Analyrrca Chrmrca Acta, 236 (1990) 469473 Elsevler Sctence Pubhshers B.V , Amsterdam

Short Communication

469

Determination of chromium by on-line preconcentration on a poly (hydroxamic acid) resin in flow-injection atomic

absorption spectrometry

AJAY SHAH and SUREKHA DEW *

Deparrment of Chemrstry, Faculty of Scrence, M S Vnruersrty, Baroda 390 002 (Indra)

(Received 16th August 1989)

ABSTRACT

A senes of poly(hydroxamlc acid) resms were used for the preconcentratlon and determmatlon of chrommm(II1)

Condltlons were optmuzed for the determmatlon of chronuum(II1) and Its separation from multi-component nuxtures

A flow mamfold was developed for the on-hne preconcentratlon and determmatlon of chronuum(II1) by atonuc

absorption spectrometry

Keywords Chronuum; Preconcentratlon, Poly(hydroxamlc acid) resm

Chromatographic methods are still widely used for the preconcentration, removal and separation

of trace metals. Among the chromatographic materials used, chelating resins are promising be- cause of their selectivity for transition and other metal ions over alkali and alkaline earth metals. Chelating resins bearing various functional groups have been reported earlier [l-3]. Various poly- meric backbones bearing hydroxarmc acid groups

[4-71 have been synthesized and used for the extraction, concentration and separation of vari- ous metal ions. In flow-injection systems, chelat-

ing resins such as Chelex-100 [8] and immobilized chelates [9-111 have been used for the enrichment

of metal ions. However, these resins and immobi- lized chelates show slow exchange rates and swell- ing and contraction with variation in the ionic strength of the solutions.

In this work, these problems were overcome by

incorporatmg a microcolumn containing poly(hy- droxamic acid) resins, as previously reported [12],

in the flow system. So far no study of the chelat- ing behaviour of chromium with poly(hydroxamic

0003-2670/90/$03 50 0 1990 - Elsewer Science Pubhshers BV

acid) resins has been reported. The conditions were optimized for the enrichment and also for the chromatographic separation of chromium from binary and quaternary mixtures. Owing to the very low water regain, the resins used do not swell or shrink, unlike Chelex-100.

ExperImental

Reagents. All reagents were of analytical-re- agent grade and were used as received, unless

stated otherwise. Deionized, distilled water was used throughout. Solutions were prepared by dis- solving appropriate amounts of uranyl acetate and chromium(II1) oxide in water and dilute nitric acid, respectively, and were standardised spectro- photometrically [13]. The synthesis and physicochemical properties of the poly(hydrox- amic acid) resins used have been reported previ- ously [ 121.

Metal ron capacity. The metal ion capacities were determined by a batch method by equilibrat- ing the resin samples (0.25 g) at pH 1.0-6.0 for 12

470 A SHAHANDS DEW

h, followed by equihbration with 25 ml of 0.2 M uranium(V1) or chromium(II1) solution for 24 h.

The kinetics of exchange, the effect of metal ion concentration on the exchange capacity, t,,, (ttme required for 50% exchange) and the percentage elution of the metal from resin with various eluents were established according to literature methods [14]. The mode of diffusion of metal ions through

the solution towards the resm was determmed by the interruption test [14,15], during which the res- in beads were removed from the solution for 10 min and then re-immersed. From the nature of the plots of percentage exchange agamst time the mode of diffusion of ions was determined as discussed earlier [15].

Chromatographlc separations. The resin sam-

ples (H+ form) were equilibrated with buffer and packed mto glass chromatographic columns for- ming a bed of length ca. 17 cm and i.d. ca. 0.5 cm. Mixtures containmg Cr(III), Fe(III), Zn(I1) and Cu(I1) m the appropriate pH buffers were passed through the columns at a flow-rate of 1 ml mm-‘. Elution was also carried out at a flow-rate of 1 ml mm-‘.

Flow mamfold. The flow manifold used for the

determmation of chromium is shown in Fig. 1, and consisted of a microtube penstaltic pump and a rotary PTFE valve (Rheodyne 5020). The con- necting PT’FE tubing was of 0.6 mm i.d. The enrichment columns (4 cm X 2.5 mm i.d.) were made of glass and filled with poly(hydroxamic acid) resin. An atomic absorption spectrometer (GB6 902, Melbourne, Australia) was used for detection, using monochromator band pass 1 nm, lamp current 2.5 mA, air pressure 2.7 X lo5 N m -2 and acetylene pressure 6.7 x lo4 N mp2. The

carrier stream was 0.2 M acetate buffer (pH 2).

b

Fig 1 Flow mamfold a, Three-way tap, b, perlstaltlc pump, c,

uqectton port, d, detector, e, preconcentratlon column

I-

)-

)- 0

I (b)

PH

Ftg. 2 Effect of pH on metal exchange capacity (a)

Uranmm(VI), (b) chrommm(II1) 0, Unsubstttuted hydrox-

an-w acid resm, 0, N-phenylhydroxamlc acid resm, 0, p-chlo-

rophenylhydroxamlc acid resm, W, m-chlorophenylhydroxarmc

acid resm, X , p-tolylhydroxamlc acid resm, A, m-tolylhydrox-

anuc acid resin, A, 3-chloro-p-tolylhydroxanuc acid resin

For preconcentration, standard chromium(II1) solutions of pH 2 were passed through the col- umns at a flow-rate of 2 ml min2. The chelated chromium was eluted with 50 ~1 of 1 M hydro-

chloric acid and measured by atomic absorptton spectrometry (AAS) at 357.9 nm. The column was further washed by passing 1 M hydrochloric acid and buffer through the system. A three-way tap was used to control the flow of standard solution and buffer or acid through the system as required.

Results and drscusslon The synthesis, structural confirmation and

physicochemical properties of the resins under study were discussed previously [12]. The results of the effects of pH on metal ion chelation for

seven different poly(hydroxannc acid) resins are given in Fig. 2. The resms are more selective to uramum than to chrornmm. The unsubstituted resin shows the highest affmny for uramum and chrommm whereas the m-chlorophenyl-sub- stituted resin shows the least affinity for both metals. It is apparent that substitution of the hydroxamic acid ligand affects the stability of the complex and the generalizatton that nucleophilic substitution increases the basicity and hence coor-

DETERMINATION OF CHROMIUM BY ON-LINE PRECONCENTRATION IN FI-AAS 471

dinating strength of the chelating group, of limited validity, can be derived. From Fig. 2 it is clearly seen that the quantitative separation of chro- mium(II1) from uranium(V1) can be achieved at pH 5.0. The kinetics of metal exchange show that equilibrium is attained after 12 h and the time

required for 50% exchange (tl,*) ranges between 3-7 min for uranium and 5-11 min for chromium. The faster rate of exchange facilitates the chro- matographic separations. It was also observed that

t,,, is inversely proportional to the metal ion

concentration. The interruption test [14,15] was used to study

the nature of diffusion of ions durmg the ex-

change phenomenon, which was observed as a particle diffusion process. The interruption pause gives time for the concentration gradients in the beads to level out. Therefore, in particle diffusion control the rate immediately after re-immersion is greater than that prior to mterruption. In film diffusion control, there is no concentration gradi- ent in the beads and the rate depends on the concentration difference across the film. The m- terruption pause does not affect the difference and hence has no effect on the rate of exchange.

Different eluents such as l-6 M hydrochloric acid, nitric acid, sulphuric acid, acetic acid, sodium chloride and 5-30’S (v/v) per&lo& acid (70%) were used for elution of metal ions. The first three acids (1M) were satisfactory for the quantitative recovery of uranium but could elute only 80% of chromium from the resins. Complete recovery of chromium from the resins was not achieved even after prolonged elution. Mixed eluents such as

aqueous methanol or acetone were not useful. The only means of recovermg chromium quantitatively from the resin was by burning the loaded resin.

Separations of multi-component mzxtures. (a)

Bznary system. The separation of chromium(II1) from uranium(V1) based on selective sorption was done by taking the mixtures in a 1 + 1 weight ratio (5 ml of a 1 mg ml-’ solution of each metal ion). The resin columns were equilibrated at pH 5.0 and separation was achieved at this pH, uranium being retained on the resin column and chromium being detected in the column effluent. The quantitative elution of uranium was effective with 25 ml of 1 M hydrochloric acid. The results

0 20 40 60 80 4-V,,H,O r\ 01 M HCI -

ml

Fig 3. Separation of chronuum(II1) from uramum(V1) usmg N-phenylhydroxanuc acid resm o, Chronuum, 0, uramum

for one representative resin are given in Fig. 3. The other resins showed similar trends with slight variations.

(b) Multi-component system. Quaternary rmx- tures (10 ml) of chronnum(III), iron(III), zinc(I1) and copper(I1) containing 2.5 mg of each metal

ion were passed through the chromatograpmc col- umns at pH 5 and a flow-rate of 1 ml mm ‘. At this pH all metal ions except Cr(II1) are retained by the columns and chromium is detected quanti- tatively m the column effluents. Chelated zmc and

copper were eluted with 0.1 M hydrochloric acid. After thorough washing with water, iron was eluted with 3 M hydrochloric acid. Cross-contarmnation of zinc and copper was observed in this sep- aration, owing to the similar distribution coeffi- cients of the two complexes (1768 and 1800, re- spectively). The results for a representative resin are given in Fig. 4. The other resins showed simi- lar trends with slight variations.

Determznation of chromzum(III) by flow-znJectzon MS. Flow-injection methods are useful for the

precise, accurate and reproducible analysis of in- complete reactions. Therefore, the flow manifold shown in Fig. 1 was developed for the determma- tion of chromium on enrichment, mainly because only 80% elution of chromium from the column

472 A SHAHANDS DEW

!40- ‘;

5

< 0

-1

A

” 20

i?

-j \,jL/i_

XX

I I

“1

X

0 A

0 10 30 50 70 90 110

d/o-u-H20--c-- 01 M HCI -w-3M HCI-

ml of eluent

Fig 4 Separation of chronuum(II1) from multi-component mixtures using N-phenylhydroxamlc acid resin. A,

chrommm(III), 0, zmc(II), x , copper( o, Iron(III).

takes place in batch processes. For calibration of chromium by direct injection a series of standard solutions (20 ~1 of lo-50 pg ml-’ solutions) were mjected into water, a carrier stream used in the manifold shown in Fig. 1, but without a nucrocol- umn. A linear calibration graph was obtained with a correlation coefficient of 0.9997 for five read-

ings. The regression equation obtained was ab- sorbance = 3.88 X 10e3 [Cr(III), pg] - 1.4 X 10e3 with a slope of (3.88 + 0.036) X 10s3 pg-’ and intercept (- 1.4 + 1.2) X 10e3. The linut of detec- tion (2 x noise) was 0.1 ng ml-‘. The relative standard deviation for six injections of 20 ~1 of 50 pg ml-’ solution was 1.3%. The dispersion coeffi- cient measured for a loo-p1 injection volume was 2.1. However, it was observed that the dispersion coefficient depends on the amount of sample in- jected and increases as the volume of injected sample decreases.

The effect of pH on metal preconcentration flow-mjectton analysis shows a similar behavtour to that observed in the batch process (Fig. 2). The effect of column length on metal ton concentra- tion was studied by using 22&cm long columns in the manifold. It was observed that a 2-cm column 1s satisfactory. However, columns 4 cm long were used in further studies. An mcreased column length had a negligible effect on the absorbance, indicat- mg very little contrtbution to nebulizer dispersion

by the column. The study of various acids of different concentrations as eluents showed that 50 ~1 of 1 M hydrochloric acid was sufficient for the elution of chromium. However, the elutlon did not exceed 90%. The effect of flow-rate on metal pre- concentration showed that 50-fold concentration can be achieved by using a flow-rate of 2 ml rnin-’ and a loo-ml sample for enrichment. Con- tmuous determinatton of chronuum during pre- concentration showed that column effluents are free from chromium. Higher flow-rates resulted in variations (ca. 5%) in the extent of preconcentra- tion. A 90-95% recovery of chromium on precon- centration was achieved. The column used was efficient even after repeated use for 2 months. The calibration graph obtained for the preconcentra- tion of 10 ~1 of 20, 40, 60, 80 and 100 ng ml-’ chromium solutions was linear with a regression

coefficient of 0.9940. Sea water from Gopnath (GuJarat, India) was

collected at various depths in PTFE-lined bottles. The sea water was filtered through acid-cleaned 0.4~pm filters and preserved by acidification. Sea water of pH 2-3 was passed through the system for the enrichment and determination of Cr(II1).

The concentration of chromium at the surface was ca. 0.17 nM, and decreased with depth to 40 pM at 225 m. The method discussed here thus shows good potential for the shipboard determination of Cr(II1). The closed environment of the flow-injec- tion systems enables analyses at picomolar levels to be achieved without significant contamination.

The authors are gratefully to the CSIR for financial support of this research project.

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DETERMINATION OF CHROMIUM BY ON-LINE PRECONCENTRATION IN FI-AAS 473

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