A Facile Spectrophotometric Method for the Determination of Cobalt ( II ) Using Iodine Monochloride Reagent

A novel, simple, sensitive spectrophotometric method is proposed for the determination of cobalt(II). Chloromine-T with iodine in acetic acid produces iodine monochloride, which oxidizes promethazine hydrochloride and diphenylamine to absorbing cations. Those would associate later with cobalt complex to form an ion pair, [Ph] [CoCl4] and [Dh] [CoCl4] 2 in acid medium. These appear to provide exceptional color stability to the systems. The results compare favorably with those of reported method. The conditions required for the determination of cobalt(II) are described and related analytical parameters are also calculated.


Introduction
Cobalt is a naturally occurring element that appears in the first transition series of Group 9 (VIII) of the periodic table along with iron and nickel 1 .Cobalt commonly occurs in the 0, +2 and +3 valence states 2 .Cobalt(II) is much more stable than Co(III) 2 .A biochemically important cobalt compound is vitamin B 12 .Vitamin B 12 is a cofactor in critical biochemical reactions and is required for good health 3 .The largest use of metallic cobalt is in super alloys 4 that are used in gas turbines and aircraft engines.

Experimental
UV-visible spectrophotometer with 10 mm matched quartz cells were used for absorbance measurements.

Chloromine-T-iodine reagent
Iodine monochloride solution was prepared by dissolving 0.6164 g of chloromine-T (Loba chemie, Bombay, India) in 5 mL of acetic acid in a dry beaker and 0.2143 g of iodine (S.D.fine chemicals Pvt. Ltd., Bombay India) separately in 5 mL of acetic acid.Both these solutions were transferred into a 50 mL volumetric flask and diluted to the mark with acetic acid.The solution was found to be 0.043 M (calculated) with respect to iodine monochloride.

Sodium diphenylamine (0.0073 mol L -1 )
Prepared by dissolving 0.2 g of Sodium diphenylamine (S.D.fine chemicals Pvt. Ltd., Bombay India) in 100 mL distilled water.All other chemicals, reagents and solvents used were of analytical grade.

General procedure
Aliquots of the standard Co(II) solutions containing 10 to55 µg/mL were transferred to a series of 10 mL volumetric flasks.To each one of these flasks 0.75 mL iodine monochloride (0.009 M) followed by 1.5 mL of promethazine hydrochloride were added.The volume was made up to 10 mL with water and mixed thoroughly.Absorbance of these solution were measured at 520 nm after 30 minutes making zero absorbance with distilled water.
Similarly for the reagent sodium diphenylamine, aliquots of the standard Co(II) solutions containing 5 to75 µg/mL were transferred into a series of 10 mL volumetric flasks.
To each one of these flasks 0.5 mL of iodine monochloride(0.043M), 1 mL of sodium diphenylamine and 1.5 mL of 1.25 M sulfuric acid were added.The volume was made up to 10 mL with water and mixed thoroughly.Absorbance of these solution were measured at 530 nm making zero absorbance with distilled water.

Results and Discussion
The method proposed here for the determination of Co(II) is based on the oxidation promethazine hydrochloride, sodium diphenylamine with iodine monochloride.Chloromine-T with iodine in acetic acid produces iodine monochloride.Iodine monochloride first oxidizes Co(II) to Co(III).Unreacted iodine monochloride oxidizes promethazine hydrochloride to absorbing cations 33 [34][35] which are appeared to be providing colour stability to the system.Ion-pair prevents the promethazine hydrochloride to further oxidation to colourless sulfoxides 36 .In case of diphenylamine, diphenylamine is first oxidized into colorless diphenylbenzidine 37 and is reversible further oxidized to diphenylbenzidine violet.Diphenylbenzidine violet undergoes further oxidation in presence of excess of iodine monochloride solution to diphenylbenzidinium cations 38

Application of the method for determination of Cobalt(II)
Various synthetic mixtures containing Cobalt were prepared and each was analyzed according to recommended procedure and the results were tested by known method.Results obtained are summarized in Table 1.They show a good agreement between the amounts added and estimated.

The effect of reagent concentrations
Various amount of 0.043 M iodine monochloride solution was added to a solution containing 30 µg of Co(II).The maximum constant absorbances were obtained by adding 0.75 mL of iodine monochloride solution (0.009 M) for promethazine hydrochloride and 0.5 mL of iodine monochloride(0.043M) solution for sodium diphenylamine.

Effect of acids on the color stability
Color stability and intensity of solutions were found to depend on the nature and concentration of acid used.The solutions with 2 M H 2 SO 4, 2 M HCl and 2 M CH 3 COOH were found to decrease the color with time and finally the solutions discharge their color completely in 10 minutes.But the solutions with 1.75 M CH 3 COOH for [CoCl 4 ] -[Ph + ] was found to be having color stable for 15 minutes.The absorbance of the colored solution was measured at various intervals of time (Table .2).The color intensity increases after 5 hours therefore it is recommended to measure the absorbance with in the first 5 hours.

Absorption curve
The absorption curve for ion-pair complexes was obtained by the recommended procedures.The absorption maxima of the colored solutions at 520 nm for [Ph + ] [CoCl 4 -] as shown in Figure 5 and for [Dh 2+ ] [CoCl 4 ] 2 -at 530 nm in Figure 6.

Calibration curves
The calibration curve for Co(II) was prepared by recommended procedures as shown in Figure 7 and 8. Linear relationships between absorbance and concentration held over range of 10 to 55 µg/mL for promethazine hydrochloride and 5 to 75 µg/mL for sodium diphenylamine and other parameters are given in Table 4.

Interferences
For the interference studies, 10 mg salt of the anion or 1 mg of the cation were added individually to a solution containing 30 µg of cobalt and determined by the general procedure.It was observed that, sulphate, acetate does not interfere.Among the cations studied, Cd(II), Zn(II), Mn(II),and V(V) did not interfere.Fe(II), Ce(IV) and Pb(II) interfered.Fe(II) and Fe(III) were masked with 2 mL of 5% sodium fluoride.Pb(II) was separated as PbSo 4 and Ce(IV) was separated as Ce(IV)phosphate.

Conclusion
Once the iodine monochloride reagent is prepared, it is stable for more than a week 41 .For a good reproducibility of the results, it is desirable to use the same iodine monochloride for the preparation of standard and test solutions of the Co(II).Persistence of color of the sodium diphenylamine solutions for than 24 h also with promethazine hydrochloride.The proposed method is economical, sensitive and the results are reproducible with a good stability in the color of the sodium diphenylamine solution and a very little stability in the color of the promethazine hydrochloride solution under the optimized condition.The proposed method offers several advantages.The procedure does not require any extraction, pH maintenance, coprecipitation, heating, centrifuge, any elaborate equipment and the method is less expensive.

Table 1 .
Determination of Cobaltl(II) in Synthetic mixture * An average of three determinations, ** Values are in µg, ▪ Fe was masked by sodium fluoride

Table 2 .
Stability of the color of the [Dh2+ ] [CoCl 4 ] 2 -complexThe effect of varying the concentration of sulfuric acid on [CoCl 4 ] 2 -[Dh 2+ ] complex.The rate of color development and the sensitivity of the reaction increased with increasing sulfuric acid concentration up to 1.25 M. Full color development occurred over this concentration and is stable for more than 5 hours.

Table 4 .
Optical characteristics of the proposed procedure