 A15 = 1.2349[Ru(III)] ng ml−1 + 0.2323

1–25 × 10−9 M 1–25 × 10−9 M 1–25 × 10−9 M

0.08 0.057 0.04

0.9978 0.9925 0.9875

1.19 1.41 1.79

a b

n = 7. n = 3.

NaOH and ionic strength of 0.5 M (NaCl) were chosen as optimum.

elsewhere [21], and corresponding to 5 min was 0.08 ng ml−1 . The detection limit decreases with increased time of analysis (cf. Table 2).

3.3. Analytical figures of merit 3.4. Study of interferences, i.e. selectivity The fixed time method was applied to obtain calibration equations. The theoretical dependence of rate on [Ru(III)] {cf. Eq. (6)} was experimentally confirmed when the graphs were plotted between At (change in absorbance at t min) and Ru(III) concentrations. It was found to be linear in the range of 1–25 × 10−9 M (0.101–2.526 ng ml−1 ) [Ru(III)]. The linear regression equations (i.e. calibration equations) relating At (t = 5, 10 and 15 min) and ruthenium(III) concentrations, correlation coefficients of calibration (r2 ), and the figures of the analytical method are given in Table 2. The general precision and accuracy for the determination of ruthenium(III) was tested by spiking Ru(III) in distilled water and performing recovery experiments. The standard deviation and percentage errors corresponding to several ruthenium(III) determinations are given in Table 3. As shown in Table 3, the error in case of calibration equation corresponding to A5 is less than that of A10 and A15 . This is because A5 is a close measure of the initial rate than A10 or A15 . Thus, a fixed time of 5 min was selected for further study, giving a good compromise between high sensitivity and short analysis time. The results in Table 3 show excellent sensitivity as well as reproducibility of this method. The first calibration curve/equation (A5 ) is therefore recommended for determination of Ru(III) in trace concentrations. The statistical detection limit calculated under optimum experimental conditions, using expression reported

To study the selectivity of the proposed method, the effect of various ions and poly-aminocarboxylates on the determination of a fixed ruthenium(III) concentration 1.2 × 10−8 M (1.21 ng ml−1 ) was examined under the optimum conditions. The tolerance limit was defined as the concentration of the added ion causing not more than ±4% relative error. The results are summarized in Table 4, which clearly indicates that most common ions do not interfere with the catalytic determination of Ru(III) up to 83-fold higher concentration of foreign ions. Polyaminocarboxylates (IDA, HEDTA and EDTA) form strong complexes with Ru(III) and thus mask its catalytic activity to maximum extent when exceeding their tolerance limit of 20.83 times. 3.5. Analytical application of the proposed method In order to validate the analytical capability, i.e. to establish the reliability and applicability of the developed method, it was applied to the determination of Ru(III) in different drinking (tap) water samples. The Ru(III) from lower to higher range (cf. Table 3) was added to three drinking water samples. This was done because ruthenium in the drinking water was below the detection limit of the proposed method. The results shown in Table 5 indicate that the Ru(III) recoveries from the drinking

Table 3 Computation of errors for determination of Ru(III) in spiked water where [KMnO4 ] = 1.0 × 10−4 M, [l-Pheala] = 2.0 × 10−3 M, [NaOH] = 1.25 × 10−2 M, I = 0.5 M (NaCl), pH 10.50 ± 0.02, temp. = 25.0 ± 0.1 ◦ C Ru(III) taken (ng ml−1 )

A5

A10

[Ru(III)]a

[Ru(III)]a

(ng ml−1 )

0.101 0.202 0.503 0.806 1.011 1.516 2.526

found + S.D.

0.103 ± 0.02 0.206 ± 0.09 0.500 ± 0.21 0.793 ± 0.39 1.021 ± 0.11 1.501 ± 0.36 2.536 ± 0.52 Average

%Error −1.98 +1.98 −0.60 −1.62 +0.98 −1.00 +0.39

A15

(ng ml−1 ) 0.099 0.210 0.508 0.798 1.028 1.523 2.541

± ± ± ± ± ± ±

found + S.D.

0.05 0.10 0.29 0.46 0.18 0.63 0.82

1.19

The ±S.D. values represent %relative standard deviation of the mean for three determinations. a Mean of three determinations.

%Error

[Ru(III)]a found + S.D. (ng ml−1 )

−1.98 +3.96 +1.00 −0.99 +1.68 +0.46 +0.59

0.099 0.211 0.510 0.792 1.040 1.530 2.538

1.41

± ± ± ± ± ± ±

0.03 0.04 0.36 0.26 0.12 0.52 0.91

%Error −1.98 +4.45 +1.39 −1.74 +2.86 +0.92 +0.48 1.79

R.M. Naik et al. / Spectrochimica Acta Part A 69 (2008) 193–197 Table 4 Effect of various interfering and non-interfering cations and anions on determination of 1.2 × 10−8 M (1.21 ng ml−1 ) [Ru3+ ] using A5 calibration curve where [KMnO4 ] = 1.0 × 10−4 M, [l-Pheala] = 2.0 × 10–3 M, [NaOH] = 1.25 × 10−2 M, I = 0.5 M (NaCl), pH = 10.50 ± 0.02, Temp. = 25.0 ± 0.1 ◦ C Foreign ions

[Foreign ions] taken × 106 M

Tolerance levels ([interfering ion]/[Ru(III)])

Co2+ Sn2+ Mg2+ Hg2+ Ba2+ Ca2+ Sr2+ Br− NO3 − Cl− SO4 2− H2 PO4 − I− IDA HEDTA EDTA

1.25 2.00 1.25 1.25 2.00 1.25 1.25 1.25 1.00 1.00 1.25 1.25 1.00 0.25 0.25 0.25

104.16 166.66 104.16 104.16 166.66 104.16 104.16 104.16 83.33 83.33 104.16 104.16 83.33 20.83 20.83 20.83

Table 5 Application of the method for the determination of ruthenium(III) in drinking water using A5 regression equation under conditions given in Table 2 Samples (drinking water)

Composition of the drinking water (ng ml−1 )

Ruthenium founda (ng ml−1 )

Recovery (%)

Drinking water-1 Drinking water-2 Drinking water-3

Ru(III) added 0.202 Ru(III) added 1.011 Ru(III) added 2.021

0.210 1.025 2.096

103.96 101.38 103.71

a n = 3.

water are quantitative. Thus, ruthenium can be determined in the specified concentration range (cf. Table 3) in mixtures containing many metal ions in relatively high concentrations. The higher recovery in each case may be attributed to synergistic effect due to presence of other cations in drinking water, which is not uncommon.

4. Conclusion The new catalytic spectrophotometric method developed for the determination of trace ruthenium(III) in water samples uses readily available reagents, and is highly sensitive, selective, reproducible and inexpensive. A comparison of the proposed procedure with other developed CKM-SPM is given in Table 1 and it has highest sensitivity over all with low limit of detection. Also many of these methods require the use of expensive as well as rarely available chemicals as substrates while some procedures use additional chemicals as activator [31] and quencher [32]. The method is successfully applied to the analysis of ruthenium(III) in drinking water.

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