A. V. L. N. S. H. Hariharan*, K. C. Purnima
Department of Chemistry,GIT, GITAM University,Visakhapatnam – 530 045India.
*CorrespondingAuthorE-mail:ahharan@gmail.com
ABSTRACT:
Synthetic resins such as Dowex M4195 [5] and diaion SP-850 [6] have also been utilized for the removal of chromium in water samples. Various spectrometric methods have been used in the solid phase extraction for chromium and other metal ions [7–10].
A survey of the literature reveals that long chain amines and surfactants are versatile extractants [11-12] for many metal ions. Long chain amines and quaternary ammonium salts have been used to study the extraction behavior of many metal ions [13]. Tribenzylamine has proved to be a good extractant for the removal of Cr (VI) from tannery effluents [14]. The equilibrium composition of trioctylmethylammoniumchloride with aqueous chromium (VI) has been investigated in detail [15]. The specific extraction of chromium with tetrabutylammoniumbromide (TBAB) using MIBK as the solvent has been applied for the determination of chromium in water samples [16]. In this method, phosphate buffer containing tetrabutylammonium bromide was used as the eluent. Venkateswaran and Palanivelu [17] reported the extraction of Cr (VI) with 95% recovery using TBAB in dichloromethane.
It was observed from the literature survey that the extraction of Cr (III) has been carried out by using different amines and other extracting systems. As there are no reports available on the extraction of Cr (III) with TBAI as extracting agent, an attempt has been made to study systematically the extraction of Cr (III) by TBAI from different acid media. The results obtained are discussed in the present communication.
MATERIALS AND METHODS:
A 0.5M Anal R grade TBAI stock solution in chloroform was prepared and diluted appropriately to get the required concentration. Chromium sulphate [Cr₂ (SO₄)₃] (E Merck grade) was used for preparing chromium stock solution (1M) and standardized titrametrically[18], with a standard solution of ferrous ammonium sulphate after oxidation of Cr(III) to Cr (VI) ( using ceric ammonium nitrate as oxidant ) with n-phenyl anthranilic acid (NPA) as indicator. All other chemicals used were of AR grade and purified according to the standard methods. Hence are used as such without any further purification.
A digital pH meter equipped with single electrode was used for pH measurements. Atomic Absorption Spectrophotometer type AAS-SVL Spectronics Model 205 was used for the determination of chromium content in the samples.
Procedure:
A mixture of 10mL of an aqueous solution of required concentration of Cr (III) [with the corresponding acid and sodium carbonate for the adjustment of pH in the range 1.0 – 8.0] was equilibrated with 10 mL of TBAI (0.025 M) in chloroform in a 150 mL separatory funnel is shaken for about 15mins. The organic and aqueous phases were then separated after allowing the mixture to settle for about five minutes. The equilibrium chromium (III) concentration in each of the organic and aqueous phases was estimated using AAS. It was clearly noticed that chromium (VI) could not be extracted under the experimental conditions.
RESULTS AND DISCUSSION:
Effect of equilibration time:
Preliminary studies on the extraction of Cr (III) by using TBAI in chloroform show that the two phase reaction is rapid and the equilibrium is reached within 15 minutes. Further increase in the time of equilibration does not affect the extraction equilibrium (Fig.1).
Effect of pH:
The variation of pH between 1.0 to 5.0 on the extraction efficiency of Cr (III) by TBAI from different acid media has been carried out. Representative information has been provided in Fig-2.
It was observed that the percentage extraction increased with increasing up to pH 5.0 in the case of nitric acid solutions. In perchloric acid media, it was found to decrease between pH 1.0 to 2.0 and increased up to pH 5.0. In the case of sulphuric acid solutions, extraction efficiency was found to decrease between pH 1.0 to 3.0 followed by increasing trend in extraction between pH 3.0 to 4.0 and then followed by gradual decrease in extraction up to pH 5.0. Hence, extractions beyond pH 5.0 were not carried out probably due to hydrolysis of chromium species also occurs. Maximum extraction efficiency – (99.81%) was obtained at pH 3.0 in sulphuric acid solutions.
Composition of the Extracted Species:
The composition of the extracted species was determined by the extraction isotherm method [19] and the distribution ratio method [20]. The maximum loading of 2.5 x 10–2 M TBAI with chromium (III) at aqueous phase pH 3.0, yielded a mole ratio of the [extractant] to [chromium (III)] org as three (Fig-3).
Extractant variation:
In the distribution ratio method a series of extractions are carried out with different concentrations of TBAI (0.005 to 0.025M), keeping the concentrations of the metal and the corresponding acid as constant in the aqueous phase. The log-log plots of distribution ratio versus TBAI from invariable concentrations at fixed pH (3.0) and metal concentration gave straight line with slope 3.0 irrespective of the acid used (Fig-4).
Effect of Anion:
Effect of various anions such as perchlorate, sulphate, nitrate and (0.05 to 1.0 M concentration) solutions using the respective sodium salts as sources of ions on the extraction of Cr (III) by TBAI has been studied. The results showed that the distribution ratio increases with increase in nitrate ion concentration, remained constant value up to 0.5M and followed by sudden decrease in extraction. In case of sulphuric acid it receaches a maximum value (0.5M) followed by gradual decrease in extraction. The reason may be due to the formation of anionic species or ions such as CrCl2+, CrCl-4, CrSO4+, Cr(SO4)2- [21]. In the case of perchlorate ion distribution ratio decreases with increase in the concentration of the ion (Fig-5).
Effect of Temperature and the Thermodynamic study:
A mixture of 10mL of an aqueous solution of Cr (III) of 1.0 x 10-3 M at a constant pH 3.0 and in H2SO4 medium is equilibrated with 10mL of 2.5 x 10-2 M TBAI dissolved in chloroform at different temperatures (Fig-6). After separating the two layers, distribution ratio D at each temperature is calculated.
From the values of equilibrium extraction constant at the temperature range investigated. Vant Hoff equation was used to calculate the enthalpy change.
logK = -ΔH/2.303RT + ΔS/2.303R
The plot of log K against 1/T is a straight line (Figure 8) where the slope gives the enthalpy of reaction (ΔH0) and the intercepts correspond to entropy (ΔS0) value.
The results show the extraction method was exothermic hence, reaction occurs spontaneously accompanied by increase in entropy i.e., ΔS>0 and decrease in enthalpy (-ΔH).
The thermodynamic parameters obtained after calculation were,
-ΔHex= 0.025 KJ mol-1
ΔGex= 20.585 KJ mol-1
ΔSex= 62.811 J mol-1
Data shows that extractions are enthalpy favored and was not favorable in case of entropy. Although the values of ΔH0are low and negative values indicate bond formation takes place during extraction. Release of water molecules in extraction has been reflected by positive values of entropy.
Extraction Mechanism:
The observed chromium (III): TBAI molar ratio of three from all the acid solutions (by distribution ratio method) could be explained as arising from the extraction of chromium (III) by the following solvation mechanism.
Cr3+ + 3X- + 3 TBAI (org) ↔CrX3.3TBAI (org)
Where X = Cl- , HSO4- , NO3- and ClO4-
Diluent Variation:
The suitability of various diluents like chloroform, nitrobenzene, toluene, xylene etc., was investigated. Maximum extraction efficiency was obtained with chloroform as diluent and also gave better and quick phase separation (Table-1).
Table 1: Effect of various diluents on extraction (H2SO4 medium)
[Cr (III)] = 1.0 x 10-3 M pH = 3.0
[TBAI] = 2.5 x 10-2 M
|
Diluent |
% Extraction |
|
CHCl3 |
99.81 |
|
Benzene |
98.2 |
|
Nitrobenzene |
95.1 |
|
Toluene |
96.3 |
|
CCl4 |
92.6 |
|
n-Hexane |
82.4 |
|
Cyclo hexane |
80.9 |
Choice of Stripping Agent:
After the extraction of chromium (III) [1.0x 10-3 M at pH 3.0] by 0.025 M TBAI, it was stripped with 10 mL reagents of various concentrations (0.1 – 1.0 M) of HCl, HNO3, H2SO4 solutions. It was observed that HCl is extremely poor stripping agent for chromium (III). On the other hand, 1.0 M HNO3 alone is a good stripping agent. However in no case HNO3 strips out all the chromium (III) in a single extraction. It was observed that 99.9% chromium (III) could be recovered from organic phase by making contact three times with equal volumes of 1.0 M HNO3.
Recovery studies in synthetic samples:
The separation of Cr (III) is possible in the presence of more than one foreign ion in the following mixtures (Table-2) with an error of not more than ±2%.The selective removal of Cr (III) in the presence of these foreign ions as they do not co-extract under the specific experimental conditions thus removing any possibility of interference in the estimation of the metal.
Table 2: Selective Recovery of chromium (III) in the presence of other ions
|
S .no. |
Sample |
Amount of cr found (µg) |
Standard deviation |
|
1. |
13 µg Cr + 10 mg Cu, Co, Cd, Mg + 1.0 M H2SO4 |
13.0,12.7,12.8, 12.9 12.9,13.0,13.0 |
0.10 |
|
2. |
27 µg Cr + 10 mg Cu, Co, Cd, Mg + 1.0 M H2SO4 |
26.5,26.6, 26.7,26.9 27.0, 27.0, 27.0 |
0.20 |
|
3. |
54 µg Cr + 30 mg Cu, Co, Cd, Mg + 1.0 M H2SO4 |
53.8, 53.7, 54.0, 53.6 54.0, 54.0,53.8 |
0.19 |
|
4. |
81 µg Cr + 10 mg Cu, Co, Cd, Mg + 1.0 M H2SO4 |
81.0 ,80.6, 80.8, 80.9 80.7, 80.8, 80.9 |
0.35 |
CONCLUSIONS:
This method requires no pre-equilibration of the extractant prior to extraction. Extraction occurs in a single step and the time required for analysis of the samples after separation is only about 15 minutes. The results are highly reproducible. Hence the method can be successfully applied to the determination of chromium (III).
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Received on 12.10.2017 Modified on 02.11.2017
Accepted on 29.12.2017 © AJRC All right reserved
Asian J. Research Chem. 2018; 11(1):171-175.
DOI:10.5958/0974-4150.2018.00035.4