by Quan Han, Xiaohui Yang, Na Yang, Huo Yanyan and Yaping He
AL
Determination of Trace Rhodium in Water Samples by Thermal Lens Spectrometry Following Cloud Point Extraction
Due to its unique physical and chemical properties, rhodium has been used as a catalyst in organic synthesis, as an alloy in thermocouples, as a raw plating material in electronic instruments and as a component in the three-way catalysts in automobile exhaust systems.1
A substantial
increase in emissions necessitates a reliable analytical technique to assess and study the impact of rhodium on the environment.
Laser thermal lens spectrometry (TLS) is an advanced molecular ab- sorption technique based on the measurement of local changes in refractive index of medium due to a rise in temperature produced by nonradiative relaxation upon the absorption of laser radiation, pro- portional to the concentration of the absorbent.2,3 determination of absorbance down to 10-8 the 10-11
The method allows and detection of analytes at mol L-1 level.4–7 from low selectivity.7
Used for separation and preconcentration, cloud point extraction (CPE) is convenient, safe, simple to use, low in cost, offers high preconcentra- tion in comparison to conventional liquid–liquid extraction and does not use toxic organic solvent—thus it is characterized as a “green” extraction technique.8–10
analytical techniques,11–19 pling of CPE and TLS.20–24
CPE has been used successfully in conjunction with many but little information is available on the cou-
Cloud point extraction/laser thermal
lens spectrometry The current study combined CPE with TLS for the determination of trace rhodium in water samples. As the ligand, 2-(3,5-dichloro-2-pyridylazo)- 5-dimethylaminoaniline (3,5-diCl-PADMA) was chosen because it is a good chromogenic reagent for the determination of rhodium,25
and the
maximum absorption wavelength of its rhodium complex (624 nm) is well matched to the wavelength of the He–Ne laser (632.8 nm); octylphenoxy- polyethoxyethanol was selected as the extractant because its cloud point is in the range of 23–25 °C.
AMERICAN LABORATORY 26 While known for its high sensitivity, TLS suffers Experimental
Apparatus Thermal lens measurements were carried out with a laboratory-built single-beam laser thermal lens spectrometer.26
A Model PS-THN-1200 con-
tinuous-wave (CW) He–Ne laser (Shanling Laser and Electronic Appliance Co. Ltd., Shanling, China) (9632.8 nm, TEM00
, 25 mW) was used as both
excitation and probe beam. The laser beam was modulated by a Model 197 chopper (Signal Recovery, part of Ametek Advanced Measurement Technology, Broussard, La.) at 10 Hz and focused onto a 5-mm quartz cell located at the confocal distance by a 150-mm focal-length lens. Signal was detected as the far-field beam intensity change through a 0.5-mm pinhole with a photoelectric device located 2.4 m from the sample cell, and was processed with a Model DS5102M digital storage oscilloscope (Beijing Rigol Electronic Co. Ltd., Beijing, China) connected to a personal computer. The thermal lens signal is given by Eq. (1)22
: (1)
Where I(0) and I(∞) are intensities of the laser beam passing though the sample cell during the thermal lens effect at initial time and steady state time; P and λ are the power and wavelength of the laser; and A, dn/dT and k are the absorbance, temperature coefficient of refractive index and conductivity of the sample, respectively.
Absorption spectra were recorded using a TU-1810 spectrophotometer (Beijing Puxi General Co. Ltd., Beijing, China) equipped with a 1-cm quartz cell. The pH measurements were performed by a pHS-2C digital pH meter furnished with a combined glass electrode (Shanghai Leici Instruments Factory, Shanghai, China). An HH-2 thermostatic water bath (Beijing Kewei Yongxing Instrument Co. Ltd., Beijing, China) maintained the tempera- ture in the CPE experiments. A Model 800 centrifuge (Shanghai Pudong Physical Instruments Factory, Shanghai, China) was used to accelerate phase separation.
MAY 2016
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