Extraction studies of Lanthanum(III) from sodium salicylate medium using trialkyl phosphine oxide

. Cyanex-923, a mixture of four trialkylphosphine oxide has been used for the extraction and separation of La(III) from sodium salicylate medium. Lanthanum(III) ion was found to be quantitatively extracted with Cyanex-923 in toluene at pH 6.0. The metal loaded organic phase is stripped with 2.0M HCl solution. Various parameters like effect of pH, Cyanex-923 concentration, sodium salicylate concentration, diluents, equilibration period and stripping agent on the extraction of La(III) has been studied. The stoichiometry of the extracted species of this metal ion was determined on the basis of the slope analysis method. The reaction proceed by solvation and the probable extracted species found were LaSal-HSal .2Cyanex-923.


Apparatus and Reagents
Trialkyl phosphine oxide extractant, Cyanex-923 used for the extraction of Lanthanum(III) was supplied by Cytec Industries Inc. Canada. It was used without further purification. A known amount of Lanthanum Oxide, La 2 O 3 , was dissolved in 5ml of hot 1:1 HCl solution and then diluted with double distilled water as per requirement. All other chemical used were of analytical grade. EQUIP-TRONIC model EQ-614 pH meter with combined electrode was used for pH measurements. ELICO UV-visible SL-27 spectrophotometer with 10 mm cortex quartz cuvettes was used for absorbance measurements.

Extraction Condition
An aliquot containing 40g of La(III) was taken in 10ml beaker, its pH was adjusted to 6.0 with dilute HCl and NH 4 OH. Sodim salicylate solution of required concentration was then added to the same solution. The solution was equilibrated with equal volume of Cyanex-923 diluted in toluene for the required shaking time of 10 minutes. After extraction, the two phases were allowed to separate, the organic phase containing the metal extracted species was stripped with 2.0M HCl. The amount of La(III) was determine spectrophotometrically by Arsenazo-I indicator (13). All the experiment was carried out at room temperature except the effect of temperature.

Effect of pH
The extraction of La(III) was carried out at various pH ranging from 1.0 to 7.0. The extraction of Lathanum(III) increases with increase in pH and it became quantitative in the pH range 5.0-7.0. After pH 7.0, the extraction decreases, hence all the extraction of La(III) with Cyanex-923 and sodium salicylate was carried out at a fixed pH of 6.0 (Fig-1).

Effect of Reagent Concentration
The concentration of Cyanex-923 diluted in toluene was varied from 1.0 x10 -3 M to 1x10 -1 M. It was found that the extraction of Lanthanum(III) increased with increase in the Cyanex-923 concentration. Quantitative extraction of La(III) was found in the range 1.0 x10 -2 M to 1x10 -1 Cyanex-923 in toluene. Therefore 1.0 x10 -2 M of Cyanex-923 was used for all the further experiments.

Effect of Sodium Salicylate Concentration
The effect of Sodium Salicylate concentration on the percentage extraction of La(III) with 1.0x10 -2 M Cyanex-923 in toluene at fix pH of 6.0 was studied in the sodium salicylate range from 1.0 x 10 -4 M -1.0 x 10 -3 M. As the sodium salicylate concentration increases the extraction goes on increasing and becomes quantitative in the range 7.5 x 10 -4 M -1.0 x 10 -3 M with Cyanex-923 in toluene. Hence all the extractions were carried out at 7.5 x 10 -4 M sodium salicylate with Cyanex-923 in toluene.

Effect of stripping agents
After extraction of Lanthanum(III) in the organic phase of Cyanex-923 diluted in toluene, it was stripped back to the aqueous phase with different strengths of acids like HCl, HNO 3 and H 2 SO 4 . It was found that 2.0 M HCl was sufficient to strip the lanthanum ions into the aqueous phase. The amount of La(III) was then determined spectrophotometrically using Arsenazo-1 method. However incomplete recovery of metal ion was observed with other acids (Table-1).

Influence of diluents and equilibration period
In order to determine the effect of nature of diluents on the extraction of La(III) various diluents like toluene, cyclohexane, xylene, chloroform, n-hexane and carbon tetrachloride are used. It was found that all the above diluents showed quantitative extraction except xylene (86.4%) and carbon tetrachloride (74.5%). Toluene was preferred as the best diluents for the extraction of La(III) since it provided better phase separation.
Extraction equilibrium was studied for different periods of shaking ranging from 1-30 mins. It was observed that 1minute of shaking period was sufficient for quantitative extraction of La(III) with 1.0x10 -2 M Cyanex-923 in toluene and 7.5 x 10 -4 M sodium salicylate. However there was no change in extraction by increasing the extraction period upto 30 mins.

Stoichiometry of the extracted Species:
The nature of the extracted species was established using log-log plots (Fig.2,3). A plot of log D versus log Cyanex-923 concentration at fixed pH and sodium salicylate concentration and a plot of log D versus log of sodium salicylate concentration at fixed pH and Cyanex-923 concentration gave a slope of 2.15 and 2.37 respectively, indicating a molar ratio of 1:2 of Lanthanum with respect to both extractant, Cyanex-923 and sodium salicylate. The extracted species must be a neutral complex with the composition LaSal-HSal. 2Cyanex-923, where Sal is