Spectrophotometric method for determination of vanadium and its application to industrial, environmental, biological and soil samples

Talanta 48 (1999) 1085 – 1094

Spectrophotometric method for determination of vanadium and its application to industrial, environmental, biological and soil samples M. Jamaluddin Ahmed *, Saera Banoo Laboratory of Analytical Chemistry, Department of Chemistry, Uni6ersity of Chittagong, Chittagong 4331, Bangladesh Received 28 May 1998; received in revised form 6 October 1998; accepted 7 October 1998

Abstract The very sensitive, fairly selective direct spectrophotometric method for the determination of trace amount of vanadium (V) with 1,5-diphenylcarbohydrazide (1,5-diphenylcarbazide) has been developed. 1,5-diphenylcarbohydrazide (DPCH) reacts in slightly acidic (0.0001 – 0.001 M H2SO4 or pH 4.0 – 5.5) 50% acetonic media with vanadium (V) to give a red–violet chelate which has an absorption maximum at 531 nm. The average molar absorption coefficient and Sandell’s sensitivity were found to be 4.23 × 104 l mol − 1 cm − 1 and 10 ng cm − 2 of Vv, respectively. Linear calibration graph were obtained for 0.1 – 30 mg ml − 1 of Vv: the stoichiometric composition of the chelate is 1:3 (V: DPCH). The reaction is instantaneous and absorbance remain stable for 48 h. The interference from over 50 cations, anions and complexing agents has been studied at 1 mg ml − 1 of Vv. The method was successfully used in the determination of vanadium in several standard reference materials (alloys and steels), environmental waters (potable and polluted), biological samples (human blood and urine), soil samples, solution containing both vanadium (V) and vanadium (IV) and complex synthetic mixtures. The method has high precision and accuracy (s = 9 0.01 for 0.5 mg ml − 1). © 1999 Elsevier Science B.V. All rights reserved. Keywords: Spectrophotometry; Vanadium determination; 1,5-diphenylcarbohydrazide; Alloy; Steel; Environmental; Biological samples; Soil samples

1. Introduction Vanadium poisoning is an industrial hazard [1]. Environmental scientists have declared vanadium as a potentially dangerous chemical pollutant that can play havoc with the productivity of plants, * Corresponding author. Fax: +880-31-610938/726310; email: [email protected].

crops and the entire agricultural system. High amounts of vanadium are said to be present in fossil fuels such as crude petroleum, fuel, oils, some coals, and lignite. Burning these fuels releases vanadium into the air that then settles on the soil. There are cases of vanadium poisoning, the symptoms of which are nervous depression, coughing, vomiting, diarrhoea, anaemia and increased risk of lung cancer, that are sometimes

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fatal [2]. Recently, vanadium has been noticed as the index element in urban environmental pollution, especially air pollution [3]. Laboratory and epidemiological evidence suggests that vanadium may also play a beneficial role in the prevention of heart-disease [4]. Shamberger [5] has pointed out that human heart-disease death rates are lower in countries where more vanadium occurs in the environment. Vanadium in environmental samples has been determined by NAA [6], ICPatomic emission spectrometry [7], AAS [8] and spectrophotometry [10 – 22]. The first two methods are disadvantageous in terms of cost and instruments used in routine analysis. AAS is often lacking in sensitivity and affected by matrix conditions of samples such as salinity. Catalytic solvent extractive methods are highly sensitive but are generally lacking in simplicity. Hence its accurate determination at trace levels using simple and rapid methods is of paramount importance. The aim of this study is to develop a simpler direct spectrophotometric method for the trace determination of vanadium. 1,5-Diphenylcarbohydrazide (DPCH) has been reported as a spectrophotometric reagent for chromium [9] and has previously been used for spectrophotometric determination of vanadium [10] but this method is solvent extractive, lengthy and time consuming and lack selectivity due to much interference. Pyridine and chloroform had been used as solvents for this extraction which can be classified as toxic and as environmental pollutants [3] and have been listed as carcinogens by the Environmental Protection Agency (EPA) [9a]. This paper reports its use in a very sensitive, highly specific non-extractive spectrophometric method for trace determination of vanadium. The method possesses distinct advantages over existing methods with respect to sensitivity [10–15], selectivity [10 – 15,18 – 22], range of determination [10–15], simplicity [10 – 13,17], speed [10,12,18], pH/acidity range [10,17 – 22], thermal stability [10–22], accuracy [10 – 15,22], precision [10–22] and ease of operation [10 – 22]. The method is based on the reaction of non-absorbent DPCH in slightly acidic solution (0.0001 – 0.001 M sulfuric acid or pH 4.0 – 5.5) with vanadium (V) to

Fig. 1. Effect of the acidity on the absorbance V(V) DPCHsystem.

produce a highly absorbent red–violet chelate product followed by direct measurement of the absorbance in aqueous solution. With suitable masking, the reaction can be made highly selective and the reagent blank solutions do not show any absorbance.

Fig. 2. Effect of reagent (DPCH:V(V) molar concentration ratio) on the absorbance of V(V)-DPCH system.

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Fig. 3. Calibration graphs: A, 1–10 mg ml − 1 of vanadium (V) and B, 10 – 30 mg ml − 1 of vanadium (V).

2. Experimental

2.1. Apparatus A Shimadzu (Kyoto, Japan) (model-160) double-beam UV/VIS spectrophotometer and Jenway (England, UK) (Model 3010) pH meter with a combination of electrodes were used for the measurements of absorbance and pH, respectively. A Shimadzu (Model 5000) atomic absorption spectrometer equipped with a microcomputer-controlled graphite furnace was used for comparison of the results.

2.2. Reagents All chemicals used were of analytical-reagent grade or the highest purity available. Doubly distilled de-ionized water and HPLCgrade acetone, which is non-absorbent under ultraviolet radiation, were used throughout.

2.2.1. DPCH solution, 4.12×10 − 4M Prepared by dissolving the requisite amount of DPCH (Merck Darmastadt, Germany) in a known volume of distilled acetone. More dilute solutions of the reagent were prepared as required. 2.2.2. Vanadium (V) standard solutions A 100-ml amount of stock solution (1 mg ml − 1) of pentavalent vanadium was prepared by dissolving 0.2269 mg of ammonium metavanadate (Merck) in doubly distilled de-ionized water containing 1–2 ml of nitric acid (1+ 1). More dilute standard solutions were prepared from this stock solution as and when required. 2.2.3. Vanadium (IV) stock solution A 100-ml amount of stock solution (1 mg ml − 1) was prepared by dissolving 390.7 mg of purified grade vanadyl sulfate (Fisher Scientific) in

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doubly distilled de-ionized water. The working standard of vanadium (IV) was prepared by appropriate dilution of this solution.

2.2.4. Other solutions Solutions of a large number of inorganic ions and complexing agents were prepared from their AnalaR grade or equivalent grade water soluble salts. In the case of insoluble substances, special dissolution methods were adopted [23]. All glassware was kept in nitric acid (1+ 1) for at least a day and then was rinsed with de-ionized water before use. Stock solutions and environmental water samples were kept in poly (propylene) bottles containing 1 ml of concentrated nitric acid.

3. Procedure To 0.1–1 ml of slightly acidic solutions containing 1–300 mg of vanadium (V) in a 10-ml calibrated flask was mixed with a 0.1 – 1.0 (preferably 0.5 ml) of 0.001 M H2SO4 (or pH 4.0 –5.5) followed by the addition of 20 – 100-fold molar excess of DPCH solution (preferably 1 ml of 4.12×10 − 4 M). After 1 min, 5 ml of acetone were added and the mixture was diluted to the mark with de-ionized water. The absorbance was measured at 531 nm against a corresponding

reagent blank. The vanadium content in an unkown samples was determined using a concurrently prepared calibration graph.

4. Results and discussion

4.1. Factors affecting the absorbance 4.1.1. Absorption spectra The absorption spectra of the vanadium (V)DPCH system in 0.001 M sulfuric acid medium was recorded using the spectrophotometer. The absorption spectra of the vanadium (V)-DPCH is a symmetric curve with maximum absorbance at 531 nm and average molar absorption of 4.23× 104 l mol − 1 cm − 1. DPCH did not show any absorbance. In all instances measurements were made at 531 nm against a reagent blank. The reaction mechanism of the present method is as reported earlier [24]. 4.1.2. Effect of sol6ent Of the various solvents (benzene, chloroform, carbon tetrachloride, nitrobenzene, isobutyl alcohol, n-butanol, isobutyl methyl ketone, ethanol, acetone and 1,4-dioxane) studied, acetone was found to be the best solvent for the system. No absorbance was observed in the organic phase with the exception of n-butanol. In 509 2% (v/v)

Table 1 Determination of vanadium in some synthetic mixtures Sample

Composition of mixture (mg ml−1)

A

V(V)

B

As in A+Zn(25)+Cd(25)+Ca(25)+tartrate

C

As in B+Mn2+(25)+NO− 3 (25)

D

As in C+CrV1 (25)+NH+ 4 (25)

E

As in D+Co2+(25)+K(25)

a b

Average of five analyses of each sample. The measure of precision is the SD.

Vanadium (V) (mg ml−1) Added

Founda

Recovery 9 SDb (%)

0.50 1.00 0.50 1.00 0.50 1.00 0.05 1.00 0.50 1.00

0.50 0.99 0.49 1.01 0.52 1.02 0.05 1.02 0.54 1.06

100 90.0 99 9 0.5 98 90.5 101 90.7 1049 1.0 102 90.8 100 90.0 102 9 0.6 108 91.5 106 91.3

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Table 2 Analysis of high-speed steel and alloys Certified reference material (composition)

BAS-CRM BCS-CRM BCS-CRM BAS-CRM a

64b High-speed steel (Cr,Mo,Vand Tc) 241/1 High-speed steel (Cr,V,W,Co, Mn, C, Si, Pand S) 220/1 High-speed steel (C,Si,S,P,Mn, Mo,V,Ni,Cr,Co, W and Cu) 10g High tensil (Cu,Sn,Zn,Pb,Ni,Fe,Al,Mn, and V)

Vanadium(%) Certified value

Founda SD

1.99 1.57 2.09 0.52

1.98 1.58 2.10 0.58

90.05 90.07 90.06 90.08

Average of five determinations.

acetonic medium, however, maximum absorbance was observed; hence a 50% acetonic solution was used in the determination procedure.

4.1.3. Effect of acidity Of the various acids (nitric, sulfuric, hydrochloric and phosphoric) studied, sulfuric acid was found to be the best acid for the system. The absorbance was at a maximum and constant when the 10 ml of solution (1 mg ml − 1) contained 0.1–1.0 ml of 0.001 M sulfuric acid (or pH 4.0– 5.5) at room temperature (2595°C). Outside this range of acidity, the absorbance decreased (Fig. 1). For all subsequent measurements 0.5 ml of 0.001 M sulfuric acid (or pH 4.75) was added. 4.1.4. Effect of time The reaction is very fast constant maximum absorbance was obtained just after the dilution to volume and remained strictly unaltered for 48 h. 4.1.5. Effect of reagent concentration Different molar excesses of DPCH were added to fixed metal ion concentration and absorbance were measured according to the standard procedure. It was observed that at 1 mg ml − 1 V-chelate metal, the reagent molar ratio 1:20 and 1:100 produce a constant absorbance of the V-chelate (Fig. 2). Greater excesses of reagent were not studied. 4.1.6. Calibration graph (Beer’s law and sensiti6ity) The effect of metal concentration was studied over 0.1–30 mg ml − 1 distributed in three different sets (0.1–1.0, 1 – 10, and 10 – 30 mg ml − 1) for convenience of measurement. The absorbance was

linear for 0.1–30 mg ml − 1 of vanadium at 531 nm (Fig. 3). The molar absorption coefficient and the Sandell’s sensitivity [25] were 4.23× 104 l mol − 1 cm − 1 and 10 ng cm − 2 of Vv, respectively.

4.1.7. Effect of foreign ions The effect of over 50 ions and complexing agents on the determination of only 1 mg ml − 1 of Vv was studied. The criterion for an interference [26] was an absorbance value varying by more than 5% from the expected value for vanadium alone. There was no interference from the following: 1000-fold amount of sulfate, sulfite, nitrate, perchloride, bromide, chloride, iodide, thiocyanide, Na, Mg, Ba,K, MnII, Zn, NH4+ , Ca or Cd; 100-fold amounts of tartrate, fluoride, CoIII, NiII or HgII; 10-fold amounts of EDTA, oxalate, citrate, CrIII, PbII azide, persulphate, phosphate, WVI, AsIII, AsV, Cs, MnVII,CrVI, FeII, FeIII, CN − , or UVI. EDTA prevented the interference of a 10fold amounts of Ag, Al, CuII or MoVI. The quantities of these diverse ions mentioned were the actual amounts added and not the tolerance limits. A 50-fold excess of iron FeII and FeIII or CuII could be masked with ammonium thiocyanate or fluoride. During the interference studies, if a precipitate was formed, it was removed by centrifugation.

4.1.8. Composition of the absorbance Job’s method [27] of continuous variation and the molar-ratio [28] method were applied to ascertain the stoichiometric composition of the complex. A V-DPCH (1:3) complex was indecated by both methods.

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4.1.9. Precision and accuracy The relative SD (n =5) was 1.5 – 0.0% for 1– 300 mg of vanadium in 10.0 ml, indicating that

this method is highly precise and reproducible. The detection limit (3 SD of the blank) and Sandell’s sensitivity (concentration for 0.001 ab-

Table 3 Determination of vanadium in some environmental water samples Sample

Vanadium, mg l−1 Added Founda

Tap water

0 100 500 0 100 500 0 100 500

1.6 101.05 502.0 8.0 107.0 509.0 1.4 102.0 501.5

0 100 500 0 100 500

11.2 112.0 511.0 14.4 115.0 513.0

0 100 500 0 100 500

5.0 104.0 505.0 6.0 106.0 507.0

0 100 500 0 100 500

18.5 119.0 520.0 20.0 119.0 5021.0

Well water

Rain water

River water Karnaphuli (upper)

Karnaphuli (lower)

Sea-water Bay of Bengal (upper)

Bay of Bengal (lower)

Lake water Kaptai (upper)

Kaptai (lower)

Drain water Karnaphuli Paper Millc

Steel Milld

Eastern refinerye

a

0 100 500 0 100 500 0 100 500

Average of five replicate determinations. The measure precision is the relative SD. c Karnaphuli Paper Mill, Chandraghona, Chittagong d Chittagong Steel Mill, Patenga, Chittagong. e Eastern Refinery, North Patenga, Chittagong. b

35.00 136.0 536.0 75.0 176.0 580.0 145.0 250.0 640.0

Recovery 9 SD (%)

SDr (%)b

90.1 99.9 9 0.2 100 9 0.1

0.31 0.35 0.42

99 90.3 100.2 90.4

0.19 0.39

100.5 9 0.1 100 9 0.0

0.14 0.00

100.7 9 0.3 100 9 0.0

0.18 0.00

100.4 90.2 99.7 90.4

0.3 0.37

99 90.01 100 9 0.0

0.08 0.00

100 90.0 100.2 90.05

0.00 0.08

100.4 90.4 100.3 90.5

0.29 0.34

99 9 0.3 100.2 90.2

0.41 0.32

100.7 90.5 102.0 90.6

0.45 0.35

100.6 9 0.3 100.9 9 0.5

0.08 0.15

102.0 9 0.6 99.2 90.5

0.49 0.55

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Table 4 Concentration of vanadium in blood and urine samples Serial No.

1

Sample

Blood Urine Blood Urine Blood Urine

2 3

a b

Vanadium, mg l−1

Sample sourceb

AAS

Proposed methoda

9.0 2.5 370.0 75.0 10.0 3.0

10.0 91.5 2.8 91.2 381.0 91.0 85.0 9 1.5 12.0 9 1.4 3.2 9 1.3

Heart-diseases Patient (male) Lung cancer Patient (male) Normal Adult (male)

Average of five determinations9SD. Samples were from Dhaka Medical College Hospital.

sorbance unit) for vanadium (V) were found to be 20 ng ml − 1 and 10ng cm − 2, respectively. The result for total vanadium were in good agreement with certified values (Table 2). The reliability of our V-chelate procedure was tested by recovery studies. The average percentage recovery obtained for addition of a vanadium (V) spike to some environmental water samples was quantitative as shown in Table 3. The method was also tested by analysing several synthetic mixtures containing vanadium (V) and diverse ions. The results of biological analyses by the spectrophotometric method were excellent agreement with those obtained by AAS (Table 4). The precision and accuracy of the method were excellent.

till the brisk reaction subsided. The solution was heated and simmered gently after addition of 5 ml of concentrated HNO3 untill all carbides were decomposed. Then 2 ml of 1:1 (v/v) H2SO4 was added and solution was evaporated carefully to dense white fumes to drive off the oxides of nitrogen and then cooled to room temperature (25–30°C). After suitable dilution with de-ionized

Table 5 Determination of vanadium in some surface soil samplesa,b Sl. No.

Vanadium (mg g−1)

Sample source

S1c

0.0295

4.2.1. Determination of 6anadium in synthetic mixtures Several synthetic mixtures of varying compositions containing vanadium (V) and divers ions of known concentrations were determined by the present method using sodium tartrate as masking agent. The results are shown in Table 1.

S2 S3

0.0401 0.0215

S4 S5

0.0265 0.0198

S6 S7

0.0310 0.0295

4.2.2. Determination of 6anadium in alloys and steels A 0.1 g amount of an alloy or steel samples containing 0.52 – 2.09% of vanadium was weighed accurately and placed in a 50 ml Erlenmeyer flask. To it, 10 ml of 20% (v/v) sulfuric acid was added, carefully covering with a watch-glass un-

S8

0.0229

S9 S10

0.0225 0.0570

Clevedon Tea Estate (Sylhet) Esturine soil (Karnaphuli) Chittagong University Campus Karnaphuli Paper Mill Marine Soil (Bay of Bangle) Steel Mill Clevedon Tea Estate (Sylhet) Bangladesh Oxygen Company (BOC) T.S.P. Complex Eastern Refinery

4.2. Applications

a

Mean = 0.03 (mg g−1) SD= 90.01 c Composition of the soil samples: C, N, P, K, Na, Ca, Mg, Fe, NO3, NO2, Zn, SO4, Mn, Mo, Co etc. b

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Table 6 Determination of vanadium (IV) and vanadium (V) speciation in mixtures V, taken (mg ml−1)

V,found (mg ml−1)

Error (mg ml−1)

V(V)

V(IV)

V(V)

V(IV)

V(V)

V(IV)

1 1:1 1.00 2 1:1 1.00 3 1:1 1.00 Mean error: V(V) = 90.0067; V(IV) = 9 0.013 SD: V(V) = 9 0.0058; V(IV) = 90.011

1.00 1.00 1.00

0.99 1.00 0.99

0.98 1.00 0.98

0.01 0.00 0.01

0.02 0.00 0.02

1 1:5 1.00 2 1:5 1.00 3 1:5 1.00 Mean error: V(V) = 9 0.013; V(IV) = 9 0.016 SD: V(V) = 9 0.0058; V(IV) = 9 0.0058

5.00 5.00 5.00

0.99 0.98 0.99

4.98 4.98 4.99

0.01 0.02 0.01

0.02 0.02 0.01

1 1:10 1.00 2 1:10 1.00 3 1:10 1.00 Mean error: V(V) = 90.016; V(IV) = 9 0.016 SD: V(V) = 9 0.0058; V(IV) = 9 0.0058

10.00 10.00 10.00

0.98 0.99 0.98

10.99 10.98 10.98

0.02 0.01 0.02

0.01 0.02 0.02

Sl. No.

V (V):V(IV)

water, the contents of the Erlenmeyer flask were warmed to dissolve the soluble salts. The solution was then cooled and neutralized with dilute NH4OH in the presence of 1 – 2 ml 0.01% (w/v) tartrate solution. The resulting solution was filtered, if necessary, through a Whatman No. 40 filter paper into a 50-ml calibrated flask. The residue (silica and tungstic acid) was washed with a small volume of hot (1 +99) H2SO4 followed by water and the volume was made up to the mark with de-ionized water. A suitable aliquot of the above solution was taken into a 10-ml calibrated flask and the vanadium content was determined as described under procedure using thiocyanide or fluoride as masking agent. The results are shown in Table 2.

The resulting solution was then quantitatively transferred into a 25-ml calibrated flask and made upto the mark with de-ionized water. An aliquot 1 ml of this preconcentrated water sample was pipetted into a 10-ml calibrated flask and the vanadium content was determined as described under procedure using thiocyanide or fluoride as a masking agent. The results are shown in Table 3. Most spectrophotometric methods for the determination of vanadium in natural and sea-water require preconcentration of vanadium [29]. The concentration of vanadium in nutral and sea-water is a few ng ml − 1 in Japan [17]. The mean concentration of vanadium found in US drinking waters is 6 ng ml − 1 [29].

4.2.3. Determination of 6anadium in en6ironmental water samples Each filtered environmental water sample (1000 ml) was evaporated nearly to dryness with a mixture of 1 ml of concentrated H2SO4 and 5 ml of concentration HNO3 in a fume cupboard and was then heated with 10 ml of de-ionized water in order to dissolve the salts. The solution was then cooled and neutralized with dilute NH4OH in the presence of 1 – 2 ml of 0.01% w/v tartrate solution.

4.2.4. Determination of 6anadium in biological samples Human blood (20–50 ml) or urine (30–50 ml) was taken into a 100-ml micro-Kjeldahl flask. A glass bead and 5 ml of concentrated nitric acid were added and the flask was placed on the digester under gentle heating. When the initial brisk reaction was over, the solution was removed and cooled. Sulfuric acid (1 ml of concentrated) was added carefully followed by the addition of 1

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ml of 70% perchloric acid and heating was continued to dense white fumes, repeating nitric acid addition if necessary. Heating was continued for at least 1/2 h. and then cooled. The content of the flask was filtered and neutralized with dilute NH4OH in presence of 1 – 2 ml of 0.01% (w/v) tartrate solution, transferred quantitatively into a 10-ml calibrated flask and made upto the mark with de-ionized water. A suitable aliquot of this preconcentrated solution was pipetted out into a 10-ml calibrated flask and the vanadium content was determined as described under procedure using thiocyanide or fluoride as masking agent. The results of biological analyses by the spectrophotometric method were found to be in excellent agreement with those obtained by AAS. The results are shown in Table 4. The abnormally high value for the lung cancer patient is probably due to the involvement of high vanadium concentrations with As and Zn. Occurrence of such high vanadium contents are also reported in cancer patients from some developed countries [2]. The low value for the heart-disease patient is probably due to a low vanadium concentration in the environment. There is an inverse correlation between human heart-disease and vanadium concentration in the environment [29].

4.2.5. Determination of 6anadium in soil samples An air-dried homogenized soil sample (100 g) was weighed accurately and placed in a 100-ml Kjeldahl flask. The sample was digested in the presence of an oxidizing agent following the method recommended by Jackson [30]. The content of flask was filtered through a Whatman No. 40 filter paper into a 25-ml calibrated flask and neutratized with dilute ammonia in the presence of 1–2 ml of 0.01% (w/v) tartrate solution. It was then diluted up to the mark with de-ionized water. Suitable aliquots 1 – 2 ml was transferred into a 10-ml calibrated flask and a calculated amount of 0.001 M sulfuric acid needed to give a final acidity of 0.0001–0.001 M H2SO4 (or pH 4.0 –5.5) was added followed by 1 – 2 ml of 0.01% (w/v) thiocyanide or fluoride solution as masking agent. Vanadium was then determined by the above procedure and quantified from calibration graph

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prepared concurrently. The results are shown in Table 5.

4.2.6. Determination of 6anadium (IV) and 6anadium (V) speciation in mixtures Suitable aliquots (1–2 ml) of vanadium (IV+ V) mixtures (preferably 1:1, 1:5, 1:10) were taken in a 25 ml conical flask. A few drops of 0.001 M sulfuric acid and 1–3 ml of 1% (w/v) potassium permanganate solution was added to oxidize the tetravalent vanadium. 5 ml of water was added to the mixtures and heated on the steam bath for 10–15 min. with occasional gentle shaking and then cooled to room temperature. Then 3–4 drops of freshly prepared sodium azide solution (2.5% w/v) was added and heated gently with further addition of 2–3 ml of water, if necessary, for 5 min. to drive off the azide cooled to room temperature. The reaction mixture was transferred quantitatively into a 10-ml volumetric flask, 1 ml of 4.12 × 10 − 4M DPCH reagent solution was added followed by addition of 0.5 ml of 0.001 M H2SO4, it was made up to the mark with de-ionized water. The absorbance was measured after 1 min. at 531 nm against a reagent blank. The total vanadium content was calculated with help of the calibration graph. An equal aliquot of the above vanadium (IV+ V) mixture was taken in a 25 ml beaker, 1 ml of 0.01% (w/v) tartrate was added to mask vanadium (IV) and neutralize with dilute NH4OH. The content of the beaker was transferred in to a 10-ml volumetric flask, then 0.5 ml of 0.001 M sulfuric acid solution was added followed by addition of 1 ml of 4.12× 10 − 4 M DPCH and made up to a volume with de-ionized water. After 1 min the absorbance was measured against a reagent blank, as before. The vanadium concentration was calculated in mg l − 1 or mg ml − 1 with the aid of a calibration graph. This gives a measure of vanadium (V) originally present in the mixture. This value was substracted from that of the total vanadium to get vanadium (IV) present in the mixture. The results were found to be highly reproducible. Occurrence of such reproducible results are also reported for different oxidation states of vanadium [31]. The results of a set of determination are given in Table 6.

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5. Conclusion The proposed method using DPCH not only is one of the most sensitive methods for the determination of vanadium but also is excellent in terms of selectivity and simplicity. Therefore this method will be successfully applied to the monitoring of small amounts of vanadium in environmental, biological and soil samples. No extraction step is required and hence the use of organic solvents, which are generally toxic pollutants, is avoided.

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