Effect of pH on the extraction recovery of bismuth. Applied condition is of 0.5 ng mL-1 Bi, 5×10-6 M Na-DDTC, 0.05% (w/v) Triton X-114 and 1% THF.
- Effect of Na-DDTC concentration
The extraction recovery as a function of the Na-DDTC concentration is shown in
. 3
. For this study, 10mL of a solution containing 0.5 ng ml
-1
bismuth in 0.05% (w/v) Triton X-114 with various amounts of Na-DDTC was subjected to the cloud point preconcentration process. At this stated concentration of bismuth, ~100% extraction was achieved for a Na-DDTC concentration of 0.8×10
-6
M. A concentration of 5×10
-6
M Na-DDTC was chosen for subsequent experiments.
Effect of Na-DDTC concentration on the extraction recovery of bismuth. Utilized condition is of 0.5 ng mL-1 Bi, 0.05% (w/v) Triton X-114, 1% THF, pH 3.0-3.5.
- Effect of Triton X-114 concentration
A successful cloud point extraction should be able to maximize the extraction efficiency through minimizing the phase volume ratio (
V
org
/
V
aqueous
), so as to improve the preconcentration factor. Triton X-114 was chosen as the nonionic surfactant due to its low cloud point temperature and high density, which facilitates phase separation by centrifugation.
. 4
highlights the effect of the surfactant concentration in the range of 0.04-0.25% (w/v) on the extraction efficiency. Triton X-114 was found to quantitatively extract the Bi-DDTC complex from the aqueous sample at surfactant concentrations above 0.05%, using a single step extraction procedure. Using more than 0.05% of surfactant, the analytical sensitivity decreased due to dilution of the sample by additional surfactant solution.
Extraction recovery of the bismuth as a function of Triton X-114 concentration. Applied condition is of 0.5 ng mL-1 Bi, 5×10-6 M Na-DDTC, 1% THF, pH 3.0-3.5.
- Selection of the dilution agent for the surfactantrich phase
High viscosity of the surfactant-rich phase (~20 cP) is drastically decreased using diluting agents. Different solvents such as methanol, ethanol and acidic solutions of methanol and ethanol were tried. None of these solvents could dissolve the extracted phase completely. However, THF dissolved the surfactant-rich phase and the extracted materials (complex of bismuth and excess of Na-DDTC) completely and the best results were obtained with this diluent.
- Effect of THF concentration
. 5
shows the effect of THF concentration on extraction efficiency. It was found that the extraction efficiency decreased at THF concentrations >3% due to dissolution of the surfactant phase and a decrease in the volume of this phase. Hence, a relatively concentrated solution (5×10
-4
M) of Na-DDTC in THF was used.
Effect of THF concentration on the extraction recovery of bismuth. Utilized condition is of 0.5 ng mL-1 Bi, 5×10-6 M Na-DDTC, 0.05% (w/v) Triton X-114, pH 3.0-3.5.
- Effect of equilibration temperature and time
The equilibration temperature above the cloud point and the incubation time were also optimized. The shortest incubation time and the lowest possible equilibration temperature are desired. The results, illustrated in
. 6
show excellent recoveries for equilibration temperature from 40 to 60 ℃.
Effect of equilibration temperature on the extraction recovery of bismuth. Applied condition is of 0.5 ng mL-1 Bi, 5 × 106 M Na-DDTC, 0.05% (w/v) Triton X-114, 1% THF, pH 3.0-3.5.
Higher temperature lead to the decomposition of Na-DDTC and the reduction of extraction yield. A temperature of 50 ℃ was hence used in all experiments. The dependence of extraction efficiency upon incubation time was studied in the range of 5-20 min. An incubation time of 5 min was sufficient for quantitative extraction.
- Interferences
Bismuth(III) ions produce an orange-brown complex with Na-DDTC, Bi(HD
2
)
3
, which is stable over the pH range 2.5-6.0. Na-DDTC is known to be highly selective for bismuth at pH 3.0-3.5.
58
Only lead, thallium and cadmium can be co-extracted with bismuth but their dithizonates are unstable in slightly acidic medium (pH 3.0-3.5). The noble metals (Pt, Pd, Au, Ag, and Hg) and copper are quantitatively extracted with Na-DDTC at pH 0.5-1.0. The results of the interference study are shown in
2
; it will be seen that bismuth recovery was essentially quantitative in the presence of foreign cations. Therefore these ions produce no interference in the extraction of bismuth.
Effect of foreign ions on the preconcentration and determination of bismuth (0.3 ng mL-1)
Effect of foreign ions on the preconcentration and determination of bismuth (0.3 ng mL-1)
- Characteristics of the method
A calibration curve was constructed by preconcentration of 10 ml of sample standard solutions with Triton X-114.
3
shows the analytical characteristics of the method. Under the optimum experimental conditions, the calibration curve for bismuth was linear from 0.05 to 0.70 ng mL
-1
.
Analytical characteristics of the method
aBismuth concentration was 0.3 ng mL-1 for which the R.S.D. was obtained. bDetermined as three times of the standard deviation of the blank signal. cCalculated as the ratio of slope of preconcentrated samples to that obtained without preconcentration.
The enhancement factor of about 195 was obtained by preconcentration a 10ml of sample. The limit of detection was calculated to be 0.04 ng ml
-1
(S/N=3). If samples with volumes larger than 20 μl are injected into graphite furnace, sensitivity can be increased but in all experiments we injected 20 μl of analyte, because, otherwise sample may spread to the low-temperature areas of the furnace, or may overflow to the filler port that results in decreased accuracy.
- Analysis of real samples
In order to validate the proposed methodology, the developed procedure was applied to the determination of bismuth in tap water, and some waste water samples. Various water samples were also analysed (
4
) in all cases the spike recoveries were excellent, showing no matrix interferences.
Determination of Bi in real samples (results of recoveries of spiked samples)
aMean of three experiments ± standard deviation. bFrom drinking water system of Saveh, Iran. cFrom Rain water of Varamin, Iran.
The analytical results shown in
4
demonstrate that the results by using the present method are consistent with those obtained by ICP-MS.
CONCLUSIONS
We have proposed the use of cloud point extraction as an alternative method for the preconcentration of Bi as a prior step to its determination by ETAAS. The method allows the determination of ultra low levels of Bi by ET-AAS.
The methodology offers a simple, rapid, sensitive, low cost, good extraction efficiency and lower toxicity than those using organic solvents. Environmental pollution is limited to a small amount of surfactant. The method gives a low limit of detection and good R.S.D.
Compared with other methods, interference from other cations is minor. The proposed method can be applied for the determination of bismuth in tap water .
Acknowledgements
We gratefully acknowledge financial support of chemistry department, faculty of science, Imam Khomeini International University, Qazvin. The authors wish to thank the chemistry department of Varamin campus Islamic Azad University and Saveh campus Islamic Azad University for financial support.
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