AL


Reagents and solutions All chemicals and reagents were of analytical grade unless otherwise stat- ed. Double-distilled water was used in the preparations. A standard stock solution of rhodium (1000 µg/mL) was prepared by dissolving 0.3248 g of spectrographically pure (NH4


)2 RhCl5·H2O in 100 mL 1 mol L-1 mol L-1 HCl. The work-


ing solutions were obtained by stepwise dilution of the stock solution with water. A 5 × 10-5


3,5-diCl-PADMA (laboratory-synthesized27 )


solution was prepared by dissolving 0.1551 g 3,5-diCl-PADMA in 1000 mL absolute ethanol. A 1% (w/v) Triton X-114 (Sigma-Aldrich, St. Louis, Mo.) water solution was made. Buffer solution of pH 5.0 was prepared from 0.2 mol L-1


HAc and 0.2 mol L-1 NaAc solution; 2.67 mol L-1


solution was prepared by mixing 8 mol L-1 volume ratio of 1:2.


HCl-alcohol HCl solution with ethanol by a


General procedure An aliquot of the working standard or sample solution containing ap- propriate amounts of rhodium, 2 mL pH 5.0 HAc-NaAc buffer solution and 80 μL of 5.0 × 10-4


mol L-1 3,5-diCl-PADMA ethanol solution was placed


in a 10-mL graduated conical centrifuge tube. The mixture was heated in a boiling water bath for 20 min. For cloud point extraction, 0.8 mL of 1% (w/v) Triton X-114 solution was added and the solution was diluted to 10 mL with water. This solution was shaken and kept in a thermostat- controlled bath at 60 °C for 10 min. Separation of the two phases was then accelerated by centrifugation at 3500 rpm for 5 min. Upon cooling in an ice bath for 10 min, the surfactant-rich phase became viscous and remained at the bottom of the tube. Bulk aqueous phase was discarded by inverting the tube. The surfactant-rich phase in the tube was dissolved with 0.45 mL 2.67 mol L-1


HCl-alcohol solution. Next, the final solution was introduced


into a 5-mm optical pathlength quartz cell to measure the thermal lens signal (Sc


) by the thermal lens spectrometer at a wavelength of 632.8 nm and chopper frequency of 10 Hz. Results and discussion


Absorption spectra and wavelength selection The absorption spectra of 3,5-diCl-PADMA and its Rh(III) (rhodium chloride) complex in the surfactant-rich phases after dissolving with HCl-ethanol


Figure 2 – Effect of pH on thermal lens signal—0.08 mL 5 × 10-4 mol L-1


3,5-diCl-PADMA; 0.8 mL 1.0% (w/v) Triton X-114; temperature: 60 °C, heating time: 10 min; 25 ng Rh(III).


solution were recorded (Figure 1). It can be seen that the reagent exhibits maximum absorption at 444 nm, while the Rh(III) complex exhibits maxi- mum absorption at 624 nm, which is well matched to the wavelength of the He–Ne laser (632.8 nm).


Effect of acidity The chelating agent 3,5-diCl-PADMA showed acid–base indicator proper- ties. Four species—H3


L+ , H2L2+ , HL+


and L—are included in the solution


because of the protonation of the ring nitrogen and two amino group nitrogen atoms. Distribution of these species in solution is determined by the acidity. pH therefore plays an important role in the formation and hydrophobicity of the chelate, as well as extraction. Figure 2 shows the influence of pH on the thermal lens signal of the rhodium complex. A maximum thermal lens signal was obtained at pH 4.6–5.2; pH 5.0 was thus chosen for subsequent experiments.


Effect of chelating agent Because 3,5-diCl-PADMA was sparse in water, ethanol was chosen as the solvent for preparing the chelating agent solution. The 3,5-diCl- PADMA concentration affects the rhodium extraction, that is, the lower the concentration, the larger the volume of solution that will be used. This means that more ethanol will enter the CPE system, which will prevent micelle formation and reduce extraction efficiency. To reduce the volume added, a higher concentration of 3,5-diCl-PADMA solution (5.0 × 10-4


mol L-1 The effect of 5.0 × 10-4


) was used. mol L-1


mol L-1 3,5-diCl-PADMA on the thermal lens signal


was investigated within the range of 10–150 μL (see Figure 3). The thermal lens signal first increased and then remained stable in the presence of 30–120 μL 5.0 × 10-4


3,5-diCl-PADMA. When an excess amount of


Figure 1 – Absorption spectra: 1) 3,5-diCl-PADMA vs water. 2) Rh(III) complex vs reagent blank. Conditions: 1.0 × 10-5 87.5 ng Rh(III), 1-cm cell.


mol L-1 3,5-diCl-PADMA; AMERICAN LABORATORY 27


3,5-diCl-PADMA was used, a gradual decrease in the thermal lens signal was observed. As 3,5-diCl-PADMA also has strong hydrophobicity, there was more 3,5-diCl-PADMA and less Rh(III)-3,5-diCl-PADMA complex in the surfactant-rich phase with increasing amounts of 3,5-diCl-PADMA. Hence, 80 μL of 5.0 × 10-4 quent experiments.


mol L-1 3,5-diCl-PADMA was chosen for subse- MAY 2016


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60