Spectrophotometric Determination of Tellurium ( IV ) with Chrome Azurol S : Application to Sea Water and Synthetic Alloys

The detailed investigation of a sensitive spectrophotometric method for the determination of trace amounts of tellurium (IV) is described. The method is based on the reaction of tellurium (IV) with chrome azurol S in the presence of cetylpyridinium chloride hydrate surfactant at a final pH of 3.1 to form a pink coloured chelate absorbing maximum at 525 nm. Beer’s law is obeyed over the range (5-50) μg/25 ml, with a molar absorptivity of (2.5 ×10) l.mol.cm and a coefficient of determination (r) of 0.9997. Sandells sensitivity index is (5.104×10) μg.cm, a relative error of (0.8-0.2%) and a relative standard deviation of (0.2 to ±1.8) %, depending on the concentration levels in the calibration curve. The method has been applied successfully to determine Te (IV) in sea water and some synthetic alloys.


INTRODUCTION
Tellurium came from the Latin word tellus that means earth, it can be found in dioxide form, the properties of tellurium included a silvery-gray colour, boiling point of 1810˚F, melting point of 841.12˚F.In air, tellurium burns in a greenish-blue flame forming the dioxide, TeO 2 is highly insoluble in water and soluble in concentrated sulfuric acid.It reacts with acids to make tellurium salts and bases to make tellurites (Mediawlki, 2012).
Tellurium's conductivity can also increase if exposed to light.Tellurium can be used in ceramics.It can be added to lead to prevent corrosion and strength (Whinnie,1994).
The impact of tellurium supply on cadmium telluride photovoltaics modules have become the lowest-cost producer of solar electricity, despite working at lower efficiency than crystalline silicon cell (Ken, 2010).
A spectrophotometric method for the determination of palladium, iron and tellurium from nitric acid media after the extraction of their p-[4-(3,5-dimethyl soxazol-yl) azophenylazo]calyx(4) arene complexes has been developed with possible synergistic effects (Kumar et al., 2008) Atomic absorption spectrometric determination of trace tellurium after hydride trapping on platinum-coated tungsten coil has been worked out.This method was applied to the determination of tellurium in several geological standard reference materials, by hyphenation of electrically heated quartz tube atomic absorption spectrometry and tellurium hydride trapping on platinum-coated tungsten coil.With a mixture of Ar and H 2 , tellurium hydride was transported to tungsten coil for trapping at 390˚C and releasing at 1200˚C (Maoyang et al., 2010).
Te (IV) and Te (VI) were determined by employing dispersive liquid-liquid microextraction combined with electrothermal atomic absorption spectrometry using palladium as a permanent modifier.Under acidic condition (pH1), only Te (IV) formed a complex with ammonium pyrrolidine dithiocarbamate (APDC) and therefore Te (IV) was determined in the sedimented organic phase while Te (VI) remained in the aqueous phase (Najafi, et al., 2010).
Determination of bismuth, selenium and tellurium in nickel-based alloys and pure copper by flow-injection hydride generation atomic absorption spectrometry with ascorbic acid prereoduction and cupferron chelation extraction was performed in phosphate buffer.For H 2 Te, the linear range for the calibration curve was 0.5-12 ng/ ml (Hui-Ming et al., 2002).
The reagent used in this investigation chrome azurol S (CAS) is trisodium salt of sulphodichlorohydroxy dimethyl fuchson dicarboxylic acid.The structural formula of the reagent is: Chrome Azurol S (MW = 605.28g/ mole) The above reagent has been used to determine indium (Poledniok and Buhl, 2006), vanadium (IV) (Starczewska, 2004) and cobalt (Mushran et al., 2004) as ternary (in the presence of surfactant) or as binary complexes.
The present study involves the determination of Te (IV) using the reagent CAS.To the best of our knowledge, such investigation has not yet been touched.

EXPERIMENTAL Apparatus
Spectral and absorbance measurements were carried out using CECIL CE7200 Aquarius spectrophotometer with 1-cm matched quartz cells.
The pH measurements were carried out using professional Beuchtop pH meter Bp3001.
This solution was prepared by dissolving 0.0125 g of tellurium dioxide (Hopkin and Williams) in 2 ml concentrated HCl solution and the volume was completed to 100-ml with distilled water in a volumetric flask.Working Tellurium (IV) (50 µg/ ml) solution.This solution was prepared by diluting 50 ml of the above stock tellurium (IV) solution to 100 ml with distilled water in a volumetric flask.Chrome Azurol S (CAS) reagent (1×10 -4 M) solution.
It was prepared by dissolving 0.00605 g of Chrome Azurol S (Merck) in distilled water and the solution is diluted to 100 ml in a volumetric flask and then transferred to a dark bottle, where it was stable for at least seven days.

Buffer solution (pH10)
This solution was prepared by mixing 17.41 ml of 0.02 M sodium bicarbonate solution and 21.89 ml of 0.01 M sodium carbonate and the volume was diluted to 100 ml with distilled water in a volumetric flask (Perrin and Dempsey, 1974).

Cetylpyridinium chloride hydrate (CPC) (1×10 -3 M) solution
This solution was prepared by dissolving 0.0358 g of cetylpyridinium chloride hydrate (CPC)(BDH) in distilled water and the volume was completed to 100 ml in a volumetric flask.A 100 µg/ ml solution of each ion tested was prepared in distilled water.

Synthetic sea water.
It was prepared as described (Herkinson, 1965) by dissolving 2.2 g NaCl, 9.7 g MgCl 2 .6H 2 O, 3.7 g Na 2 SO 4 , 1 g CaCl 2 , 0.65 g KCl, 0.17 g NaHCO 3 and 0.023 g H 3 BO 3 in about 500 ml of distilled water, then the solution was diluted to 1 l with distilled water in a volumetric flask.

RESULTS AND DISCUSSION Preliminary Investigations
For subsequent investigations, 50 µg of Te (IV) were taken in a 25 ml final volumes.Upon the addition of CAS reagent to Te (IV) solution, an orange-red species was formed against the corresponding yellow blank solution.

Optimization of Experimental Conditions
The effect of various factors that may affect the analytical characteristics of the coloured species has been investigated and optimal experimental conditions are selected.

Effect of pH
The effect of pH of the final reaction solutions is studied by adding different volumes of (0.01) M HCl or NaOH to the mixture containing 50 µg of tellurium (IV) and 5 ml of 1×10 -4 M CAS.The spectrum was scanned for each coloured solution against its corresponding reagent blank, blank against distilled water and the final pH was measured.The results obtained were given in Table (1).From the results in Table ( 1), it can be shown that pH values (2.96-3.48)were favorable for the formation of the coloured compound.Furthermore, the wavelengths of maximum absorption of both standard and blank were constant.The pH was 3.1 although 11 ml of 0.01M NaOH solution was added because the element was originally dissolved in some strong acid medium.
At the optimum amount of NaOH (11 ml of 0.01M ) solution, buffers of pH10 (sodium carbonate + sodium bicarbonate) and pH12 (NaOH, KCl buffer) (Perrin and Dempsey, 1974) have been tried.pH10 gives more useful results than pH12 and, therefore, were incorporated in the subsequent experiments.Also, pH10 gave more useful results in terms of colour contrast while NaOH solution gave lower blank value.The experimental results were represented in Table (2).According to the results in Table 2, 6 ml of pH10 buffer solution was chosen for the subsequent work, since it gave a good absorbance for the standard and lower blank value.
For the final pH of 3.1, pH10 and pH12 have been selected for the examination.These higher pH values buffers were chosen because the Te element was initially dissolved in acid solution (see experimental).The effect of the amount of buffer solutions on the absorbance of the final coloured solutions was shown in Table (2).

Effect of Reagent Amount and The Concentration of Tellurium (IV)
The formation of the coloured complex has been completed using 7 ml of 1×10 -4 M CAS, the results were shown in Table (3).The coefficient of determination correlates that 7 ml of 1× 10 -4 M CAS was the optimum amount for the determination of (5-50) µg Te (IV) ion.

Effect of Surfactants
The effect of different types of surfactants (cationic, anionic and non-ionic) with different concentrations on the intensity of the coloured complex with different orders of addition has been examined and the results were shown in Table (4).
The presence of CPC surfactant afforded both hyperchromic effect and bathochromic shift (515 nm to 525 nm) which are both admired from the analytical point of view.

Effect of time and CPC Surfactant
The effect of time on the colour intensity of the Te (IV)-CAS chelate produced from different amounts of tellurium (IV) ion has been explored.The maximum formation of the coloured complex took place after 5 min, and remained stable for about 60 min at least in the presence of CPC, while in the absence of CPC lower absorbencies were observed and less efficient stability was attained.The results are shown in Table ( 5)

Final Absorption Spectra
The absorption spectra have been obtained on solution containing 50 µg Te (IV), 1 ml of 1×10 -3 M CPC, 6 ml pH10, 7 ml of 1× 10 -4 M CAS, and the absorption spectra of the coloured complex are shown in Fig. 1 at 525 nm, while the reagent blank gives maximum absorption at 423 nm.

Accuracy and precision
To check the accuracy and precision of the calibration graph, tellurium (IV) was determined at three different concentrations.The data given in Table 6 indicate that the results are satisfactory.

Nature of the complex
Job's method of continuous variations and mole-ratio method (Hargis, 1988) was used to evaluate the combining ratio between Te (IV) ion and CAS chelating agent.The results revealed that the ratio of tellurium (IV) to CAS is 1:2.Also, Job's method of continuous variations was used to determine the proportion of CPC in the complex.The results revealed that the components react in a ratio of 1: 4 Te to CPC.The structural formula of the chelate may be postulated as follows:

Effect of organic solvents
Different organic solvents were used to examine their effects on the Te (IV)-CAS coloured complex.The results are shown in Table ( 7) and (Fig. 3).As shown in Table (7), water seems to be still the most favorable solvent from the analytical point of view.

Study of interferences
In order to assess the analytical applications of the proposed method, the effects of foreign ions were examined by adding known amounts of each foreign ion to a solution containing 50 µg of tellurium (IV), and then determining the latter using the recommended procedure.The results obtained are summarized in Table (8).The results in the above table indicate that a high negative and positive errors were observed in the case of Al (III), Bi (III), Th (IV), Ba (II), Mn (II), Pb (II), Se (IV), Cu (II), SCN -and CO 3 ( -II) due to the competition of these ions with Te (IV) for the reagent (appearance of blue colour) there by depleting the reagent available for the Te (IV) ion.

Application of the method
The method was applied to the determination of tellurium ion in synthetic composite mixture first and in sea water second.The results are listed below: 1. Determination of tellurium (IV) in synthetic composite mixture Mixtures containing cations and anions and 50 µg of tellurium (IV) were analyzed for tellurium.The results are given in Table (9).Other composite mixtures were tried but results were unsuccessful.It can be observed Table (9) that the method gives a successful recovery of tellurium (IV), and applicable for all cases except for mixture (III)

Determination of Tellurium (IV) in sea water
The following Table shows the results obtained for the determination of Te (IV) in sea water which was prepared in the laboratory (Henriksen, 1965).The results in Table (10) were satisfactory to some extent and recovery ranged from 89.7 to 103.2%.

Comparison of the method
Table (11) shows the comparison between the analytical variables of the present method with another method (Marczenko, 2000).

Fig. 1 :
Fig. 1: Absorption spectra of 50 µg Te (IV) treated as under optimum experimental conditions and measured (A): against blank, (B): against distilled water and (C): blank measured against distilled water.

Table 7 :
Fig. 3: Effect of organic solvents on the spectral properties of Te (IV) -CAS chelate.

Table 2 : Effect of buffers on absorbance Absorbance / mL of buffer added pH10 at λ 515 nm* pH12
*With buffers, λ max shifts from 506 nm to 515 nm a bicarbonate carbonate buffer.

Table 6 : Accuracy and Precision
*For six determinations

Table 10 : Determination of Te (IV) in sea water sample
*Average of three determinations.