Analyst, February 1995, Vol. 120 555

Determination of Trace Amounts of Nickel by Chelating Ion Exchange and On-line Enrichment in Flow Injection Spectrophotometry

Rajesh Purohit* and Surekha Devit Department of Chemistry, Faculty of Science, M . S . University of Baroda, Baroda 390 002, India

Trace amounts of nickel were determined after preconcentration at batch levels and by flow injection using chelating resins. Various experimental conditions were optimized for the preconcentration procedure. The batch procedure for preconcentration was highly dependent on pH. In the continuous-flow method, nickel was eluted from the resin with 20 pl of 2 moll-1 hydrochloric acid and was determined at 445 nm as the nickel-dimethylglyoxime complex. Column chromatographic separations of copper and nickel did not show any cross-contamination. The flow method was also used for the determination of nickel in various brands of chocolate and vegetable oil after preconcentration

Keywords: Nickel; preconcentration; separation; flow system; chocolate and vegetable oil

Introduction

The enrichment of metal ions by ion exchange continues to be the most popular method for the recovery of trace levels of metal ions. Flow injection (FI) has contributed to the development of several semi-automated methods for this purpose owing to its numerous advantages over conventional methods.l-5 On-line preconcentration has mainly been carried out on activated alumina,6 8-hydroxyquinoline3,7-* and its derivatives5 immobilized on silica gel or glass, Chelex 100,lJ resin 1229 and tri(pyridylmethy1)ethylenediamine. 1 0

Chelating resins containing 8-hydroxyquinoline have been known for the last 80 years but have limitations such as low exchange capacity, low chemical stability and slow exchange. Attempts have been made to overcome these limitationsl1.12 and we have reported the general properties and synthetic routes for these resins in previous papers. 13-14 The potential of these chelating resins in the separation and determination of copper.13 zinc14 and lead15 has already been reported.

The separation and determination of nickel, an essential trace element, by a continuous-flow method was first reported by Kuroda and Mochizukil6 in 1981. To date, about 25 reports have appeared concerning the determination of nickel by FI methods but only a few involve determination after precon- centration.17-20 In the present paper the use of novel ion exchangers for the determination of nickel at nanogram levels in chocolate and vegetable oil is described. The results obtained by both conventional batch and continuous-flow procedures are compared.

’ Permanent address: Gujarat Jalseva Training Institute, Gandhinagar 382 015, India. I To whom correspondence should be addressed.

Experimental

Analytical-reagent grade chemicals and high purity de-ionized water were used to prepare the metal ion solutions. Working solutions of nickel were prepared by appropriate dilution of a stock standard nickel solution. Acetate buffers of different pH were prepared from 0.2 mol 1-1 sodium acetate and acetic acid. Dimethylglyoxime (DMG) solution (0.5% m/v) was prepared as required.

Synthesis of Chehting Resins

The synthesis and physico-chemical properties of resins containing 8-hydroxyquinoline (8HOQ), and resorcinol (R) or hydroquinone (Ha) , and formaldehyde (F), 2-furaldehyde (Fu) and benzaldehyde (B) as cross-linking agents, have been described elsewhere. 13,14 The physico-chemical properties of the resins used are presented in Table 1.

Preconcentration of Nickel

The resins were used for batch and column chromatographic studies of nickel exchange. Optimization of the pH for preconcentration at the batch level was carried out according to a literature procedure,21 using 0.2 moll-1 acetate buffer of pH 2-8. Nickel was determined complexometrically in the supernatant solution and also by eluting the chelated metal from the resin with 0.1-2 moll-’ hydrochloric acid. From the exchange kinetics, the value of t1/2 (time required for 50% exchange of nickel) for the batch procedure was determined.21

The rate constant and activation energy for the exchange of nickel(i1) were calculated by means of kinetic studies at 30,35, 40 and 45°C. The mode of diffusion of nickel through the solution towards the resin was determined by the interruption tesPJ2 and by the method reported by Native et al.23 During the interruption test, resin beads were interrupted by remov- ing them from the solution for a short period of time (10 min) and then re-immersing them in the solution. From the nature of the graph of percentage exchange versus time, the mode of diffusion of the ions was determined as described by Helfferich. 21

Various eluents such as acids, alkalis, electrolytes and complexing agents of different strengths were tested for the quantitative elution of nickel in batch experiments.

Chromatographic Separations

Chromatographic columns of 10-18 cm length and 0.7 cm id were prepared from the following resins: 8HOQFR (8-hydroxyquinoline-formaldehyde-resorcinol); 8HOQFHQ (8-h ydroxyquinoline-formaldehyde-hydroquinone) ; 8HOQFuR (8-hydroxyquinoline-2-furaldehyde-resorcinol);

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556 Analyst, February 1995, Vol. 120

8HOQFuHQ (8-hydroxyquinoline-2-furaldehyde-hydroqui- none); and 8HOQBR (8-hydroxyquinoline-benzaldehyde- resorcinol). The breakthrough and column capacities and interstitial volumes were determined according to literature methods .21,24

Binary mixtures of copper and nickel (1 + 1) were passed through the columns at the optimum pH, at a flow rate of 1 ml min-1 followed by washing of the columns with de-ionized water. The chelated metal was eluted with the appropriate eluting reagent and fractions of the column effluents were analysed for both metal ions.

On-line Preconcentration of Nickel

A continuous-flow manifold for the preconcentration of nickel at nanogram levels was constructed as shown in Fig. 1 using 4 X 0.2 cm microcolumns, a Tokyo Rikakikai microtube peristaltic pump, a Rheodyne RH 5020 injection valve, 0.5 mm id poly(tetrafluoroethy1ene) tubing and an ultraviolet- visible spectrophotometer with a 20 pI flow cell as the detector. The absorbance-time response was recorded on an x-t chart recorder at 445 nm. The conditions, such as variation in pH, column length, flow rate and metal and reagent concentration, were optimized for the preconcentration and determination of nickel. The chelated nickel was eluted from the column by injecting 20 yl of 2 mol 1-1 hydrochloric acid followed by acetate buffer solution of pH 8 to avoid carryover of the sample to the next determination.

Determination of Nickel in Chocolate and Vegetable Oil

The determination of nickel in various chocolates and vegetable oils was carried out by using a dry digestion procedure. Samples (1, 5 and 10 g) of different brands of chocolate and vegetable oil were heated to 800 "C in a furnace. The residue was dissolved in 5 ml of 2 mol 1 - 1 hydrochloric acid. The insoluble matter was filtered off and the filtrate was made up to 25 ml with de-ionized water.

A 5 ml aliquot of the solution was used for the preconcentra- tion and determination of nickel as described earlier. The results obtained were compared with those obtained by atomic absorption spectrometry (AAS).

Results and Discussion

The physico-chemical properties of the resins under study are given in Table 1. The sodium exchange capacities and the rate of exchange were found to be lower and slower for resins with 2-furaldehyde and benzaldehyde as cross-linking agents. The

exchange rate was faster for resins containing resorcinol than for those containing hydroquinone. This may be due to the difference in the complexing capacities of the two monomers. Maximum exchange of nickel was found to occur at pH 6-6.5, irrespective of the nature of the resin (Fig. 2). However, the exchange capacity and distribution coefficient were found to be functions of the type of resin. The selectivity order of the resins for nickel, based on the exchange capacity or distribu- tion coefficient, is: 8HOQFHQ > 8HOQFR > 8HOQFuHQ > 8HOQFuR > 8HOQBR. Decreased selectivity was ob- served for resins containing cyclic cross-linking agents.

The results of the effect of the nickel ion concentration on the exchange capacity, presented in Fig. 3, indicate that the exchange of nickel is a concentration-dependent phenom- enon. All the resins, irrespective of their structure, show saturation for exchange at a metal concentration of 0.1 mol 1-1.

In the study of the diffusion of nickel ions by means of the interruption test,21,22 it was observed that the exchange of nickel is a particle diffusion process. In the interruption test, the interruption allows time for the concentration gradients in the beads to level out. Therefore, in a particle diffusion controlled process the exchange rate increases immediately after interruption. In film diffusion, no concentration gradient exists in the beads and the rate depends on the concentration difference across the film and, hence, interruption does not affect the exchange rate. The results obtained by the method of Native et al.23 also support the particle diffusion process for the exchange of nickel.

Thermodynamics and Rate of Exchange

The exchange rates for the uptake of nickel by the resins were determined at 30, 35, 40 and 45°C. The rate constant, k , for initial exchange was calculated using the equation for a first-order reaction

dc kt --=kcand-log(a-f)= -

dt 2.303 where a is the initial concentration of nickel ion, c is the concentration of nickel at time t, and f is the fraction of ions exchanged on the resin. The plots of -log (a - f) versus time (t) were observed to be non-linear and did not pass through the origin. Hence, the rate constant was calculated from the initial rate of the reaction by using the mirror method,2s in which a mirror, when oriented normal to the curve, shows a reflection which is a smooth continuation of the curve. A tangent is then drawn perpendicular to the normal and k is calculated from the slope of the line. The activation energy

Table 1 Physico-chemical properties of the resins*

Property 8HOQFR 8HOQFHQ 8HOQFuR 8HOQFuHQ SHOQBR Moisture content/g g- l 0.03 True density/g ml-I 1.40 Void volume/ml g- 0.78 Sodium exchange

capacity/mmol g-1 4.60 tIl2 for sodium/min 7.00 Ni" exchange capacity at

pH 6/mmol g- 0.72

exchange/min 3.00

coefficient for 0.1 moll-' nickel at pH 6)

for nickel

kd (distribution

7.33

0.08 1.50 0.66

5.00 4.00 1

0.76

13.00

8.25

0.64 1.25 0.53

2.80 10.00

0.59

8.00

6.01

1.98 1.16 0.49

3.30 12.00

0.66

90.00

6.84

1.51 1.18 0.63

2.60 24.00

0.51

60.00

5.40 * Abbreviations represent resins synthesized using 8-hydroxyquinoline (SHOQ), resorcinol (R) , hydroquinone (HQ). formaldehyde (F),

2-furaldehyde (Fu) and benzaldehyde (B).

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Analyst, February 1995, Vol. 120 557

( B E ) required for complex formation was determined using the Arrhenius equation

k = Ae-AEIRT

where R is the gas constant, A is the frequency factor and T is the temperature. From the slope of the plot of log k versus 1/T, the value of A E was calculated and is given in Table 2. From the results it was observed that an increase in the exchange rate with temperature results in higher rate constants. It was also observed that the energy required for the formation of the nickel complex is higher than that required for copper,13 zinc,14 cadmium14 and lead.15

Metal solution n I

De-ionized water I

DMG 2

Fig. 1 FI manifold for on-line determination of Nil1 using DMG as reagent. 1, Two-way stopcock; 2, peristaltic pump; 3, injection valve; 4, microcolumn containing resin; 5 , reaction coil; and 6, detector (ultraviolet-visible spectrophotometer).

0 2 4 6 8 0.6

(b) -:- 0.4

2 % 9

0.2

0 2 4 6 8 1 0

PH Fig. 2 Effect of pH on nickel exchange. (a) Batch procedure; (b) FI method. 0 .8HOQFR; x, 8HOQFHQ; 0,8HOQFuR; A , 8HOQ- FuHQ; and 0,8HOQBR.

I I? 0

2 4 6 8 1 0 PH

Fig. 2 Effect of pH on nickel exchange. (a) Batch procedure; (b) FI method. 0 .8HOQFR; x, 8HOQFHQ; 0,8HOQFuR; A , 8HOQ- FuHQ; and 0,8HOQBR.

Elution of Nickel From Resins

Elution of nickel from the resins was carried out with various eluting agents of different strengths. It was found that nickel can be quantitatively eluted with 0.5-1 mol I-' hydrochloric acid; 0.2-1.0 mol 1-1 sulfuric acid can elute nickel from 8HOQFR, 8HOFuR, 8HOQFHQ and 8HOQBR but not from 8HOQFuHq. Citrate and tartrate were unable to elute nickel from the resins.

Binary Separations

The separation of copper and nickel from binary mixtures containing 400 and 1000 pg ml-1 of each metal ion was carried out under the conditions given in Table 3, using columns of the resins under study. The flow rate of the solution through each column was maintained at 1 ml min-1. Chelated nickel and copper were eluted with 0.1 and 3 mol 1-1 hydrochloric acid, respectively. The column effluents were analysed for both metal ions by using appropriate masking reagents. The results obtained are given in Fig. 4 and Table 4. No cross-contamina- tion was observed during separation.

FZ System

A calibration graph for the determination of nickel was constructed by injecting a series of standard nickel ion solutions into the carrier stream of the flow manifold illustrated in Fig. 1, without the microcolumn. The percentage recovery after preconcentration was determined from the calibration graph.

The determination of nickel at nanogram levels was carried out by using the FI manifold shown in Fig. 1. Preconcentration of nickel was achieved by passing 5 ml of 1 x 10-7 moll-1 (50 ng ml-1) nickel solution at a flow rate of 3 ml min-1 through microcolumns containing the chelating resins 8HOQFR, 8HOQFHQ, 8HOQFuR, 8HOQFuHQ and 8HOQBR. Con- tinuous monitoring of the column effluent showed quanti-

0.8

7 0.6 CJI - E" E .- a 0.4 1

2 8 Q

0.2

0 0.05 0.10 0.15 0.20 0.25 Concentratiorimol I-'

Fig. 3 Effect of Ni" ion concentration on exchange process. Amount of resin, 0.25 g; reaction volume, 25 ml; equilibration time, 24 h. Symbols as in Fig. 2.

Table 2 Rate of exchange and activation energy for nickel

Rate constant, k/lW s-l Activation energy,

Resin 30°C 35°C 40°C 45°C AEIJmol-1 8HOQFR 3.2 3.7 3.8 6.5 3699.3 8HOQFHQ 3.1 3.4 3.6 5.2 3578.1 8HOQFuR 1.6 2.3 2.5 3.6 4063.0 8HOQFuHQ 2.6 2.5 3.2 3.3 1354.3 8HOQBR 2.0 2.3 2.8 3.9 3941.7

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558 Analyst, Februpry 1995, Vol. 120

(d) - 0.1 mol I-' HCI - 3 mol I-' HCI -+

60 I (a) 1

204 0 25 50 75 100 60

0.1 moll-1 HCI - 3 moll-1 H c ~ L( 40 t

0 20 40 60 80 100 60

(c) I Y g? 4 0 1

40 t 0 25 50 75

0.1 mot I-' H C 1 - t 3 mol I-' HCI 40

0 25 50 75 100 Volume of effluentlml

tative uptake of nickel by the resin in the column. After washing the columns with acetate buffer of pH 7.5-9.0, the chelated nickel was eluted by injecting 20 1-11 of 2.0 mol 1-1

hydrochloric acid. The dispersion was calculated by nebulizing 5 pg ml-1 of nickel solution and injecting 20 p1 of the solution into the flow manifold, with and without the microcolumn. The introduction of the microcolumn did not affect the peak height. The FI peak height, in comparison with the steady- state response, gave a dispersion coefficient of 2.1 for the manifold. The dispersion of the system was observed to be dependent on the volume injected and increased with a decrease in the volume injected. On elution of nickel, sharp peaks were obtained indicating faster exchange kinetics.

In comparison with the conventional spectrophotometric analysis, the proposed FI method was observed to be highly sensitive, with detection limits at the ng ml-1 level.

Optimization of pH for the preconcentration of nickel was carried out by passing metal ion solutions of pH 2-10 through the microcolumns containing the synthesized resins. The chelated metal was eluted by injecting 20 1-11 of 2.0 rnol 1-1

hydrochloric acid. From the results presented in Fig. 2, it was found that preconcentration of nickel by the FI system mainly takes place in the alkaline region, unlike the batch procedure, and is less dependent on the pH.

Optimization of the column length for preconcentration was carried out by using microcolumns of 2, 3, 4 and 6 cm length and 0.2 cm id. A flow rate of 3 ml min-1 was maintained throughout. From the results obtained it was observed that 4

0 20 40 60

Fig. 4 Separation of copper and nickel from mixtures. Concentra- tion of nickel and copper, 10 mg each; volume of solution, 25 ml; flow rate, 1 ml min-I. 0, Nickel; and 0, copper. (a) 8HOQFR; (b ) 8HOQFHQ; (c) 8HOQFuR; ( d ) 8HOQFuHQ; and (e ) 8HOQBR.

Amounting Fig. 5 Calibration plots for Ni". Flow rate, 3 ml min-1; column length, 4 cm; resin, 8HOQFuHQ; pH, 7.5-9.0; eluent, 20 p1 of 2 rnol 1-1 HCI. Symbols as in Fig. 2.

Table 3 Column characteristics

Property 8HOQFR 8HOQFHQ 8HOQFuR 8HOQFuHQ 8HOQBR Column length/cm 17.50 17.50 9.00 10.50 9.30 Interstitial volume/ml 9.20 9.80 4.80 5.20 4.60 Breakthrough capacity/mequiv g-1 0.10 0.09 0.09 0.08 0.09 Total capacity/mequiv g-' 0.50 0.62 0.39 0.43 0.48 Degree of utilization* 0.20 0.14 0.23 0.19 0.19

* Breakthrough capacity/total capacity.

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Analyst, February 1995, Vol. 120 559

Table 4 Efficiency of recovery of nickel and copper from binary mixtures

Amount of Ni’+/mg Amount of Cu’+/mg

Resin Taken Recovered* Recovery (%) Taken Recovered* Recovery (%) 8HOQFR 4.0 3.99k0.021 99.8 4.0 4.01 kO.021 100.2 8HOQFHQ 4.0 4.03 k 0.024 100.8 4.0 4.06 k 0.028 101.5 8HOQFuR 4.0 4.01 k 0.021 100.2 4.0 3.97k0.025 99.2 8HOQFuHQ 4.0 3.97 k 0.023 99.2 4.0 3.91 k0.031 97.8 8HOQBR 4.0 3.92f0.028 98.0 4.0 3.99k0.024 99.8

8HOQFR 10.0 9.98 f 0.030 99.8 10.0 10.02 k 0.026 100.2 8HOQFHQ 10.0 9.94 f 0.031 99.4 10.0 9.99 k 0.020 99.9 8HOQFuR 10.0 10.06 k 0.030 100.6 10.0 9.99 * 0.023 99.9 8HOQFuHQ 10.0 9.93 k 0.029 99.3 10.0 9.93 k 0.027 99.3 8HOQBR 10.0 9.99 f 0.025 99.9 10.0 9.96 k 0.022 99.6

* Values given are the means of five measurements k standard deviations.

Table 5 Determination of nickel in chocolate and vegetable oil

Nickel found Nickel found Sample by proposed by AAS*/ No. Name of product method*/pg g- * pg g- * Chocolates-

1 Cadbury, India 0.5 k 0.02 0.45 f 0.018 2 NestlC, India 7.3 0.02 8.0 f 0.02 3 Amu1,India 2.5 f 0.03 2.1 k 0.02

1 Dalda, India 0.0 0.0 2 Sunflower, India 0.0 0.0 3 Madhuram, India 0.0 0.0 * Mean of nine determinations k standard deviation.

Vegetable oils (ghee)-

cm long columns are efficient for preconcentration and that columns longer than 6 cm show a high degree of dispersion. The optimum flow rate was found to be 3.0 ml min-1.

Elution of chelated nickel was carried out by injecting 20 1-11 of various acids of different strengths: 2 mol 1-1 hydrochloric acid was found to give quantitative elution of nickel from all the resin columns studied. Elution of nickel using 2 mol 1-1

hydrochloric acid gave a narrow peak of comparable height, indicating that the analyte concentration lies in a relatively narrow eluent range.

A calibration graph for the preconcentration of 5 ml of nickel solutions with concentrations of 10-70 1-18 ml-1 was obtained for each resin after elution of nickel with 20 1-11 of 2 mol 1-1 hydrochloric acid. The results were recorded in triplicate as shown in Fig. 5. The regression coefficients were 0.9948, 0.9982, 0.9936, 0.9972 and 0.9926 for 8HOQFR, 8HOQFHQ, 8HOQFuR, 8HOQFuHQ and 8HOQBR, respectively. The lowest detection limit (based on twice the standard deviation of the blank signal) was found to be 10 pg ml-1 after preconcentration of 5 ml of solution. The relative standard deviations for n = 5 were in the range 0.4-2.8%.

Analysis of Chocolate and Vegetable Oil The determination of nickel in different brands of chocolate and vegetable oil was carried out as described earlier. The results obtained are given in Table 5. Nickel was not detected in Dalda, Sunflower and Madhurum vegetable oil brands from India.

References

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Paper 4/01 908C Received March 30, 1994

Accepted September 21, 1994

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