THERMAL LENS SPECTROMETRY continued
Used for separation and
preconcentration, cloud point extraction is convenient, safe, simple to use, low in cost, offers high preconcentration in
comparison to conventional liquid–liquid extraction and
does not use toxic organic solvent.
Figure 4 – Effect of Triton X-114 on the thermal lens signal—0.08 mL 5 × 10-4
= 5.0; 25 ng Rh(III).
Effect of Triton X-114 concentration The nonionic surfactant Triton X-114 was selected as the extractant because it is commercially available in a highly purified homogeneous form, inexpensive, offers lower toxicity and has a cloud point of 23–25 °C. The high density of the surfactant-rich phase facilitates separa- tion by centrifugation. Figure 4 shows the variation of the thermal lens signal as a function of the concentration of Triton X-114. The maximum thermal lens signal was obtained in the volume range of 0.4~1.0 mL 1.0% (w/v) Triton X-114. At lower concentrations, the analytical signal is low probably due to the inability of the assemblies to entrap the hy- drophobic complex quantitatively. With an increase in Triton X-114 to above 1.0 mL, the signals decrease because of the increasing volumes of the surfactant phase. Therefore, 0.8 mL 1.0% Triton X-114 was used in the proposed method.
Effect of equilibrium temperature and time Equilibrium temperature and time are critical parameters to achieve effec- tive phase separation. In general, the optimal equilibration temperature of CPE is 15–20 °C higher than the cloud point temperature. The influ- ence of equilibrium temperature on extraction efficiency was studied in the range of 40–80 °C. When equilibrium temperature was higher than 50 °C, good phase separation was obtained and the thermal-lens signal remained stable. Therefore, a temperature of 40 °C was used throughout. Dependence of the thermal-lens signal on equilibration time was studied in the 5–30 min range. An equilibration time of 10 min was adequate to achieve quantitative extraction, and the increase in equilibration time had no significant effect on the thermal lens signal.
Effect of the solvent The color reaction of the chelating agent 3,5-diCl-PADMA with Rh(III) was reported in detail elsewhere.24
reacts with 3,5-diCl-PADMA to form a 1:2 violet-red complex (RhL2, λmax H2 2+, λmax
In the pH range of 4.0~5.2, Rh(III) =
554 nm). The complex can be changed to another blue di-proton spe- cies (RhL2
= 614 nm) having much higher absorption after
acidification with HCl. Optimal concentration of HCl was found to be 0.72~6.0 mol L-1 in Figure 5.
As a themooptical analytical technique, the sensitivity of TLS is similar to the thermophysical properties of the solvent.3,28,29
As seen in Eq. (1),
the intensity of thermal lens signal is proportional to the temperature- dependent refractive index (dn/dT) and thermal conductivity (k). Due to its low dn/dT and k values, water is a poor solvent for TLS. Organic solvents have much better thermooptical properties than water because of the higher dn/dT and k values.
Figure 3 – Effect of 3,5-diCl-PADMA on thermal lens signal—0.8 mL 1.0% (w/v) Triton X-114; temperature: 60 °C, heating time:10 min; pH = 5.0; 25 ng Rh(III).
AMERICAN LABORATORY 28
In order to match the wavelength of the He–Ne laser (632.8 nm) and en- hance the thermal lens signal, the surfactant-rich phase after CPE should be dissolved with HCl and organic solvent. The organic solvent should be environmentally friendly, readily soluble with HCl and have a higher signal enhancement factor. In addition, the maximum absorption wavelength of the complex (RhL2
H2 MAY 2016 2+) should be as close as possible to the wavelength . Complexation of Rh(III) with 3,5-diCl-PADMA is shown mol/L 3,5-diCl-PADMA; temperature: 60 °C, heating time: 15 min; pH
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