In the present study the effects of several important parameters influencing the flotation step such as type of the organic phase, pH of the solution, concentration of Eriochrome cyanine R, surfactant, floatation, relaxation and complexation time were investigated.
Effect of Eriochrome cyanine R concentration
The selection of optimum concentration of Eriochrome cyanine R is one of the important parameter in the flotation method. The effect of Eriochrome cyanine R concentration used for the floatation of thorium was evaluated in the range from 0.4 × 10−5 mol L−1 to 1.7 × 10−5 mol L−1. The maximum absorbance occurs to Eriochrome cyanine R concentration above 1.5 × 10−5 mol L−1 therefore, a concentration value of 1.5 × 10−5 mol L−1 was chosen for further investigation ().
Figure 1. Effect of Eriochrome cyanine R concentration. Experimental condition: pH = 4, volume of buffer = 5 mL, concentration of surfactant = 1.5 × 10−6 mol L−1, volume of n-hexane = 11 mL, volume of methanol = 2 mL, standing time = 4 min. (more ...)
Effect of pH and volume of buffer
demonstrates the influence of pH and volume of buffer (mL) on the absorbance efficiency within the range of 1–10. An optimum volume of acetate buffer of 5 mL with pH = 4 was obtained.
Figure 2. Effect of pH and volume of buffer. Experimental condition: Eriochrome cyanine R concentration = 1.5 × 10−5 molL−1, concentration of surfactant = 1.5 × 10−6 mol L−1, volume of n-hexane = 11 mL, volume of (more ...)
Effect of surfactants concentration
The effect of surfactant concentration is shown in . The volume of surfactants (Brij-35, CTAB and CPC) with the concentration was investigated in the range of 1 × 10−6 to 8 × 10−6 mol L−1. The differences observed in the signals at various surfactant concentrations are also exposed in . At lower concentrations of surfactant, the efficiency was low probably due to the inadequacy of the assemblies to entrap the complex quantitatively. According to this result, all further experiments were carried out at the optimum concentration 1.5 × 10−6 mol L−1 of Brij-35.
Figure 3. Effect of surfactants concentration. Experimental condition: Eriochrome cyanine R concentration = 1.5 × 10−5 molL−1, pH = 4, volume of buffer = 5 mL, volume of n-hexane = 11 mL, volume of methanol = 2 mL, standing time = 4 min. (more ...)
Effect of volume of the n-hexane
The effect of volume of the n-hexane on the flotation process was examined in the range of 4–17 mL. By increasing the n-hexane, the volume of the absorbance of extracted content increased up to 11 mL. For thorium, a similar pattern was observed in the volume ranged between 10–17 mL. Therefore 11 mL of n-hexane was selected for subsequent experiments ().
Figure 4. Effect of volume of the n-hexane. Experimental condition: Eriochrome cyanine R concentration = 1.5 × 10−5 molL−1, pH = 4, volume of buffer = 5 mL, concentration of surfactant = 1.5 × 10−6 mol L−1, volume (more ...)
Effect of type and volume of organic solvent
The effect of type and volume of organic solvent (methanol and acetonitrile) on absorbance was also investigated. A volume of 2.5 mL of methanol provided better results compared to acetonitrile ().
Figure 5. Effect of type and volume of organic solvent. Experimental condition: Eriochrome cyanine R concentration = 1.5 × 10−5 molL−1, pH = 4, volume of buffer = 5 mL, concentration of surfactant = 1.5 × 10−6 mol L−1 (more ...)
For enhancing the repeatability and efficiency, it is necessary to choose a time in which equilibrium is reached between the organic phase and the aqueous sample. The influence of standing time on the complex formation was studied over a time period of 2–9 min. A standing time of 4 min was needed in order to obtain maximum absorbance ().
Figure 6. Standing time. Experimental condition: Eriochrome cyanine R concentration = 1.5 × 10−5 molL−1, pH = 4, volume of buffer = 5 mL, concentration of surfactant = 1.5 × 10−6 mol L−1, volume of n-hexane = 11 mL, (more ...)
Conformity with Beers law and figure of merit
Under mentioned optimized conditions, linearity, precision, and limit of detection (LOD) were used for validation of method. The calibration curve was constructed for thorium over the concentration range of between 6–230 ng mL−1. The correlation coefficient (R2) was 0.998 (n = 7), showing the plot was linear for target compound. In order to determine the precision of the analytical procedure, 10 consecutive analyses were performed at about 150 and 30 ng mL−1 level. The relative standard deviation for 150 and 30 ng mL−1 of thorium were determined to be 3.26% and 4.41%, respectively.
The limit of detection for thorium was defined as the concentration of analyte which gave a signal of 3σ above the mean blank signal (where σ is the standard deviation of the blank signal). The LOD for thorium was found to be 1.71 ng mL−1.
Effect of foreign ions
The influences of some cations and anions on the determination of thorium were investigated in detail. A relative error of not greater than ±5% in the recovery at a concentration of 30 ng mL−1 thorium was observed. The tolerance limits of a foreign species were as follows: 1000-fold excess of K+, NH4+, SCN−, and NO3−; 500-fold excess of Mn2+, ClO4− and HPO42−; 400-fold excess of Ba2+ and SO42−; 200-fold excess of CO32−; and 100-fold excess of Hg2+, MoO42− and WO42− did not interfere with the determination of thorium in this method.
Application of real samples
The proposed method was applied to determine thorium in the natural water and urine samples. summarizes the average recovery of thorium in the fortified river waters and urine samples. The water samples were spiked with 20, 30 and 100 ng mL−1 of standard solution of thorium. The recoveries of from the spiked water samples varied in the range of 93.8%–100.3%.
Determination of thorium in 60 mL of water samples and in 5 ml of urine sample (n = 5).
Urine samples were kindly donated by volunteers. Urine was filtered using Whatman No. 42 filter paper and centrifuged. Into a set of 100 mL volumetric flasks, separate aliquots of urine (5 mL) were spiked with varying amounts of thorium (20, 30 and 100 ng mL−1). Finally, the extraction was carried out under the most appropriate conditions. The recoveries of from the spiked urine samples varied in the range of 96.1%–102.8%. The result indicated that the proposed method was applicable for quantitative determination of thorium in water and urine samples.
The characteristics of the proposed method were compared with other methods used for determination of thorium. compares the limit of detection (LOD), relative standard deviation (RSD), linear range (LR), extraction time, recovery and matrix using optical chemical sensor,2
ICP-MS and spectrophotometric detection,4
spectrophotometric determination with pyrimidine azo dyes and cetylpyridinium chloride17
and Supercritical fluid extraction.18
The proposed method provided similar quantification extraction efficiency, with advantages of being faster than many other mentioned techniques.
Comparison of different methods for the determination of thorium.