A Facile Spectrophotometric Method for the Determination of Periodate Using Azure B

Foi desenvolvido um método simples, rápido e sensível para a determinação de periodato em amostras de água de rio e em solução. O método é baseado na reação do periodato com iodeto em meio ácido, com liberação de iodo. O iodo liberado diminui a intensidade da cor violeta do azure B, medida em 644 nm. O decaimento da absorbância é diretamente proporcional a concentração de periodato e obedece a lei de Beer na faixa de 0,2 – 5,5 μg mL. A absortividade molar, a sensibilidade de Sandell e os limites de detecção e de quantificação encontrados foram 1,09x10 L mol cm, 1,75x10 μg cm, 0,07 μg mL and 0,20 μg mL, respectivamente. As condições ótimas de reação e os parâmetros analíticos foram avaliados. O efeito de íons interferentes é descrito. O método proposto foi aplicado com sucesso na determinação de periodato em amostras de águas de rio e em solução.


Introduction
The determination of iodine compounds, particularly periodate, I(VII) is important in organic chemistry, biology and life sciences, but it is rather difficult to determine I(VII) at low concentrations.2][3][4][5][6] Therefore simple, rapid, sensitive and accurate method is required for the determination of trace amounts of periodate in different samples.
In this work a new spectrophotometric method is described for the determination of trace quantities of periodate based on the reaction with azure B. The proposed method is rapid, simple, precise and accurate and is suitable for the determination of periodate in solution and different river water samples.

Experimental
Apparatus A Secomam Anthelie NUA 002 UV -Visible spectrophotometer with 1 cm quartz cell was used for the absorbance measurements and a WTW pH 330, pH meter was used.

Reagents
All chemicals used were of analytical reagent or chemically pure grade and double distilled water was used throughout the study.A standard stock solution (1000 μg mL -1 ) of periodate was prepared by dissolving 0.1205 g of potassium periodate in 100 mL.Hydrochloric acid ( 2 mol L -1 ), 2% potassium iodide, acetate buffer of pH 4 were used.A 0.05% solution of azure B was prepared by dissolving 0.05 g of azure B (molecular weight = 305.83 ) in 25 mL methanol and made up to 100 mL with distilled water.

Procedure
Sample solution containing 0.2 -5.5 μg mL -1 of periodate was transferred into a series of 10 mL calibrated flasks, potassium iodide ( 2% (m/v), 1 mL) then hydrochloric acid ( 2 mol L -1 , 1 mL) were added and the reaction mixture was gently shaken until the appearance of yellow color, indicating the liberation of iodine.Azure B ( 0.05%, 0.5 mL ) was then added and the reaction mixture mixture shaken for 2 minutes.The pH of the medium was adjusted by using acetate buffer of pH 4, then the contents were diluted to 10 mL with distilled water and mixed well.The absorbance of the resulting solution was measured at 644 nm against reagent blank.The amount of the periodate present in the volume taken was computed from the calibration graph.

Determination of periodate in river water samples
An aliquot of the spiked samples containing not more than 5.5 μg mL -1 of the periodate was treated with 1 mL of 2% potassium iodide reagent followed by 1 mL of 2 mol L -1 , HCl.The mixture was shaken well until the appearance of yellow color and then 0.5 mL of 0.05% azure B solution was added.The pH of the medium was adjusted by using acetate buffer of pH 4, then the contents were diluted to 10 mL with distilled water and mixed well.The absorbance of the resulting solution was measured at 644 nm against reagent blank .

Results and Discussion
The absorption spectrum of the azure B ( violet colored species) is presented in Figure 1 and the reaction mechanism is represented in Scheme 1.

Effect of iodide concentration and acidity
The effect of iodide concentration and acidity on the decolorization was studied with 1 μg mL -1 periodate solution.The oxidation of iodide to iodine was effective in the pH range 1.0 to 1.5, which could be maintained by adding 1 mL of 2 mol L -1 HCl in a final volume of 10 mL .The liberation iodine from KI in an acid medium was quantitative.The appearance of yellow color indicates the liberation of iodine.Although any excess of iodide in the solution will not interfere.It was found that 1-1.2 mL of 2% KI and 1 mL of 2 mol L -1 HCl were sufficient for the liberation of iodine from iodide by periodate and 0.5 mL of 0.05% azure B was used for subsequent decolorization.Effect of concentration of potassium iodide in reaction system is presented in Figure 2.

Effect of pH
The variation of absorbance of known concentration of the periodate with pH of the medium was studied.A series of buffer solution differing by pH 1 was prepared, and using each of these buffer, system was studied.The maximum absorbance value was found at pH 4. Hence, the pH was maintained at four throughout the study by using acetate buffer of pH 4. Effect of pH on color intensity is shown in Figure 3.
of interfering species were established at those concentrations that do not cause more than ± 2% error in absorbance values of periodate at 2 μg mL -1 (Table 1).The results indicated that Cu(II), Fe(III), Ce(IV), Se(IV) and iodate were found to interfere.However, the tolerance level for these ions may be increased by the addition of 1 mL of 1% EDTA.

Application
The proposed method was applied to the quantitative determination of periodate in solution and various river water samples, the results are presented in Table 2 and 3 respectively.Statistical analysis of the results by t-and Ftests showed no significant difference in accuracy and precision of the proposed and reference method. 17The precision of the proposed method was evaluated by replicate analysis of samples containing periodate at three different concentrations.

Analytical data
The adherence to Beer's law was studied by measuring the absorbance values of solutions varying periodate concentration.A straight line graph was obtained by plotting absorbance against concentration of periodate.Beer's law obeyed in the range of 0.2-5.5 μg mL -1 of periodate.The molar absorptivity and Sandell's sensitivity for colored system was found to be 1.09 10 5 L mol -1 cm -1 , 1.75 10 -3 μg cm -2 respectively.The detection limit (D L = 3.3 σ/ S) and quantitation limit (Q L = 10 σ/ S) [where σ is the standard deviation of the reagent blank ( n = 5 ) and S is the slope of the calibration curve] for the periodate determination were found to be 0.07 μg mL -1 and 0.20 μg mL -1 respectively.Ringbom's plot for the periodate determination is shown in Figure 2.

Effect of interfering ions
The effect of various ions at μg mL -1 levels on the determination of periodate was examined.The tolerance limits Table 1.Effect of diverse ions in the determinations (2.0 μg mL -1 ) of periodate

Table 2 .
Determination of periodate in river water samples Average ± Standard deviation; b RSD-Relative standard deviation; c Tabulated t-value for four degree of freedom at P(0.95) is 2.78; d Tabulated F-value for (4,4) degree of freedom at P(0.95) is 6.39. a

Table 3 .
Determination of periodate in potassium periodate solution using azure B as a reagent Tabulated t-value for four degree of freedom at P(0.95) is 2.78.
a Average ± Standard deviation; b RSD-Relative standard deviation; c