Determination of V(V) by a kinetic stopped-flow FIA method with spectrophotometric detection

Analytica Chimica Acta 366 (1998) 287±293

Determination of V(V) by a kinetic stopped-¯ow FIA method with spectrophotometric detection M.E. Palomeque, A.G. Lista, B.S. FernaÂndez Band* Department of Chemistry and Chemical Engineering, FIA Laboratory, Universidad Nacional del Sur, 8000-BahõÂa Blanca, Argentina Received 22 September 1997; received in revised form 13 January 1998; accepted 19 January 1998

Abstract In the present paper, two automatised methods to determine V(V) in water samples are described. A continuous ¯ow system based on the classical reaction for the determination of vanadium(V) with 3,30 dimethylnaphtidine in acid medium, by using a stopped-¯ow technique and spectrophotometric detection was developed (Method A). It was possible to increase the kinetic of the reaction obtaining a higher sample throughput (34 hÿ1), lower detection limits (0.2 mg lÿ1) and a high sensitivity. In order to determine still a lower concentration levels of V(V), a preconcentration step has been introduced in the above system (Method B). This manifold has a lower detection limits (0.032 mg mlÿ1) and a high sensitivity (4 times greater than Method A). Both methods show a good reproducibility (RSD: Method Aˆ1.9% and Method Bˆ2.3%). # 1998 Elsevier Science B.V. Keywords: Vanadium(V); Stopped-¯ow; Preconcentration; Spectrophotometric; FIA

1. Introduction The biological effects of vanadium(V) involve normalisation of sugar levels, participation in various enzyme systems as an inhibitor and a cofactor [1] and catalysis of the oxidation of various amines [2]. It has been proved that the vanadium in approximately 0.5 mg lÿ1 inhibits cholesterol synthesis and increases the oxidation of fatty acids of liver phospholipids [2]. Vanadium in trace amounts is an essential element for cell growth but it can be toxic at higher concentrations and its toxicity depends on its oxidation

*Corresponding author. Fax: 54-91-551-447; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00116-0

state, being vanadium(V) more toxic than the other forms [3]. Therefore, its accurate determination at trace levels using simple and rapid methods is of a paramount importance. Spectrophotometry is a common technique used for vanadium determination owing to it's simple technique and it has a good sensitivity [4±10]. The classic reaction for the determination of vanadium(V) with 3,30 dimethylnaphtidine (3,30 DMN) [11] by using the batch method needs 30 min to attain the chemical equilibrium before doing the spectrophotometric detection. The main purpose of this work was to developed an alternative procedure for V(V) determination using a stopped-¯ow injection system in order to obtain a

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higher sample throughput, lower detection limit and a high sensitivity (Method A) by using the classic above mentioned reaction. The spectrophotometric signal was detected at 544 nm. On the other hand, with the purpose of determining still lower concentration levels of vanadium, a preconcentration step has been introduced on the above FIA system (Method B). 2. Experimental 2.1. Apparatus 2.1.1. Method A Spectrophotometric determination was performed on UV-V Perkin Elmer Lambda 2S coupled with a FIAS 300 Perkin Elmer, with a Hellma 178-010-QS ¯ow cell with an inner volume of 18 ml. All the reaction coils were made of PTFE tubing (i.d. 0.5 mm). A Julabo U3 thermostat was used. The determinations related to the reference method were carried out in a SHIMADZU 1000III emission spectrophotometer with ICP. 2.1.2. Method B A Hewlett Packard 8452A diode array spectrophotometer, with a Hellma 178-010-QS ¯ow cell with an inner volume of 18 ml was used. The propulsion system consisted of two Gilson Minipuls 3 peristaltic pumps. A Rheodyne 5041 and Rheodyne 5011 injection valves were used. All the reaction coils were made of PTFE tubing (i.d. 0.5 mm). SBSR reactor was made with the same tubing (i.d. 0.8 mm). A Julabo U3 thermostat was used.

The mini-column consisted of a piece of glass tube (5.5 cm3.5 mm i.d.) ®lled with Chelex 100 resin (chelating resin iminodiacetic acid, 50±100 dry mesh, sodium form). 2.2. Reagent and solutions Analytical grade reagents were always used as well as distilled, deionized water. A 99 mg mlÿ1 vanadium stock solution was prepared by dissolving 0.3771 g lÿ1 of Na VO3 (Merck) in water. The 3,30 DMN (Merck) solution, 410ÿ4 M, was prepared daily by dissolving weighed portion in 10 ml of acetic acid and then making up the volume to 50 ml with water. 2.2.1. Method A The vanadium calibration solutions containing 0.5 M of phosphoric acid, were prepared by appropriate dilution of stock solution. 2.2.2. Method B The vanadium calibration solutions were prepared by dilution in water of the stock solution. 3. Procedure 3.1. Method A A developed double channel FIA manifold is presented in Fig. 1. The vanadium sample is injected into a H3PO4 carrier, merging with a reagent (3,30 DMN) stream at con¯uence ``a''. The chromogenic reaction started inside the reactor (R) when the ¯ow is stopped and the plug reaction inside the reactor is

Fig. 1. FIA manifold for the Method `A'. P: peristaltic pump; IV: injection valve; D: spectrophotometer; W: waste.

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Fig. 2. FIA manifold for the Method `B'. P: peristaltic pump; PC: packed column; D: spectrophotometer; W: waste.

kept at 600.18C during 85 s, in order to increase the extent of the reaction. After this time, the ¯ow is restored and the improved signal is detected at 544 nm. 3.2. Method B The ¯ow injection system with internally coupled valves incorporating a Chelex 100 resin mini-column instead of the loop in (V1) valve is shown in Fig. 2. The resin inside the column is regenerated by passing HNO3 ¯ow through it during 60 s, then passing a stream of water before loading the sample (Fig. 3). Sample is loaded during 2 min, after this time V1 is switched to allow a stream of water to wash the column during 1 min. Then the (V2) valve loaded with a ®xed volume of eluent (NH4OH) is switched and the eluent passes through the column washing out the vanadium ions. The sample plug is carried out by the eluent merging and mixing with H3PO4 stream inside a SBSR reactor (R1) in order to obtain a suitable acid medium, then it merges with a 3,30 DMN stream and inside the reactor (R2) at that moment the ¯ow is stopped. The reaction plug was processed similarly on Method A mentioned above.

Fig. 3. Resin regeneration system and sample loading.

4. Results and discussion 4.1. Optimisation of chemical and FIA variables The variables in¯uencing the performance of the methods were optimised by the univariant method. 4.1.1. Method A Stopped-¯ow injection system was ®rst developed in order to improve the sensitivity, to decrease the detection limit and to obtain a higher sample throughput as regards to the classic method [11]. As the reaction between 3,30 DMN and vanadium is produced in acidic medium, different acids were studied (H2SO4, H3PO4 and HCl), phosphoric acid was found to be the best acid for the system. Then it was proved the acid concentration (0.01±0.7 mol lÿ1) and the optimum value was 0.5 mol lÿ1. Lower concentration of H3PO4 resulted in unstable signal and at higher concentration the signal was not improved. The reagent concentration was tested from 110ÿ4 to 610ÿ4 M and a 410ÿ4 M was selected. The system is dependent on temperature variations. In the tested temperature range (20±608C), the signal increased as the temperature increased, so the selected temperature was 608C. The range of the variables FIA studied and their optimum values are listed in Table 1. The calibration curve was linear over the range 0.2± 7.2 mg lÿ1 of V(V) (Aˆ0.003‡0.09 (V(V) mg lÿ1); r2ˆ0.999; LODˆ0.19 mg lÿ1 V) [12]. The relative standard deviation calculated for 11 samples contained 4.0 mg lÿ1 V(V) injected by triplicate was 1.9%. The sample throughput was 34 hÿ1.

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Table 1 Optimisation of FIA variables

Table 2 Optimisation of chemical and FIA variables

Variable

Studied range

Optimum value

Variable

Studied range

Optimum value

Reactor length (R) Reagent and carrier flow rates (q) Sample volume (Vs) Delay time Stopped-flow time

30±500 cm 0.7±2.6 ml minÿ1

180 cm 1.1 ml minÿ1

50±300 ml 11±18 s 0±190 s

100 ml 15 s 85 s

HNO3 conc. Eluent volume (V2) SBSR length (R1) Reactor length (R2) Reagent and carrier flow rate (q1) H3PO4 flow rate (q2) V(V) loading flow rate time Temperature Delay time Stopped-flow time Inner diameter of the column Length of the column

0.5±2 M 50±300 ml 3±15 cm 30±500 cm 0.7±2.6 ml minÿ1

1M 100 ml 8 cm 180 cm 1.3 ml minÿ1

0.7±2.6 ml minÿ1 1±4 min

1.9 ml minÿ1 2 min

20±608C 28±32 s 0±190 s 1.5±5.0 mm

608C 30 s 85 s 3.5 mm

3.0±8.0 cm

5.5 cm

4.1.2. Method B In order to reach a determination of lower concentration levels of V(V) than the above proposed Method A, a continuous ¯ow injection system with on-line chelex 100 packed column as a preconcentration reactor, has been developed (Fig. 2). According to the Riley and Taylor paper [13], vanadium was completely retained by the Chelex 100 resin at pHˆ5±6 and ammonia was an ef®cient eluting agent for it. The resin in its hydrogen form was packed inside a glass tube (3.5 mm i.d.) with a resin's length of 5.5 cm for the column. The reagent concentration was selected by the same way as Method A (410ÿ4 M). The eluent concentration was investigated over the range 10±30% v/v in water. The best signal was obtained when a concentration of 20% v/v was adopted and the return to the baseline was good. As the H3PO4 stream merges with the eluent stream (NH4OH), to obtain an optimum acid medium, we tested H3PO4 concentration (0.5±3 mol lÿ1) selecting 2 mol lÿ1. For a ®xed ¯ow rate and loop volume in (V2), the time of vanadium ¯owing through the column was controlled, as the time between the injection and the elution were also controlled. All the other variables tested are presented in Table 2. The calibration graph showed linear over the range 0.04±1.50 mg mlÿ1 V(V) (Aˆ0.101‡0.427 [V(V) mg mlÿ1]; r2ˆ0.999; LODˆ0.032 mg mlÿ1 V) [12].The relative standard deviation calculated for 10 samples containing 0.80 mg mlÿ1 V(V) injected by duplicate was 2.3%. The sample throughput was 12 hÿ1.

Table 3 Interfering ions tolerance limits Ions tested

Tolerance limit (mg/l) Method A

Tolerance limit (mg/l) Method B

Clÿ SOÿÿ 4 NOÿ 3 NOÿ 2 Fÿ ‡ K Na‡ Ca‡‡ Mn‡‡ Fe‡‡‡ Mg‡‡

>480 400 >10 1 160 >340 >320 >160 1.5 1.5 >100

>450 >600 20.5 5 90 >500 300 180 3 Interfere a 80

a

This interference is eliminated adding Fÿ.

4.2. Interferences The effect of foreign ions on the determination of 1.6 mg lÿ1 (Method A) and 0.8 mg lÿ1 (Method B) of vanadium was studied. The common ions in their normal concentration in water samples do not interfere (Table 3). The interference of Fe3‡ is eliminated by adding NaF 0.08 M to the water sample. 4.3. Determination of vanadium in natural waters In order to asses the quality of the results obtained with the developed ¯ow injection systems for the determination of vanadium in natural waters, several

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0.014 for the slope and ÿ0.0220.011 mg mlÿ1 for the intercept (ˆ0.05, 6 degrees of freedom) were obtained [14]. In both cases, the results indicated good agreement between the two methods.

samples were analysed by proposed procedures and by ICP±AES. The recovery values obtained from vanadium (V) added to natural water samples are shown in Table 4 (Method A) and Table 5 (Method B). From a linear regression between the obtained results with Method A and ICP±AES, con®dence limits: 1.020.01 for the slope and ÿ0.0340.036 mg lÿ1 for the intercept (ˆ0.05, 10 degrees of freedom) were obtained [14]. The obtained results by Method B were compared with ICP±AES method, the con®dence limits 1.034

5. Conclusions The developed ¯ow injection system with spectrophotometric detection proposed for V(V) determination in natural waters is an alternative to other

Table 4 Recovery from V(V) added to natural water samples Proposed method

Reference Method (ICP±AES)

Sample

Vanadium added (mg/ml)

V found (mg/ml)

Recovery (%)

V found (mg/ml)

Recovery (%)

1a 1 1 1

None 0.50 2.00 4.00

Ð 0.49 1.99 4.07

Ð 98.0 99.5 101.8

Ð 0.50 2.01 3.99

Ð 100.0 100.4 99.8

2a 2 2 2

None 0.50 2.00 4.00

Ð 0.51 2.03 4.05

Ð 102.0 101.5 101.3

Ð 0.51 2.05 4.04

Ð 102.0 102.4 101.0

3a 3 3 3

None 0.50 2.00 4.00

Ð 0.49 1.96 4.13

Ð 98.0 98.0 103.3

Ð 0.50 1.97 4.06

Ð 100.0 98.5 101.5

4a 4 4 4

None 0.50 2.00 4.00

Ð 0.50 1.97 4.01

Ð 100.0 98.5 100.3

Ð 0.50 2.01 4.03

Ð 100.0 100.5 100.7

5b 5 5 5

None 0.50 2.00 4.00

Ð 0.51 2.09 4.03

Ð 102.0 104.5 100.8

Ð 0.51 2.04 3.98

Ð 102.0 102.0 99.5

6b 6 6 6

None 0.50 2.00 4.00

Ð 0.51 2.07 4.03

Ð 101.2 103.5 100.8

Ð 0.50 2.02 4.02

Ð 100.0 101.2 100.5

a b

Samples obtained from different places of Colorado River (Rio Negro State, Argentina). Samples of well water.

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Table 5 Recovery from V(V) added to natural water samples Proposed method a

Reference method (ICP±AES)

Vanadium added (mg/ml)

V found (mg/ml)

Recovery (%)

V found (mg/ml)

Recovery (%)

1a 1 1 1

None 0.05 0.50 1.00

Ð 0.052 0.49 1.01

Ð 104.0 98.7 101.2

Ð 0.053 0.51 0.99

Ð 106.0 102.0 99.8

2a 2 2 2

None 0.05 0.50 1.00

Ð 0.053 0.53 1.06

Ð 106.0 106.0 106.0

Ð 0.053 0.53 1.05

Ð 106.0 106.0 105.0

3a 3 3 3

None 0.05 0.50 1.00

Ð 0.05 0.51 1.03

Ð 100.0 102.0 103.0

Ð 0.051 0.504 1.01

Ð 102.0 100.8 101.0

4a 4 4 4

None 0.05 0.50 1.00

Ð 0.049 0.49 1.02

Ð 98.0 98.5 102.0

Ð 0.05 0.503 1.01

Ð 100.0 100.5 101.0

5b 5 5 5

None 0.05 0.50 1.00

0.06 0.10 0.58 1.04

Ð 90.9 103.6 98.2

0.06 0.11 0.57 1.05

Ð 100.0 101.7 99.1

6b 6 6 6

None 0.05 0.50 1.00

0.09 0.13 0.61 1.08

Ð 92.9 103.4 99.1

0.08 0.12 0.60 1.09

Ð 92.3 103.4 100.9

Sample

a b

This samples were obtained from different places of Colorado River (Rio Negro State, Argentina). Samples of well water.

methodologies. It provides good quality results in terms of accuracy and precision (RSDˆ1.9%). The most important advantages as regards the batch method are the sample throughput (34 hÿ1) and the lower detection limit (0.2 mg lÿ1). A second FIA manifold based on the same reaction with a preconcentration step, can be applied to the determination of low levels of V(V). It has a lower detection limits for the determination of vanadium (0.032 mg mlÿ1), a good sensitivity (4 times greater than Method A) and reproducibility (RSDˆ2.3%).

Acknowledgements The authors express their gratitude to the Consejo Nacional de Investigaciones Cientõ®cas y TeÂcnicas (CONICET) de la RepuÂblica Argentina for the ®nancial support. References [1] D.C. Grans, M. Shaia Gottlicb, J. Tawara, R.L. Bunch, L.A. Thiesen, Anal. Biochem. 188 (1990) 53.

M.E. Palomeque et al. / Analytica Chimica Acta 366 (1998) 287±293 [2] H.E. Stokinger, in: Patty's Industrial Hygiene and Toxicology, 3rd edn., vol. 2A, Wiley-Interscience, New York, 1981, p. 2028. [3] B. Patel, G.E. Henderson, S.J. Haswell, R. Grzeskowiak, Analyst 115 (1990) 1063. [4] T. Yotsuyanagi, J. Ito, K. Aomura, Talanta 16 (1969) 1611. [5] S. Nakano, M. Tago, T. Kawashima, Anal. Sci. 5 (1989) 69. [6] H. Hoshino, T. Yotsuyanagi, Chem. Lett. 8 (1984) 1445. [7] C.W. Fuller, J.M. Ottaway, Analyst 95 (1970) 28. [8] A. Sevillano-Cabeza, J. Medina-Escriche, F. Bosch-Reig, Analyst 237 (1984) 207.

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[9] J. Miura, Anal. Chem. 62 (1990) 1424. [10] M. Palomeque, A. Lista, B. FernaÂndez Band, submitted to An. Asoc. Quim. Argent., 1997. [11] R. Belcher, A.I. Netten, W.I. Stephen, Analyst 76 (1951) 430. [12] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Ellis Horwood, London, 2nd. edn., 1993, p. 103. [13] J.P. Riley, D. Taylor, Anal. Chim. Acta. 40 (1968) 479. [14] Avances en QuimiometrõÂa PraÂctica, Dr. R. Cela (Ed.), Universidad de Santiago de Compostela, EspanÄa, 1994, p. 157.