ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT 2001 © The Japan Society for Analytical Chemistry
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Simultaneous Voltammetric Determination of Bromide and Iodide in Salts Habib H.I. IBRAHIM, 1† Hassan N.A. HASSAN,1 and Stöß M.2 1†
2
Microanalytical Chem. Lab., National Research Centre, El-Tahrir Str., Dokki, Cairo, Egypt, （Email:
[email protected]） Vliesstoffwerk Christian Heinrich Sandler GmbH & Co. KG. Lamitzmühle 1, D-95126 Schwarzenbach, Saale, Germany.
A simple, high selective and sensitive procedure for simultaneous trace analysis of bromide and iodide in complex matrices was developed. Bromide and iodide in table salts with contents down to 2.4 and 0.6 ìg/ml, respectively, were oxidized to bromate and iodate with sodium hypochlorite solution for half an hour. After destroying the excess of oxidant by heating and adjusting the acidity to pH 3, cathodic differential pulse voltammetric measurements were carried out using hanging mercury drop electrode. The two reduction peaks of halates were well-separated and present six-folds amplification for the corresponding amounts of halides. (Received on August 9, 2001; Accepted on September 13, 2001) Iodide has been determined as an essential trace element necessary for synthesis of thyroid hormone. A daily requirement of iodide is 80-150 ìg/adult. Goiter may result from iodine deficiency. In the USA, table salt has been supplement with 100 mg of potassium iodide per gram of sodium chloride in order to prevent goiter. On the other hand, bromide has no known biological role. In proper dosage the bromide ion provides central depressant action. Excessive continued dosage may elicit a toxic condition, brominsin.1 Several methods for trace analysis of bromide and iodide in sea water or table salts were based on their oxidation individually followed by measuring their higher oxidation states directly either by polarography for iodate,2,3 by low power surfatron microwave induced plasma-AES for bromine4 or by solvent sublation spectrophotometry for iodine,5 or indirectly either by reaction with organic reagents for subsequent colorimetry for bromine,6-8 by reaction with reductants and subsequent ion chromatography for bromide,9 iodide,10 or by measuring the residual of oxidizing agent iodimetrically. 11 Apart of the polarographic measurements after oxygen flask combustion of authentic mixtures of halogenated organic compounds,12 none of these methods has been described for simultaneous determination of the cited ions in real samples using hanging mercury drop electrode and different modes of sweep.
Experimental Apparatus All cathodic voltammograms were recorded on a Metrohm VA693 processor equipped with a Metrohm Stand VA694 involving three potentiometric electrodes. The working electrode was hanging mercury drop electrode HMDE, while the auxiliary electrode was platinum wire. The reference electrode was Ag/AgCl in 3M/L KCl. All standard solutions of potassium bromide, iodide, bromate and iodate were of analytical reagents and freshly prepared in double distilled water. An acidic mixture solution of 0.04 M each of acetic, phosphoric and boric
acids were also prepared. The modified universal buffer series of Britton-Robinson, pH2-12, was also prepared as given by Britton.13 Procedure An appropriate amount of table salt, ca. 3 gm, was dissolved in 100 ml distilled water. A 5 ml of the test solution was then transferred to a 10 ml measuring flask containing 0.5 ml sodium hypochlorite solution (1.7%). The mixture was first left for 30 min to complete the oxidation at room temperature, ca. 25ºC, and then heated for 10 min in water bath to decompose the excess of the oxidant. After cooling, 1 ml of acidic solution was added and completed to the mark with distilled water. The pH of the solution was shown ca. 3-3.5 in most test samples. The solution was then transferred to a voltammetric vessel and pure nitrogen gas was purged through it for 2 min at 1 atmospheric pressure. The solution was allowed to rest for 10 s, the cathodic differential pulse sweep at HMDE was started from 0 to – 1300 mV with scan rate 60 mV/s and pulse amplitude 50 mV. The experiment was triplicated with a fresh mercury drop on the electrode in order to check the reproducibility within a maximum value of 5%. To determine the actual concentration of bromide or iodide in the sample, 0.05 µl of standard bromate and iodate solutions were added so that the resulting sequential peaks were nearly duplicated and triplicated, respectively. A blank was made to determine the concentration of any bromate or iodate in the hypochlorite solution.
Results and discussion Oxidation of bromide and iodide Among different oxidizing agents used for the oxidation of halide, sodium hypochlorite has been proved to be the strongest one. Depending on the acidity of reaction medium, different forms of oxidation states were yielded.14 Bromide was oxidized to bromate with excess hypochlorite in a weakly acidic solution, pH range 5.5-7, containing as much as 10% chloride. At the pH range 9-10, the reaction was
ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
Factors affecting on the cathodic voltammetric measurements of halate In all the following studies, unless stated otherwise, the solutions used were contained 0.6 and 2.4 µg/ml iodate and bromate, respectively, and the differential pulse mode of sweep was applied. 1. Effect of pH The peak height and position were clearly dependent on the pH values as shown in Fig. 1 and the bromate peak potential was disappeared at pH>4. The large half-width potential of bromate and iodate were interpreted by the highely irreversible nature of the electrode reaction. The buffer solution with pH 3 was selected for simultaneous determination of bromide and iodide. This effect was in good agreement with that obtained by Temerk et al.15
2. Effect of Pulse Amplitude ∆E The peak height was increased linearly with the pulse amplitude up to 90 mV (Fig. 2). The distance between two peak positions was maximized at ∆E=50 mV. 3. Effect of Scan Rate ν The peak height was increased linearly with scan rate up to 100 mV/s (Fig. 3) and in the same time the peak position was shifted slightly to more negative potential indicating the irreversible reduction character. The separation between the two peaks was approximately the same at different ν values and the 60 mV/s was selected as optimum parameter. 4. Effect of Different Modes of Sweep Based on the selected measuring parameters, viz., pH 3, ∆E = 50 mV, ν = 60 mV/s, different modes of cathodic sweeps, viz., direct current tast DCT, differential pulse DP, square wave SW and first harmonic current AC1, were studied as given in Fig.4.
Optimum ∆E value
-300
-350
potential, mV
completed in few minutes to form hypobromite quantitatively, whereas in the range 12-14, several hours were required. In acidic medium, below pH 3, other reactions involve the simultaneous formation of chlorate and bromate. On the other hand, iodide was oxidized first to iodine and then to iodate in the presence of bromide or chloride. Obviously, the pH range and the presence of chloride were essential to accomplish the hypochlorite oxidation of bromide and iodide to the corresponding halate ions. The reaction time needed for complete oxidation of iodide in different authentic saline media to iodate was 30 min,3 which has been confirmed experimentally in the present work enough to oxidize iodide and also bromide in brine solutions using 0.5 ml of 1.7 % sodium hypochlorite solution. The excess hypochlorite was then destroyed with a mild reducing agent such as formate, sulfite, hydrogen peroxide…etc., under conditions that do not reduce the halate and do not interfere with the current peak. Alternatively, the excess oxidant can be removed easily by boiling the solution for 10 min.13 After cooling to the room temperature, the sample was then transferred to voltammetric vessel where the bromate and iodate present a well-known six-folds amplification relative to the halide ions during the electrode reaction:
-950
Iodate 0.6 µg/ml Bromate 2.4 µg/ml
-1000
current, nA
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60 55 50 45 40 35 30 25 20 15 10 5 0 10
20
30
40
50
60
70
80
90
Pulse Amplitude, ∆ E
Fig. 2 Variation of pulse amplitude with current and potential using 0.6 µg/ml bromate, 2.4 µg/ml iodate solutions, pH 3 and differential pulse mode at ν = 60 mV/s.
Optimum pH value 200 half-width potential, mV
50 0
Iodate 0.6 µg/ml Bromate 2.4 µg/ml
-400
Potential, mV
100
-200
potential, mV
Optimum ν value
-300 150
-350
-950
-600
Iodate 0.6 µg/ml Bromate 2.4 µg/ml
-1000
-800 -1000
30
Current, nA
-1200
current, nA
30 25 20 15
25 20 15
10 5
10
0
2
3
4
5
6
7
pH
Fig. 1 Variation of pH values of Britton-Robinson buffer solutions with peak current and peak potential using 0.6 µg/ml bromate, 2.4 µg/ml iodate solutions, pH 3 and differential pulse mode at ∆E = 50 mV and ν = 60 mV/s.
5 20
40
60
80
100
Scan rate, ν mV/s
Fig. 3 Variation of scan rate with current and potential using 0.6 µg/ml bromate, 2.4 µg/ml iodate solutions and differential pulse mode at. pH 3 and ∆E = 50 mV.
ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT
-450
DCT
65
350
DP
-350 -300
current, nA
60 -400
-250 -200 -150
55
300
50
275
525
SW
325
AC1
500
iodate bromate
475
3.6 3.6 3.6 3.0 2.4 1.8 1.2 0.6
450 425 45
250
40
225
375
35
200
350
30
175
325
25
150
300
20
125
15
100
10
75
5
50
400
9.6 7.2 4.8 4.8 4.8 4.8 4.8 2.4
µg/ml
275 250
-100
225 200
-50 0
175
0 -300 -600 -900-1200
0 -300 -600 -900-1200
-300 -600 -900 -1200
-300 -600 -900 -1200
potential, mV
Fig. 4 Application of different modes of sweep using 0.6 µg/ml bromate, 2.4 µg/ml iodate solutions pH 3 and differential pulse mode ∆E = 50 mV and ν = 60 mV/s. Response of peak height vs. concentration of each ion is linear for all modes of sweep, as illustrated in Fig.5 and the regression parameters were given in Table 1. 100
150
90
DCT
SW
125
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Table 2. Trace analysis of bromide and iodide existing in various table salts using differential pulse voltammetry. Salt Bromide, % Iodide, % El-Nasr Saline Co. (Sailor), 0.003±5.1×10-4 * Egzpt El-Nasr Saline Co. (El-Max), 0.004±4.8×10-4 * Egzpt Rock salt deposits, 0.197±4.1×10-3 0.023±3.2×10-3 Yemen * The two salts were produced and marketed by evaporating seawater in large basins. Quantitative analysis of bromide and iodide in table salts Accordingly, the determination of bromide and iodide in different sources of table salts was accomplished by employing the optimum conditions of differential pulse mode, pH 3, ∆E = 50 mV and ν = 60 mV/s and using the standard addition method. The results were given as shown in Table 2.
References
80 70
current, nA
100
SW
1.
60 75 DCT DP AC1
50
2.
50 40
DP
3.
30 25
4.
20 10
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
1
2
iodate, µg/ml
3
4
5
6
7
8
9 10
bromate, µg/ml
Fig. 5 Variation of peak currents vs. concentration of bromate and iodate using pH 3 solution and different modes of sweep at ∆E = 50 mV and ν = 60 mV/s. Table 1 Regression parameters obtained on various sweep modes and different concentrations of iodate and bromate. DCT DP SW AC1 Iodate
Slope 15.231 13.386 38.792 17.96 Intercept -3.661 -0.5 -2.58 -13.839 R2 0.996 0.998 0.996 0.962 SD
1.297
0.914
3.369
Bromate Slope 9.811 Intercept -1.722
3.888 0.499
6.434 1.458
R2
0.992
0.996
0.994
SD
2.117
0.581
1.223
4.466
The best mode to yield the two well-separated peaks and to give minimum error of different concentrations of iodate and bromate was differential pulse voltammetric sweep. The response of peak height with concentration of bromate on employing the AC1 mode was experimentally not correlated totally for unknown reasons.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
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