INTRODUCTION:

Chromium salts are extensively used in industrial processes such as metal Pickling, electro plating, leather tanning, Aluminum anodizing etc. Several workers^1-8 reported the extraction of metals by high molecular weight as well as tertiary amines including chromium. Fewer studies appeared in literature on the extraction of chromium (VI) by amines especially in different acid media. In the present communication the results obtained on the extraction of Chromium (VI) by Tribenzyl amine (TBA) from hydrochloric, sulphuric and phosphoric acid solutions has been discussed.

EXPERIMENTAL:

A 0.25 M stock solution of TBA (Mol. wt. 287.398) in chloroform was prepared and diluted appropriately to get the required concentration. Chromic acid (E.Merck) was used for preparing Chromium (VI) stock solution (0.5M) used as such without any further purification. All other chemicals used were of AnalaR grade or samples purified according to the standard methods. Double distilled water was used throughout.

CHROMIUM (VI) EXTRACTION:

An aliquot (10ml) of chromic acid and mineral acid was equilibrated with an equal volume of TBA in chloroform (0.05M) pre-equilibrated with 0.1M mineral acid.

The chromium (VI) concentrations in the aqueous phase before and after extraction was estimated spectrophotometrically⁹ by measuring the absorption of Cr (VI) – DPC complex at 540nm, using Shimadzu UV-Visible Spectrophotometer type UV-260. The equilibrium chromium (VI) concentration in the organic phase was determined by taking the difference in the initial chromium (VI) concentration and the equilibrium chromium (VI) concentration in the aqueous phase.

RESULTS AND DISCUSSION:

The results obtained on the variation of distribution ratio as a function of aqueous phase concentration of mineral acid (HCl, H₂SO₄ and H₃PO₄) are presented in Fig-1. In the case of hydrochloric and sulphuric acid solutions the extraction of chromium (VI) by TBA in chloroform as a function of acidity, the distribution ratio (K_d) increased with increasing the concentration of the acid up to 0.75 M & 0.25 M acidity respectively followed by a gradual fall up to 3.0 M, beyond which chromium (VI) under goes reduction to chromium (III). The extractions are nearly quantitative from both the acid solutions (Fig -1). On the other hand the extractions are partial from phosphoric at acid solutions with a maximum extraction at 0.25 M followed by a gradual fall in K_d with increasing acidity.

COMPOSITION OF THE EXTRACTED SPECIES:

The composition of the extracted species with all the acid systems was determined by the extraction isotherm method¹⁰ and distribution ratio method¹¹. In the extraction isotherm method the limiting ratio of the metal to TBA was found to be unity irrespective of the acid used in the study. (Fig-2).

The log-log plots of K_d Vs. TBA from various acid solutions gave straight lines. With hydrochloric acid solutions the log-log plot gave straight line of unit slope (Fig-3). On the other hand, the slope analysis of the distribution data in sulphuric and phosphoric acid solutions indicates that the solvation number is two.

The individual chromium (VI) species can be identified on the U.V. region^12-14. The absorption spectra of chromium (VI) in the organic phase obtained by extraction from different acid media exhibit absorption bands at 245, 285 and 355 nm. A comparison of the ratio of the molar extinctions (e) at 285 nm and 355 nm a function of acid molarity is taken as a criterion for identifying the species as chloroform has cut off point at 245nm. These ratios are independent of the concentration of all the acids used in the study. The peak ratios are constant ie., unity from hydrochloric and two from other acid systems confirming the presence of a single species (Table-1). This is in conformity with the observation of Tuck and walters¹⁵using TBP as extractant.

Table-1: Percentage Extraction of chromium (VI) by TBA from various acid solutions

[Cr(VI)] = 0.0005M

[TBA] = 0.035 M

Acid (M)		Molar Extinction (E) at		(_{285 /}_{355 )}
Acid (M)		285nm	355nm	(_{285 /}_{355 )}
HCl	0.1	1185	1065	1.11
	0.5	1110	1085	1.02
	1.0	1250	1170	1.07
H₂SO₄	0.1	1980	1010	1.96
	0.5	1970	995	1.98
H₃PO₄	0.1	1960	1025	1.92
	0.5	1875	985	1.90
	1.0	1905	955	1.99

Based on the result obtained, the observed chromium: TBA molar ratio of two from all the acid solutions (by distribution ratio method) could be explained as arising from the extraction of chromium (VI) by the following salvation mechanism.

From Hydrochloric acid solutions:

TBA H⁺₊CrO₃Cl^-_aq Û TBA HCrO₃ Cl

From other acid solutions:

2(TBA) H⁺_(aq)+ Cr₂O₇^-2 Û (TBAH)₂ Cr₂O_7
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The decrease in the distribution ratio at higher acidities (>3.0M) can be explained as due to the greater extractability of the mineral acid over the chromium (VI) species.

VARIATION OF DILUENTS:

Besides chloroform various diluents used in the present study are benzene, xylene, toluene, carbon tetra chloride, hexane, cyclo hexane, nitrobenzene, dichloro methane which are of wide verities in their chemical nature and dielectric constant. Maximum extraction efficiency was achieved with chloroform as diluent (Table-2). Hence the same diluent was used in all these studies. With, dichloromethane, cyclo hexane and toluene low % extraction was noticed (70 to 80%).

Table-2: Effect of Diluents on Extraction

[Cr(VI) = 1.0 x 10^-3 M [TBA] = 5.0 x 10^-2 M

(From HCl medium)

Diluent	Dielectric constant	% extraction
Benzene	2.28	99.65
Xylene	2.38	92.68
Toluene	2.3	80.4
CCl₄	2.24	93.75
CHCl₃	4.8	94.05
n-Hexane	1.90	85.7
Cyclo hexane	2.02	80.55
Nitrobenzene	36.10	88.3
Dichloro methane	8.08	68.15

CHOICE OF STRIPPING AGENT:

The extraction of chromium (VI) from the organic phase (TBA) has been tried with 10 ml reagents of various concentrations (0.01 – 0.1 M) of HCl, HNO₃, NaCl, NaNO₃and NaOH solutions. It was observed that HCl, NaCl, and NaNO_3, are extremely poor stripping agents for chromium (VI). On the other hand, 1.0 M NaOH alone is a good stripping agent. However in no case NaOH strips out all the chromium (VI) in a single extraction. It was observed that 99.8% chromium (VI) could be recovered from organic phase by making contact three times with equal volumes of 1.0 M NaOH_.

Table-3: Selective Recovery of chromium (VI) in the presence of other ions

Sample		Results	Standard deviation
1.	27 µg Cr + 10 mg Cu, Co, Ni + 1.0 M H₂ S0₄	27.0 26.8 26.8 26.8 26.9 27.0 27.0	0.09
2.	54 µg Cr + 10 mg Cu, Co, Ni + 1.0 M H₂ S0₄	53.5 53.7 53.7 53.7 54.0 54.0 54.0	0.19
3.	54 µg Cr + 30 mg Cu, Co, Ni + 1.0 M H₂ S0₄	53.8 53.7 54.0 53.5 54.0 54.0 53.8	0.17
4.	54 µg Cr + 10 mg V + 1.0 M H₂ S0₄	54.0 54.0 53.8 53.9 53.7 5.8 54.0	0.11

EFFECT OF DIVERSE IONS:

The effect of several foreign ions on the extraction of chromium (VI) with TBA was studied following the general extraction procedure. The tolerance limit was set at the amount of diverse ion required to cause ± 2% error in the recovery of chromium (VI) (13.5 mg/ 20 ml). The results show that the ions such as Al (III), Ba (II),Cu (II), Ce (IV), Mn (II), Co (II), Th (IV), Zn (II), Acetate, Ascorbate, Chlorate, Oxalate Selenite, Tellurite, Phosphate and Tartarate, are tolerated in the ratio 1:200 and the ions such as Ca (II),Pb (II), Pd (II), Ru (III), U (VI), Sr (II), VO₃^-,WO₄^2-_, and malonate^2-, do not interfere even if present in the ratio 1:100. The ions showing small tolerance limit in the ratio 1:50 are Fe (III) and F^-. It is thus possible, to extract chromium (VI) in the presence of large number of cations and anions. The average recovery of chromium (VI) was 99.7 + 0.3 %. The relative standard deviation and relative error calculated from ten repeated determinations with 13.5 mg of Cr (VI) were found to be ± 1.02 % and ± 0.8 % respectively.

The separation of chromium (VI) is possible in the presence of more than one foreign ion in the mixtures with an error of not more than 2%.

The precision and accuracy of the method of extraction for recovery of chromium has been tested by analyzing synthetic samples and chrome alloys. It is done by extracting chromium from synthetic samples (composition Cr = 0.25 – 0.45 g/lt).

About 0.5 gm of a chrome alloy (Type 303, 410 & 416) was dissolved in 10ml aquaresia. The solution was evaporated and extracted with 20ml of dil. hydrochloric acid solution to dryness. The resulting solution was made up to 100ml.An aliquot (10ml) of the filtered solution was taken. The interference due to Ni (II) & Mn (II) were removed by precipitating them as complex. Then precipitate was filtered and quantitatively washed for complete recovery of chromium. It was made up to 100ml.10.0 ml of the solution was extracted with an equal volume of 0.05M TBA in chloroform followed by stripping with 1.0 M NaOH and estimated the chromium content as Cr(VI) –DPC complex as per the procedure described earlier. The results are presented in Table -4.

Table – 4: Analysis of chromium in alloys and industrial wastes

Sample	Chromium (VI) added (g/l)	Chromium found after recovery extraction (g/l)	% Recovery
Synthetic sample
1	0.25	0.248	99.2%
2	0.30	0.293	97.6%
3	0.35	0.344	98.3%
4	0.40	0.398	99.5%
5	0.45	0.443	98.4%
Stainless steel alloy type	Chromium Present	Chromium Found
303	17.0	16.90	99.4
410	11.5	11.30	98.3
416	14.0	13.78	98.4

CONCLUSIONS:

The developed method was applied to the extraction separation and determination of chromium in alloys as well as synthetic samples. The average recovery % of chromium was found to be 98.7% each determination requires a minimum amount of 10 minutes time.

ACKNOWLEDGEMENTS:

Thanks are due to Dr. V. Muralidhara Rao, Retd. Professor,School of Chemistry, Andhra University, Visakhapatnam for his valuable suggestions. Thanks are also due to Principal, GIT and Management of GITAM University for providing necessary facilities.

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Received on 14.06.2010 Modified on 28.06.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 1035-1037

Extraction of Chromium (VI) From Aqueous Acid Solutions by Tribenzylamine

INTRODUCTION: