ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
POTENTIOMETRIC DETERMINATION OF CADMIUM USING COATED
PLATINUM AND PVC MEMBRANE SENSORS BASED ON N,N'-BIS(SALICYLALDEHYDE)PHENYLENEDIAMINE (SALOPHEN)
© 2015 Mohammad Mirzaei1, Hadi Behrooj Pili
Department of Chemistry, Shahid Bahonar University of Kerman Kerman P.O. 76169, Iran 1E-mail: m37mirzaei@gmial.com Received 28.08.2013; in final form 14.10.2014
The construction and performance characteristics of novel PVC membrane (PME) and coated platinum (CPtE) cadmium ion selective electrodes based on N,N'-bis(salicylaldehyde)phenylenediamine (salophen) are described. The electrodes exhibit linear responses along with near Nernstian slopes of 28.9 ± 0.4 (PME) and 29.2 ± 0.6 (CPtE) mV/decade of concentration within the Cd2+ ion concentration range of 5.7 x 10-8 to 3.7 x 10-3 M for PME and 3.0 x 10-8—3.0 x 10-3 M for CPtE. These sensors are applicable in a pH range of 2.5—7.5. The lower detection limits by PME and CPtE are 3.2 x 10-8 and 1.6 x 10-8 M, respectively. They have a response time less than 14 s and can be used practically for a period of at least 2 months without any measurable divergence in results. The electrode can also tolerate partially non-aqueous media (ethanol, methanol and acetone) up to 30%. The electrodes showed excellent selectivity towards Cd2+ ion over a wide range of alkali, alkaline earth, and transition metals ions. They were successfully applied to the direct determination of Cd2+ ions in tap water, aqueduct water, and river water and soil sample. In addition the electrodes were used as an indicator electrode in potentiometric titration of Cd2+ ion with EDTA.
Keywords: potentiometric, PVC membrane, sensor, salophen, cadmium.
DOI: 10.7868/S0044450215060080
Cadmium is one of the high toxic transition metals and should be handled with care due to its harmful to the environment and human health. Cadmium occurs naturally in the environment in its inorganic form, and anthropogenic sources have further contributed to background levels of cadmium in soil, water and living organisms. The general population is exposed to cadmium from multiple sources, including smoking, but in the non-smoking general population food is the dominant source. Experimental and epidemiological studies have provided substantial evidence that low levels of long-term exposure to cadmium can contribute to an increased risk of cancer. In the body of animals, cadmium is primarily bound to metallothionein and these complexes are filtered in the kidney so that cadmium accumulates in the renal cortex [1—3]. There are several sources of human exposure to cadmium, including employment in primary metal industries, production of certain batteries, some electroplating processes and consumption of tobacco products [4]. The permissible limits of cadmium discharge in wastewater and drinking water are 0.1 and 0.05 mg/L, respectively. Several analytical methods, including atomic absorption spectrometry (AAS), cold vapor AAS or AAS—ETA (electrothermal atomiza-
tion) [5—7], inductively coupled plasma—optical emission spectroscopy [8], anodic stripping voltam-metry [9, 10], chromatography (usually HPLC) [11], photometry [12] have been utilized for determination of Cd2+ at low concentration level. These methods give accurate results but are not very convenient for large scale monitoring. Potentiometric ion selective sensors are known as excellent low cost tools for selective, sensitive and rapid determination of a vast variety of ana-lytes in different fields of application. They are extremely versatile tools to chemical sensing science in which selectivity can be chemically adjusted by incorporating different ionophores into the membrane phase. Thus, during last three decades, intensive efforts have been made to develop good ion-sensors for Cd2+ determination. A number of sensors based mainly on Ag2S—CdS mixture [13—15], cadmium chelates [16], petrol compound [17], crown ethers [18—20] and variety of other ionophores [21] have been reported. However, most of these methods show some limitations in one or more of their working activity range, selectivity, response time, pH range and lifetime. Thus, the development of reliable ion selective sensors for determination of Cd2+ ion has a considerable importance for environment and human health. To improve
the analytical selectivity, it is essential to search novel carrier compounds that would interact with Cd2+ ion with high selectivity.
In this work, N,N'-bis(salicylaldehyde)phenylene-diamine is used as an ionophore, and it is introduced as a selective carrier for cadmium ion in polymeric membrane electrode and coated platinum electrode.
EXPERIMENTAL
Reagents. All reagents were of analytical grade. Chloronaphthalene (CN), dioctyl phthalate (DOP), dibutyl phthalate (DBP) and dimethyl sebacate (DMS) were obtained from Aldrich (Milwaukee, WI, USA). High molecular weight polyvinylchloride (PVC) powder, tetrahydrofuran (THF), sodium tet-raphenylborate (NaTPB) and potassium tetrakis(p-chlorophenyl)borate (KTpClPB) were purchased from Merck (Darmstadt, Germany) or Fluka (Buchs, Switzerland). The chloride and nitrate salts of metals were obtained from Merck (Darmstadt, Germany) and were used without further purification except for vacuum drying over P2O5. Doubly distilled deionized water was thoroughly used to prepare all the metal ion solutions . N, N' -bis(salicylaldehyde)phenylenediamine (Scheme) was used as a ligand and was synthesized according to the method described in reference [22].
Chemical structure of N,N'-bis(salicylaldehyde)-phenylenediamine (salophen).
Apparatus and potential measurements. Atomic absorption spectrophotometry measurements were made on AA220 spectrometer (Varian, Australia) under the recommended conditions based on the manufacturer's instructions. All potentiometric measurements were made with a 713 pH—mV meter (Metrohm, Swiss) at laboratory ambient temperature. All electromotive force (emf) measurements with the PME and CPtE were carried out with the following cell assemblies:
Platinum surface|PVC membrane|Test solution|Salt bridge (1 M KNO3)|3 M KCl|Ag/AgCl (CPtE);
Ag/AgCl, 3 M KCl|Internal solution, 1.0 x 10-3 M Cd(NO3)2|PVC membrane|Test solution|Salt bridge (1 M KNO3)|3 M KCl|Ag/AgCl (PME).
All the emf observations were made relative to an Ag/AgCl double junction electrode (Azar electrode, Iran) as an external reference electrode and an Ag/AgCl electrode was used as the internal reference electrode under magnetic stirring. Activities were cal-
culated according to the Debye—Huckel procedure, using following equation [23]:
lgf= -0.51U2[^1/2/(1 + 1.5^1/2)] + 0.2^, where | is the ionic strength and z is the valence of ion.
Preparation of the electrodes. The PVC-based membranes were prepared by dissolving appropriate amounts of ionophore along with anionic additives (NaTPB or KTpClPB), plasticizer (DBP, DMS, CN or DOP) and PVC in THF (Table 1). The components were added in terms of weight percentages. After complete dissolution of all the components and thorough mixing, homogeneous mixture was poured into glass rings (20 mm i.d.) and placed on a smooth glass plate. The solution was then allowed to evaporate for 24 h at ambient temperature. The master PVC membrane of about 0.4 mm thickness was sectioned with a cork borer (5 mm diameter) and attached to a polyethylene pipette as a body of the electrode (5 cm length and 4 mm i.d. on top), with PVC—THF slurry. The membrane was placed carefully at the bottom of the tube and filled with an internal filling solution, 1.0 x 10-3 M Cd(NO3)2. The electrode was finally conditioned by soaking in cadmium nitrate solution (1.0 x 10-2 M) for 24 h till reproducible and stable potential were achieved.
In order to prepare the coated platinum disk electrode, Pt wire with area 0.0314 cm2 was washed with 30% HNO3 and dried overnight. This was placed in a 10 cm long, 3 mm i.d. Pyrex tube sealed at one end. One end of the wire was then sealed in the glass by heating coil temperature. The sealed end was polished with sandpaper until the wire cross section was exposed. Electrical connection to the unsealed end of the Pt wire was made with a Cu wire. The working surface of the platinum disk electrode was polished with a nano-Al2O3 powder (Fluka) on a polishing pad, sonicated in doubly distilled water and dried in air. Membrane solution was prepared by thoroughly dissolving ionophore, PVC and plasticizer in about 5 mL THF. The resulting mixture was evaporated slowly at ambient temperature until an oily concentrated mixture was obtained. The coating process of polished platinum electrode was performed by dipping electrode three times into the membrane solution. After coating, the membrane was air-dried until a thin film was formed, and the electrode was allowed to stabilize overnight. The electrode was finally conditioned for 12 h in a 1.0 x 10-2 M Cd (NO3)2 solution.
RESULTS AND DISCUSSION
Potential response. Nitrogen and oxygen donor atoms in the structure of ligand with high lipophilic character, salophen were expected to act as suitable ion carriers in the PVC membrane with respect to special transition and heavy metal ions. Thus, in the preliminary experiments, the ligand was used as a neutral carrier for the preparation of polymeric membrane
Table 1. Optimization of membrane (PME) ingredients
No. Composition of membrane sensors (%, w/w) Slopea, Linear range, M Detection
PVC plasticizer ligand additive mV/decade limit, M
1 33 65 (DBP) 0 2 (NaTPB) - - -
2 32 63 (DBP) 3 2 (NaTPB) 35.5 ± 0.5 8.8 x 10-7-4.3 x 10- -3 6.3 x 10-7
3 32 63 (DOP) 3 2 (NaTPB) 24.6 ± 0.3 5.1 x 10-6-4.3 x 10- -3 3.4 x 10-6
4 32 63 (DMS) 3 2 (NaTPB) 21.2 ± 0.5 8.2 x 10-6-6.3 x 10- -3 6.6 x 10-6
5 32 63 (CN) 3 2 (KTpClB) 23.2 ± 0.7 4.8 x 10-6—5.6 x 10- -3 3.2 x10-6
6 32 63 (DPB) 3 2 (KTpClB) 27.5 ± 0.5 4.8 x 10-7-4.3 x 10- -3 1.2 x 10-7
7 42 54 (DBP) 0 4 (KTpClB) - - -
8 41 53 (DBP) 2.7 3.3 (KTpClB)
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