Talanra, Vol. 36, NO. 6, pp. 669-671, 1989 Printed in Great Britain. All rights reserved

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SHORT COMMUNICATIONS

INDIRECT DETERMINATION OF IODATE BY ATOMIC-ABSORPTION SPECTROPHO’TOMETRY

DEBASIS CHAKI~ABORTY and ARABINDA K. DAS* Department of Chemistry, University of Burdwan, Burdwan-713104, India

(Received 27 January 1988. Revised 4 July 1988. Accepted 15 January 1989)

Summary-An indirect method for determination of trace iodate in certain high-purity chemicals by atomic-absorption spectrophotometry (MS) is described. Iodate forms a stable ion-association complex [Hg(dipy),J(IO,), in neutral medium, which can be extracted into methyl isobutyl ketone with 99% efficiency. The extract can be analysed for mercury (and hence indirectly for iodate) by flameless AAS. The limit of detection for iodate by this method is 7.5 ng. Apparent recove#es of 92-112% have been obtained for spikes of 0.25-0.70 /le of iodate.

The usual methods for determination of iodate include spectrophotometric,‘” kinetic,“i2 spectro- fluorometric,” and liquid chromatography*4 procedures. A possible method for the indirect determination of iodate by atomic-absorption spectrophotometry (AAS) has been described by Kirkbright and Johnson, Is but lacks specificity and is only useful when no other oxidizing species are present.

In this paper, a rapid, sensitive and simple flameless AAS method for the indirect determination of iodate is proposed. In neutral solution, iodate and mercury(I1) forms a stable ion-association complex with 2,2’-bipyridyl (bipy) extractable into methyl isobutyl ketone (MIBK). The mercury(I1) (and hence iodate) in the extract can be determined accurately by flameless AAS after stripping with 4A4 nitric acid. The method is highly sensitive and convenient and can be used for routine determination of iodate in certain high-purity chemicals.

EXPERIMENTAL

Reagents Mercury(H) standard solution, (5.00 mg/ml). Prepared by

dissolving Hg(NO,), . H,O in doubly distilled water con- taining 1 ml of 2M nitric acid per 100 ml, and diluted further with doubly distilled water as required.

Iodate standard solution (500 pgglml). Prepared by dis- solving potassium iodate in doubly distilled water, and diluted further as required.

2,2’-BipyridyQbipy) solution, 0.06 g per 100 ml of water.

Instrumentation A Shimadzu model 646 atomic-absorption spectro-

photometer with MVU-IA mercury analyser, was used under the following conditions: mercury hollow-cathode lamp current 6 mA, wavelength 253.7 nm, band-width 0.38 nm.

Procedure One ml of 10 pg/ml Hg*+ solution, 0.5 ml of 0.06% bipy

solution, 1.0 ml of sample solution, 0.5 ml of O.lM citrate-O.2M phosphate buffer solution @H 7.0) and 5 ml of MIBK were added to a separatory funnel in that order. The mixture was shaken for 1 min and let stand for 2 min to allow the phases to separate. The aqueous phase was discarded and the organic phase was washed with 2 ml of doubly distilled water. The washings were discarded and the organic phase was treated with 5 ml of 4M nitric acid. The concentration of H2+ was determined by flameless AAS after addition of 20 ml of 1M sulphuric acid, 20 ml of doubly distilled water and 2 ml of 10% stannous chloride solution. A reagent blank was prepared similarly.

RESULTS AND DISCUSSION

Optimization

The pH of the solution strongly affects the for- mation and extraction of the ion-association com- plex. The percentage of extraction is constant and maximal over the pH-range 6.95-7.05. Seven solvent systems were investigated, viz. iso-amyl acetate (37.2%), I-butanol (45.6%), benzene (71.6%), ethyl acetate (SS.OO/), n-butyl acetate (85.9%), chloroform (94.0%) and MIBK (99.8%), the degree of extrac- tion being shown in parenthesis. MIBK is the most efficient. Increasing the concentration of 2,2’-bipyridyl in the aqueous phase increases the degree of extraction, but the net extraction remains practically constant with 0.4-1.0 ml of 0.06% bipy solution, so a volume of 0.5 ml is recommended. The absorbance of the blank solution remains unaffected by the addition of bipy to the aqueous phase. The shaking for 1 min is sufficient for complete extraction and the phase separation takes about 2 min.

Effect of foreign ions

The determination of 1 pg of iodate is possible (within f2%) in the presence of various ions, such as Br- (2.50 mg); NO;, NO;, citrate (1.00 mg each); BrO; (500 pg); acetate (450 pg); PO:-, oxalate

669

670 SHORT COhUdUNICATTONS

Table 1. Determination of iodate in various samples

10, reported*, 10, found?, Samule lUlla, It UEllcK) It

1 NaNO,, G.R.; S.M. (Baroda) 25 28 2 NaNO,, A.R.; Basynth (Calcutta) 50 55 3 KNO,, A.R.; Basynth (Calcutta) 50 63 4 KNO,, A.R.; BDH (Bombay) 50 50 5 KI, A.R.; Glaxo (Bombay) 200 207 6 KI, G.R.; Merck (Bombay) 200 201

+Guaranti maximum content, as on the label. tAverage of 5 determinations.

(400 pg each); AsO:- (350 c(g); SO;- (325 rg); WO:- (310 pug); Fe3+, Cd*+, Cl-, VO, (300 fig each); A13+ (280 pg); Zn*+(250 pg); SCN-, SO:-, I- in presence of Cu*+, tartrate, Pb*+, Ba*+, Mn*+, MOO:- (200 pg each); Co*+ (170 pg); UOi+, Ni*+ Cu*+, F- (150 fig each); S,q-, S*- (100 pg each); Ag+ (80 pg); EDTA

(50 rg). To show the selectivity of the proposed method

in the presence of more than one foreign ion in the trace amounts stated on the labels of the bottles of NaNO,, KNO, and KI, studies on synthetic mixtures were made. The method was tested for IO; (1 pg) in the following mixtures: I, Fe3+ (150 rg) + Ba*+ (200 pg) + UO$+ (150 pg); II, MoOi- (150 pg) + Co*+ (150 pg) + Zn*+ (400 pg); III, Cu*+ (150 pg)+ I- (150 j@+CH,COO- (450 pg); IV, s*- (100 rg) + so:- (400 /4g) + PO:- (300 pg); v, SCN- (150 rg) + WO:- (200 pg) + AsO:- (270 pg); VI, Br- (300 pg) + NO; (110 pg) + VO; (200 fig); VII, Cl- (250 j& + C,O:- (250 c(g) + SO:- (150 pg); VIII, $O$- (100 pg) + Mn*+ (160 pg) + BrO; (500 pg), and found satisfactory.

Composition of the complex

To find the molar ratio of IO; to Hg*+ in the complex formed under the experimental conditions, the procedure was applied to a series of solutions with tied IO; concentration and increasing mercury con- centration. The mole ratio was found to be 1.9, in reasonable agreement with the expected value of 2. Therefore, the species extracted is presumably

[Hg(bipy)i+ IW; 12.

Analytical figures of merit

The calibration graph was linear up to 1.18 pg of

iodate and the limit of detectionI was 7.5 ng. The characteristic mass corresponding to 0.0044 ab- sorbance was 10.5 ng. The method was found to be more sensitive than the earlier methods.3*4*G’2 The relative standard deviation for 10 determinations of 1 .O p g of iodate was 2.4% and that for 15 deter- minations of 1.1 pg of iodate in the sample of G.R. NaNO, from S.M. (Baroda) was 6%.

Applications

The method was applied to the determination of IO; in different salts of high purity, e.g., NaNO,, KNO, and KI (Table 1).

Samples of NaNO, and KNO,. Each of the samples was dried in an oven at 50” then placed in a desic- cator, and 100 g of the dried sample were recrystallized from 50 ml of water by heating and cooling. The more soluble NaI03 or KI03 was retained in the mother liquor. A stock solution (50 ml) was made from the mother liquor by dilution with doubly distilled water. Iodate was determined in 1 .O ml of the stock solution by the recommended procedure.

Samples of KI. The samples were dried as above and 25-g portions were dissolved in doubly distilled water, and the solutions were diluted to volume in 100~ml standard flasks. Aliquots (1.0 ml) were anal- ysed. To each aliquot 35 ml of 0.05M silver nitrate were added, followed by 5 ml of 2M ammonia solution. The silver iodide was filtered off. The filtrates from five aliquots were combined (if 5 ml of the solution were used there would be risk of loss of iodate by adsorption on the large amount of silver iodide formed). The combined solution (-200 ml) was evaporated to a little less than 10 ml, and accurately diluted to 10 ml with doubly distilled

Table 2. Recovery of IO; from various samples

Sample IO, added, IO, found, Recovery,

Icg Pg %

1 NaNO,, G.R.; SM. (Baroda) 0 0.21 - 0.25 0.44 92 0.50 0.77 112

3 KNO,, A.R.; Basynth (Calcutta) 0 0.26 - 0.30 0.55 97 0.60 0.88 103

5 KI, A.R.; Glaxo (Bombay) 0 0.24 - 0.40 0.62 95 0.70 0.91 98

SHORT COMMUNICATlONS 671

water. The iodate in 1.0 ml of this solution was determined.

Recovery

Apparent recoveries of 92-l 12% were obtained for the determination of iodate in samples spiked with 0.25-0.70 pg of iodate (Table 2) to each of the following sample solutions. For sample No. 1,4.0 ml of stock solution were diluted to 10 ml and a l.O-ml aliquot of this solution was taken for recovery stud- ies. For sample No. 3, 2.0 ml of stock solution were diluted to 10 ml and a l.O-ml aliquot of the solution was taken for recovery studies. For sample No. 5, 1.0 ml of the final lo-ml solution was used directly for recovery studies.

Acknowledgemenr-The financial support of the University of Burdwan is gratefully acknowledged.

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