INTRODUCTION:

Sulphur (thio) compounds are being increasingly employed in the field of analytical chemistry for developing new methods of quantitative estimation due to their ability to form complexes with a large number of metal ions^1-2. Such interactions are expected to bring significant changes in the polarographic characteristics of the metal ions, providing a sound base for the development of new amperometric methods. Organic sulphur compounds containing – SHgp. are also known to give anodic wave at d.m.e.

Thioglycolic acid (merceptoacetic acid) formulated as HSCH₂COOH and abbreviated as TGA in paper has often been used as a complexing agent and analytical reagent^3-5. However its capacity to act as an amperometric reagent has by and large remained unexplored. This present work was undertaken with a view to unravel this aspect of the acid and a unique amperometric method has been evolved for estimating of trace amount of Fe(III) and Fe(II) individually and in presence of each other.

MATERIALS AND METHODS:

Amperometric titrations have a very wide field of applicability and are characterized by high selectivity, rapidity and reliability. This technique does not require determination of the capillary characteristic and also the temperature need not to be adjusted. The equipment (manual polarograph) is not very expensive. All the titrations were performed at d.m.e. Vs S.C.E. using a Toshniwal manual polarograph. Analytical grade chemicals and double distilled water were used. We used hydrated sulphates of ferrous and ferric ion as source compound of Fe(II) and Fe(III) respectively. Stock solution of ferrous sulphate was standardized amperometrically.

Fresh ferrous sulphate solution was prepared in oxygen free water every alternative day. A 5% strength H₂SO₄ was always maintained in stock iron solution to check hydrolysis. TGA (merceptoacetic acid) solution was prepared a fresh daily and standardized. Amperometric estimation of each iron solution of any particular strength was carried out at least thrice. Purified nitrogen was employed for deaeration.

RESULTS AND DISCUSSION:

Determination of Fe(II):

Amperometric determination of Fe(II) was successfully carried out with the help of TGA in 0.1M KCl medium. TGA gives a reversible, one electron oxidation wave in this medium with the limiting region stretching from -0.0V to -0.15V (Fig. 1). Some Fe(II) compounds are known to give 2-electron reduction wave of the metal species in this medium⁶ (E_1/2= -1.3V), but ferrous sulphate did not give any such wave as hydrogen evolution started rather early presumably due to the fact that the iron solution contained 5% sulphuric acid.

Fig. 1.Polarogram of 0.5 mM TGA in 0.1 M KCl (Residual Current Excluded).

The titrations were carried out at -0.01V using Fe(II) as titrant, at which only TGA wave is present. If any Fe(III) present in ferrous sulphate solution then its reduction was taken with H₂S has as described by Vogel⁷. H₂S was boiled off after the reduction. Total absence of Fe(III) was checked by running a polarogram of the solution which did not show any Fe(III) wave. Since Fe(II) did not give its wave. The titration resulted in upside down L-shaped amperometric curve and Fe(II):TGA titrimetric molar ratio of 1:2 (Fig. 2).

Fig. 2.Amperometric titration curve of Fe(II) – TGA system in 0.1M KCl medium.

Titration voltage – -0.10V, conc. of TGA – 0.5mM, conc. of Fe(II) in the titrant – 5mM (279.23 ppm).

Determination of Fe(III):

Cathodic wave of Fe(III) as well as anodic wave of TGA were carried out at -0.10 volt, which fell in the diffusion region of both the species. Fe(III) gives a single reduction wave in 0.1M KCl medium⁸ with the limiting region starting from >0.0V. The electrode reaction involved in the reduction of Fe(III) to Fe(II)⁸. TGA also gave its one electron anodic wave in 0.1M KCl medium with the limiting region from -0.05V to -0.20V, E_1/2 being -0.30V.

Amperometric titrations were carried out at this potential using TGA solution as well as Fe(III) solution as titrant. On titrating Fe(III) solution with TGA at -0.10V, cathodic current was progressively compensated by the anodic current of TGA, until it reached zero value. Addition of further quantities of TGA led to appearance of anodic current. This increased linearly with further addition of TGA but slope of this line is slightly different from that of other arm of the titration graph. The two arms of the titration curve thus intersected each other at the residual current line. At this point, the molar concentration ratio of Fe(III):TGA was found to be 1:1 (Fig. 3).

Fig. 3.Amperometric titration curve of Fe(III) – TGA system in 0.1M KCl medium.

Titration voltage – -0.10V, conc. of TGA – 5mM, conc. of Fe(III) in the titrant – 0.5mM (27.92 ppm).

During titration of TGA with Fe(III) solution, initial anodic current decreased, approached zero and went to cathodic current but the titrimetric molar ratio remained the same. This method enabled the estimation of ferric solutions in the concentrations range of 5mM through 0.05mM (279.2 to 2.79 ppm).

There was no reduction in cathodic current of the Fe(III) wave, when titrations were carried out at a more negative potential (0.80V) where TGA was totally absent. Thus, indicating that the titrations were not a consequence of any chemical reaction. It is worth mentioning that the ration of id/c of TGA and Fe(III) is 1:1. Thus, confirming the current compensation phenomenon.

Table 1: Amperometric determination of Fe(II) and Fe(III) in mixture.

Composition of mixture Fe(III) + Fe(II) (ppm)	Fe(III) content in mixture determined after masking Fe(II) with 0.2M (NH₄)₂SO₄ (ppm)	%age error	Total iron content determined as Fe(III) (ppm)	Fe(II) content in mixture calculated (4-2) (ppm)	%age error
279.23 + 279.23	279.23	0.00	556.28	277.05	0.78
279.23 + 13.96	279.23	0.00	293.08	13.85	0.77
55.85 + 279.23	55.85	0.00	335.00	279.15	0.02
55.85 + 13.96	55.85	0.00	69.68	13.83	0.93
27.92 + 27.23	27.92	0.00	305.95	278.03	0.42
27.92 + 13.96	27.92	0.00	41.70	13.78	1.2
13.96 + 55.85	13.96	0.00	69.30	55.34	0.91
13.96 + 13.96	13.96	0.00	27.68	13.72	1.71

Table 2: Safe limit of the concentration of foreign ions in the cell solution with respect to the concentration of the metal species.

Foreign Ions	Determination of Fe(II)	Determination of Fe(III)
Foreign Ions	Determination of Fe(II)	Anodic titrations	Cathodic titrations
Clˉ	A	A	A
NO₃ˉ	A	A	A
CH₃COOˉ	Equal	Equal	Equal
S₂O₃²ˉ	1/20^th	1/20^th	1/20^th
OX²ˉ	A	A	Equal
WO₄²ˉ	A	A	1/10^th
Mg(II)	A	A	A
Al(III)	Equal	Equal	Equal
V(V)	a	A	1/10^th
Cr(III)	Equal	Equal	Equal
Mn(II)	A	A	A
Mn(VII)	a	A	a
Fe(II)	-	Equal	1/20^th
Fe(III)	1/10^th	-	-
Co(II)	5 times	Equal	A
Ni(II)	A	A	A
Cu(II)	1/20^th	A	1/20^th
Zn(II)	A	A	A
Mo(VI)	1/20^th	1/20^th	1/20^th
Cd(II)	1/20^th	1/20^th	Equal
Hg(II)	a	A	1/20^th
Cr(VI)	a	A	1/10^th

A = No interference even in the presence of 20 times excess of the foreign ion with respect to the metal species determined.

a = Serious interference even when foreign ion is 1/20^th of metal ion concentration.

Ampeormetric determination of Fe(III) in presence of Fe(II):

Fe(III) can be estimated in presence of Fe(II) directly by masking Fe(II) with 0.2M (NH₄)₂SO₄. (NH₄)₂SO₄did not allow it to react with TGA while its presence in the Fe(II) solution did not affect the Fe(III) wave or its amperometric estimation in any way. Thus, the titration of TGA solution with ferrous sulphate in presence of 0.2M (NH₄)₂SO₄did not bring about any reduction in the height of the TGA wave. This formed the basis for developing a method of determining Fe(III) in presence of Fe(II). Ammonium sulphate was added to the mixture solution of Fe(III) and Fe(II) before titration. This yielded quite accurate results for Fe(III) in its concentration range lied (5mM to 0.25mM) even when Fe(II) was present in concentrations twenty times greater than that of Fe(III). Presence of Fe(II) in still higher concentration of ammonium sulphate in the cell solution found to be good for Fe(III) concentration range given above.

Determination of Fe(III) and Fe(II) in mixture:

A satisfactory method for estimating both the iron contents in a mixture was also developed. The total of the mixture was amperometrically determined after oxidizing its Fe(II) contents with nitric acid⁹. Then, nitric acid was ultimately completely eliminated from the iron solution by concentrated H₂SO₄.

In a separate experiment, Fe(III) contents, only was determined by masking Fe(II) content of the mixture with ammonium sulphate as describe earlier. Then by subtracting this from total iron content, concentration of Fe(II) in the mixture was calculated.

Checking of interference of foreign ions and selectivity:

Study of interference of foreign ions in the new titrimetric methods was carried for three concentrations of each metal species viz. 5mM, 1mM and 0.5mM. A large number of foreign ions were quite well tolerated. Such foreign species did not interfere even when present together in the cell solution. Table 1 indicates amperometric determination of Fe(II) and Fe(III) in mixture while Table 2 includes safe limits of foreign ions in the cell solution.

REFERENCES:

1. Cheney GE, Fernando Q, and Frieser, H. Some metal chelates of mer- captosuccinic acid.J Phys Chem.1959; 63(12): 2055-2057.

2. Lenz GR and Martell AE. Metal chelates of mercaptosuccinic and α,α'-dimercaptosuccinicacids.Inorg Chem. 1965; 4(3): 378-384.

3. Domingo A and Molina J. Thioglycolic acid as a reagent for trace determination of Zn(II),Cd(II)and Hg(II).An Quim Ser.1983; B79-2: 174.

4. Banerjee S. Spectrophotometric determination of titanium with tannin and thioglycollic acid and its application to titanium-treated steels and ferrous and non-ferrous alloys.Talanta1986; 33(4): 360-362.

5. Banerjee S and Dutta RK. Spectrophotometric determination of niobium and its application to niobium-stabilized stainless steel.Talanta. 1974; 21(10): 1091-1094.

6. Meites L.Polarographic Techniques (2^nd Edition),Interscience Publishers, New York, 1965; pp. 628.

7. Yogel AI.A Text Book of Quantitative Inorganic Analysis (3^rd Edition), ELBS and Longman’s London, 1961; pp. 292.

8. Meites L.Polarographic Techniques (2^nd Edition)”, Interscience Publishers, New York, 1965; pp. 5-59.

9. Yogel AI. A Text Book of Quantitative Inorganic Analysis (3^rd Edition), ELBS and Longmans, London, 1961; pp. 471.

Received on 07.12.2010 Modified on 01.01 2011

Asian J. Research Chem. 4(4): April, 2011; Page 613-615