Simultaneous Determination of Cobalt and Iron Made Easy: Results from HPSAM

A novel protocol using H-point standard addition method (HPSAM) has been developed for simultaneous determination of cobalt and iron metals. We have herein focused on measurement of concentration of an analyte (cobalt) in the presence of other serving as the interfering species (iron). 4-[3-hydroxy-3-(4-methylphenyl) triaz-1-ene-1-yl] benzene sulfonamide (HTSN) was employed as the reagent in the presence of Triton x-100 which served as surfactant). The analytical signals were measured using two different wavelengths, l1(390) and l2(418). The analytical utility of adopted method has been tested and verified under the optimized conditions such as-pH of the medium, colour development of complex and reagent concentration etc. The molar absorptivity was found to be74413 and 41691 L mol-1 cm-1 with excellent root mean square value 0.9992 and 0.9984 for Co (II) and Fe (III), respectively. The RSD values were1.01 and 2.56 for Co and Fe, respectively. The whole methodology is simple, rapid and cost effective. We have successfully applied this method the analysis of Co (II) and Fe (III) in synthetic samples.


INTRODUCTION
Iron and cobalt metals have some similar chemical behavior and appear together or alone in many environmental and biological samples, especially in alloys.Iron is an essential element required for proper functioning of numerous metabolic activities like oxygen transport, enzyme activity, regulation of cell growth and differentiation 1 .On the other hand, cobalt is an essential microelement and the only metal found in vitamins (Vitamin B12) 2 .Cobalt compounds are efficient catalysts and are often used in a number of industrial processes.A number of methods have been projected for the simultaneous determination of different metal ions like derivative spectrophotometry 3 , atomic emission spectrophotometry 4 , H-point standard addition [5][6][7][8][9][10] and the partial least square method [11][12][13] .
H-point standard addition method (HPSAM) was developed way back in the year 1988, by Bosch-Reig and Compins-Falco [14][15][16] for simultaneous determination of metals.It is a simple bivariate chemometric technique based on dual wavelength spectrophotometry and modification of standard addition method [17][18][19][20] .This method is advantageous owing to its ability towards removal of both proportional and constant error, produced by the sample matrix [20][21][22] .Besides, it offers better resolution of the overlapping spectra of metal complexes 23 .This method enables calculation of concentration of one species in the presence of other interfering species 24 .Thus, the aim of the present paper is to find out the possibilities of spectrophotometric methods for simultaneous determination of Co(II) and Fe(III) with synthesized chromogenic reagent in aqueous media via HPSAM.
In the present method, two wavelengths were selected to measure different analytical signals of analyte and interfering species 25,26 .Under such optimized conditions, a calibration graph was constructed according to the standard addition method.The calibration lines for both the selected wave-lengths crossed at a point, called the H-point which indicated the additive effect of analyte and interferent concentration.

Materials
All chemicals used were of analytical grade and were employed directly without any further purification.Double distilled water was utilized throughout the work.Stock solutions of CoSO 4 .7H 2 O and Fe(NO 3 ) 3 were prepared in 100 ml volumetric flask and a few drops of conc.H 2 SO 4 and conc.HNO 3 , respectively, were added to prevent hydrolysis.Both solutions were subsequently standardized.Perchloric acid and Tris buffer solutions were used for adjusting the pH.The entire work was carried out using HTSN chelating reagent and Triton X-100 and all solutions were freshly prepared as per the need.

Apparatus
An Elico SL-210 double beam UV-Vis spectrophotometer was used for analyzing the spectrophotometric work and 1 cm quartz cuvettes were employed for absorbance measurements.Absorbance spectra were recorded in the range 350-600nm.The pH measurements were carried out using CHEMLINE Digital pH meter with ATC CL 120 (with combined glass electrode).

Preparation of HTSN Reagent
This method includes coupling of aryl hydroxylamine with diazonium salt of sulfanilamide in acetate buffer medium at pH5-6 with temperature between 0-5 0 C.The products of the coupling reaction were washed with distilled water and recrystallized using ethanol as solvent.Pale yellowish crystals of hydroxytriazene was obtained.The purity of the compound was checked using thin-layer chromatography (TLC) and melting point detection.The characterization of the compound was pursued with FT-IR, 1 H NMR and mass spectra.

General Procedure
The entire experiment was conducted using standard solutions of Co(II) and Fe(III) ions.Aliquots of varying concentration of Co(II) and Fe(III) with 5 mL of 1×10 -3 M HTSN solution, 2.0 mL of Triton X-100 (10% v/v, surfactant to solubilize the reagent) and 2.0 mL of Tris buffer (1%) were taken in a 10 mL volumetric flask.The volume of the solutions was made up to the mark with double distilled water and was left for at least 10 min at 25 0 C. The absorption spectra were recorded at 406 and 395nm wavelength against reagent blank for the individual calibration of Co(II) and Fe(III).

H-point standard addition method
All steps of the HPSAM procedure were performed in a systematic way.During the first stage of the procedure, mixtures of synthetic solutions with different concentration were prepared from standard solutions of cobalt and iron.Further, 0.4 mL of synthetic mixtures, 5 mL of HTSN reagent, 2 mL of tris buffer (1%) and 2 mL of triton X-100 (10%) with subsequent standard additions of additive Co(II) ranging from 1x10 -5 to 5x10 -5 were taken in a 10 mL volumetric flask and the solutions were diluted with double distilled water.The solutions, thus obtained had varying concentration of Cobalt, while the concentration of iron was kept constant.The samples were dispensed to a quartz cell and the absorbance was measured at the two selected wavelengths mentioned in HPSAM system.
For preparing the H-point graph, the absorbance data obtained at the two selected wavelength (l 1 and l 2 ) were used with the standard addition of Co(II) concentration solution.The two straight lines were plotted for absorbance and concentration, both derived lines were allowed to cross at a point by extrapolation, and the intersection point of these two lines was termed as CH and AH respectively.The Co(II) ion concentration was evaluated from CH and the concentration of Fe(III) was evaluated from AH.

RESULT AND DISCUSSION
The simultaneous determination of Co(II) and Fe(III) with reagent in the presence of Triton X-100 by the use of suggested method has been performed.All the work was carried out in micellar media, which solublized the insoluble component in water and also aided in eliminating the extraction step using organic solvent.

Individual calibration
Hydroxytriazene is an excellent chelating agent.Hence, the present work describes simultaneous determination of Co(II) and Fe(III) using hydroxytriazene by HPSA method.The entire metal determination procedure was performed using UV-spectrophotometer under optimal conditions.Individual calibration of Co(II) and Fe(III) with reagent were measured in the wavelength range 380 to 600nm against reagent blank solution.The wavelength at which maximum absorbance was observed, selected as the working l max for individual binary complex.The working wavelength in this case for Co(II) was 402.5nm and for Fe(III), it was found to be 395nm.HTSN reagent forms colored complexes with Co(II) and Fe(III).It was observed that complexation reaction of Co(II) with the ligand HTSN was faster than that of Fe(III) under same conditions.The absorption spectra for the individual colored Co(II)-HTSN and Fe(III)-HTSN complexes overlapped with each other, which was not suitable for the spectrophotometric measurements of the ions.Since the spectra overlapped, Co(II) and Fe(III) could not be determined simultaneously.To address this issue, HPSA method was used as an accurate and simple method for simultaneous determination of both metals.The absorption spectra of Co(II) and Fe(III) complexes with reagents are shown in Figure 2.

Effect of pH
The influence of the medium pH on spectra of complexes of Co(II)-HTSN and Fe(III)-HTSN is the most significant parameter for further study.pH study was carried out using perchloric acid and tris-buffer in the pH range of 3-10 as shown in Fig. 3.It was observed that analytical signals varied with pH.Absorbance of Fe-hydroxy complex increased till pH 7, after which the absorbance decreased, whereas the Co complex showed maximum absorbance within7 to 9 pH range.Therefore, a pH range of 7-8 was selected for further study.

Fig. 3. Effect of pH on absorption spectra of Co(II)-HTSN and Fe(III)-HTSN complex
Effect of surfactant Since, the obtained complexes were insoluble in water, Triton X-100 was selected as the working micellizing agent to dissolve the insoluble part in water.The effect of surfactant on optimum absorbance of Co(II) and Fe(III) complexes was studied.The obtained results revealed that the absorbance of the solutions increased upon usage of surfactant and it also increased the solubility and stability of the complexes in aqueous media.

Validation of Beer's law
For the validation of Beer's law, sixteen set of each binary complex of Co(II) and Fe(III) with reagent at variable concentration in 1:10 ratio were prepared under optimized conditions and absorbance at selected wavelengths were recorded.The results have been presented in Fig. 4. It was observed that the absorbance increased with the increase in concentration of reagent.Thus, the exact concentration of reagent with Co(II) and Fe(III) was used for the study of HPSAM.
of various concentration of Co analyte, whereas the signals of Fe(III) do not change.The Co(II) ion concentration was determined by the HPSAM.These can be expressed by following equations.
A 390 =M 390 C Co +b o +b (1) Where, A 390 and A 418 are the two analytical signals, two straight lines obtained intersect at H-point (-C Co , A Fe ), shown in Figure 5.
The Co ion concentration was calculated by HPSAM system using the equation of two straight lines, whereas the concentration of Fe was determined by the analytical signal from the calibration graph as shown in Figure 4.

H-point standard addition method
The selection of two wavelengths l 1 and l 2 werethe main requirements for HPSAM.The essential condition is that absorbance of analyte must be linear with concentration and the interfering signal must remain unaffected with change in analyte concentration.The analytical signal of mixture which contain analyte and interfering species should be equal to the sum of the individual signals of the two compounds, the difference in the slopes of the two straight lines should be as large as possible to get good accuracy.In the proposed method, Co(II) and Fe(III) ions were used as analyte and interfering species, respectively.A known concentration of Co ion was consecutively added to the mixture, after the addition of analyte and the absorbance was measured at selected wavelength (l 1 =390 and l 2 =418nm) against reagent blank.As mentioned above in Fig. 1, at the two selected wavelengths, the Co(II)-HTSN complexes signal increases linearly with the addition Reproducibility of the method To investigate the reproducibility and success of this method, total five replicate samples were prepared for determination of Co(II) and Fe(III) with the addition of analyte concentration.The binary mixture solution was poured into a1 cm quartz cuvette and the spectra were screened at selected wavelength.The analytical results obtained from HPSAM were checked and tested by the repeatability of the process.The temperature was kept constant while performing the spectrophotometric work.

CONCLUSION
The proposed method was established for the simultaneous determination of Co and Fe.In addition, to check the accuracy of this method several synthetic samples were prepared with different concentration of Co ion and the spectra of both complex (Co(II)-HTSN and Fe(III)-HTSN) were plotted simultaneously at selected wavelength (l 1 and l 2 ) as shown in Fig. 4. The results obtained from the sample containing the interfering ion varied from each other, so two calibration graphs were intersected at a common point (H-point) and it provided sufficient data for reliable determination of Co(II) and Fe(III).Thus, HPSAM shows satisfactory analytical results for the simultaneous determination of Co and Fe using HTSN reagent in aqueous medium and the developed method is simple, cost effective, rapid and precise.However, to enhance the reproducibility, the same process could be repeated with different synthetic ligands.