PVC-Membrane Electrodes Based on 18-Crown-6 and Dibenzo-18-Crown-6 Ethers for Determination of Silver

Turk J Chem 31 (2007) , 449 – 456. c TÜBİTAK
PVC-Membrane Electrodes Based on 18-Crown-6 and Dibenzo-18-Crown-6 Ethers for Determination of Silver Mohsen Mousa ZAREH1∗, Magda A. AKL2 , Ahmd Khlil GOHNEIM1 and Mohmed Hussin ABD EL-AZIZ3 1 Department of Chemistry, Faculty of Science, Zagazig University, Zagazig-EGYPT e-mail: 2 Department of Chemistry, Faculty of Science, Mansura University, Mansura-EGYPT 3 Central Laboratory, Egyptian Railway, Cairo-EGYPT Received 29.12.2006 PVC-membrane electrodes for Ag+ based on either 18-crown-6 (I) or di-benzo-18-crown-6 ether (II) were applied for the first time. The effect of solvent mediator on the electrode performance was discussed. Nernstian slope values were 59 and 58.9 mV/decade for both I and II electrode types, respectively. The linear concentration range was 10−5 -10−1 M. The detection limit was 7.9 × 10−6 M. The working pH ranges were 5-9 or 4-9, depending on the concentration of Ag+ ion. The electrodes showed good selectivity towards most of the common alkali, alkaline earths, and transition metal cations. The electrodes were applied successfully for the determination of silver in photographic wastewater and alloys that are part of railway engines. The recovery range was 96%-100%, and the relative standard deviation was (0.95-2.7) for n = 4. The obtained results were compared to those of an atomic absorption spectrophotometric method. Key Words: Ag+ determination, silver electrode, PVC-membrane electrode. Introduction Many ionophores and chelating agents were applied for preparing silver selective electrodes. Among them are calixarenes such as calix-4-arene1,2 and calix-6-arene.3 Some other chelating agents were also applied for the preparation of ion-selective electrodes like sulfanyl benzoic acid,4 and 5,10,15-tris(pentaflourophenyl)corrole.5 It is well known that crown ethers work as chelating agents for many cations. The crown-ether complexes have been extensively applied to different metal cation-selective electrodes.6,7 Several types of crown ether-metal were previously used to prepare Ag+ -selective electrodes. Of them are dibenzo-16-crown5,8 azathioethers crown containing 1,10-phenanthroline unit,9 14-membered thia-crown ether,10 macrocyclic dithia-crown ether,11 16-crown-5,12 15-crown-5,13 15-crown-6,14 14-crown-4,15 and hexathia-18-crown-6.16 ∗ Corresponding author 449 PVC-Membrane Electrodes Based on 18-Crown-6 and..., M. M. ZAREH, et al., In the present work, an extension of the studies about crown ethers as ionophores for Ag+ -selective electrodes will add some information about their behavior, especially selectivity and sensitivity. Accordingly, 18-crown-6 ether and its dibenzo-derivative were tried for Ag+ -selective electrodes. The present electrodes were advantageous, since Hg2+ did not show any interference with the measurements like that for 16-crown5.12 In addition, they are characterized by small selectivity coefficient values. They showed a wider linear range and working pH range (10−1 -10−5 M and 4-9 pH) than those for hexathia-18-crown-6 (3.2 × 10−3 -6 × 10−6 M and 2-7.5 pH).16 Their response times were the same. They can be applied easily to actual samples containing silver. O O O O O O O O O O O O 18-crown-6 (I) Dibenzo-18-crown-6 (II) Figure 1. 18-Crown-6 (I) and dibenzo 18-crown-6 (II) ethers. Experimental Section Reagents and materials 18-Crown-6 (18CE6) (Fluka), and dibenzo 18-crown-6 (DB18CE6) were the main ionophores for preparing the membranes. The plasticizers were dioctylphenylphosphonate (DOPP) (Aldrich), dioctylphthalate (DOP) (Fluka), or nitrophenyl octylether (NPOE) (Fluka). Polyvinylchloride (PVC) (Fluka) was the membrane matrix. The membrane components were dissolved in tetrahydrofuran (THF) (Fluka). To perform the + 2+ selectivity studies, analytical-reagent grade nitrates of Na+ , K+ , NH+ , Hg2+ , Cu2+ , Zn2+ , 4 , Ag , Ni Co2+ , Cd2+ , Fe3+ , Sr2+ , Mg2+, Ca2+ , and Al3+ were used. To prepare silver 500 mg/L stock solution, 0.787 g of dried silver nitrate (Aldrich) was dissolved in bi-distilled water containing 1 mL of concentrated HNO3 . The solution was made up to the mark in a 1-L standard flask and stored in amber light bottles in a dark place. The solution was standardized amperometrically against 0.01 N standard KCl solution. Less concentrated solutions were obtained by careful dilution of the standard stock solution using 0.1 M NaNO3 . Double distilled water was used throughout. Equipment The potentiometric/pH-measurements were carried out (at 25 ± 1 ˚C) on a digital pH-meter (model 5986) Cole-Parmar (sensitivity ± 0.1 mV) coupled with a channel selector. The atomic absorption measurements were performed using a Perkin-Elmer spectrometer (model 2380). 450 PVC-Membrane Electrodes Based on 18-Crown-6 and..., M. M. ZAREH, et al., Electrode preparation and potentiometric measurements The electrode construction depends mainly on the membrane constituents. The membrane composition was 1% (w/w) ionophore [either 18CE6 (I), or DB18CE6 (II)], 33% (w/w) PVC, and 66% (w/w) solvent mediator (DOPP, DOP, DDP, or NPOE). These components were dissolved in distilled THF and poured into glass rings of 24 mm i.d. resting on a glass plate. After solvent evaporation, the obtained membrane was cut out into disks of 7 mm i.d. The membrane disks were mounted in electrode bodies (Type IS 561, Philips, Eindhoven, Netherlands) for potentiometric measurements. After electrode adjustments, it was soaked for 24 h in 10−2 M solution of AgNO3 . The electrode body was filled with the inner filling solution [AgNO3 10−3 M + NaNO3 10−1 M]. The potentiometric measurements were carried out in the presence of 0.1 M NaNO3 using the procedure described previously.17 The cell potential was recorded corresponding to each Ag+ -concentration. A calibration graph has been constructed for the cell potential versus –log [Ag+ ]. The following is a representation of the cell assembly: Ag-AgCl Double Junction Reference || Sample solution || Membrane | Inner filling | Ag-AgCl P ot The potentiometric selectivity coefficient (KAg+,X Z+ ) values for different common cations were ob- tained by the separate solution method18 (10−3 M solutions for both silver and interferent). The pH adjustments of the tested solutions to the electrode optimum pH value were achieved with 0.1 M NaOH or 0.1 M HNO3 . Preparation of silver samples Samples (0.1 g) from the piston pin and carrier alloy from the turbocharger of EC, C, F, FB, and 710 General Motors-series engines (Egyptian Railway) were taken. Each sample was completely dissolved in 10 mL of hydrochloric acid (1:1) by heating on a water bath. Then it was added to a minimum volume of (1:1) HNO3 . The mixture was boiled to remove oxides of nitrogen.19 The mixture was cooled, transferred to a 50-mL volumetric flask, and diluted with deionized water. Then the pH was adjusted to 5-8 with 0.1 N NaOH. Photographic wastewater samples were collected from several local ph (...truncated)