Extraction of Trace Amounts of Copper(II) on a Membrane Filter Using 5,10,15,20-Tetraphenyl-21H,23H-porphinetetrasulfonic Acid for the Determination by Solid-Phase Spectrophotometry
Solid-phase spectrophotometry using a membrane filter is based on the extraction of a desired trace element as a colored species on the filter, and a direct absorbance measurement of the resulting colored thin layer by spectrophotometry.     The formation of a hydrophobic species is required for a quantitative extraction of the desired trace element on the filter. 5 A combination of Zephiramine (benzyldimethyltetradecylammonium chloride) and a large excess of perchlorate ion is
... hlorate ion is useful for the extraction of water-soluble anionic species 3 , in which an ion-association complex formed between Zephiramine and the anionic species is extracted together with the ion-pair precipitate formed between Zephiramine and perchlorate ions. Porphyrins are highly sensitive reagents for the determination of trace metals 6 , and copper at ppb levels was determined without preconcentration. 7-10 The sensitivity of the determination of copper with porphyrins was further improved by solvent extraction 11 and solidphase extraction using ion-exchange resin 12,13 and membrane filter; 14 however, a method of higher sensitivity is still required for the determination of copper in smaller sample volumes. The aim of this study was to develop a method of solid-phase spectrophotometry for the determination of trace amounts of copper at sub-ppb levels. 5,10,15,20-Tetraphenyl-21H,23H-porphinetetrasulfonic acid (TPPS) was selected for this purpose, and the condi-tions to extract the copper(II)-TPPS complex without extraction of the excess TPPS were investigated. Suspension-introduction graphite-furnace atomic absorption spectrometry 15 was also examined and the results of the determination of copper were compared with each other. Experimental Apparatus A Shimadzu UV-Visible recording spectrophotometer (Model UV-160A) was used for measuring the absorbance of a copper(II)-TPPS complex on a membrane filter. A Hitachi Zeeman effect GFAAS (Model Z-8270) with an auto-sampler (Model SSC-300) was used for measuring the atomic absorption of copper in a resin suspension. A Toyo KG-25 filter holder was used with a membrane filter of cellulose nitrate (0.45 or 0.65 µm pore size, Toyo Advantic). A Branson Ultrasonic Cleaner B-42 was used for preparing the resin suspension. Reagents All reagents used were of analytical grade. Doubly distilled water was used. 15 A stock copper(II) solution (100 mg l -1 ) was prepared by dissolving copper sulfate pentahydrate (Wako Pure Chemical) in 0.1 M hydrochloric acid. A working solution (100 µg l -1 ) was prepared from the stock solution by dilution with 0.1 A highly sensitive solid-phase spectrophotometric method for the determination of trace amounts of copper(II) using 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (TPPS) is described. The copper(II)-TPPS complex was extracted on a membrane filter from an acidic solution containing Zephiramine (benzyldimethyltetradecylammonium chloride) and a large excess of perchlorate ion, leaving excess TPPS in the solution. The solid-phase absorbance of the copper(II)-TPPS complex on the membrane filter was measured at 417 nm. A linear calibration graph was obtained for up to 20 ng of copper(II) in a 20 ml sample volume; the limit of detection was 0.15 ng, based on three-times the standard deviation of the blank value. The proposed method was applied to the determination of copper in tap, river and sea water samples. The results are in fair agreement with those obtained by suspension-introduction graphite-furnace atomic absorption spectrometry using ammonium pyrrolidinedithiocarbamate and finely divided anion-exchange resin.