ELETROANALYTICAL STUDIES ON SOME IONIC SENSORS FOR HEAVY METALS

Abstract

The rapid growth of industry and technology has placed heavy responsibility on analytical chemists. They are required to analyze large number of raw materials and products as well as monitor the environment to protect the human and animal life. No doubt a large number of instrumental methods are available which provide sensitive, many times specific and reproducible results. However, these methods generally require chemical manipulations of the samples and large infrastructure back up. The ideal situation for analytical chemists is a technique which provides cost effective, convenient, fast and selective analysis of various components in different samples. Good ISEs, if available provide such procedures of analysis. Moreover, the analysis by ISEs is adaptable to small sample volumes, to turbid and coloured solutions, can be non-destructive and many a times online monitoring could be also possible. It is for these reasons the ISEs find applications in various environmental, industrial and biologically important areas. Thus, availability of good ISEs in the hands of analytical chemists is highly desirable. The ISEs simply consist of solid or liquid membrane which separates two appropriate solutions of different concentrations and allows the transport of a particular cationic or anionic species. This restricted transport of ions results in generation of potential difference across the membrane which is used for the determination of ionic concentration. Thus, the materials which favour selective transport are used in the membrane for the generation of potentials and are commonly called as electroactive components of the membranes. Various materials (i) such as solid electrolytes, inorganic and organic ion exchangers, insoluble salts of multivalent metals, ligands and their metal chelates, crown ethers, cryptands, calixarenes and other macrocycles etc. have been used as electroactive phase of the membranes employed in developing ISEs. As a result of using different materials for preparing membranes, ISEs have now become commercially available for hydrogen, ammonium, some alkali, alkaline earth and heavy metal ions and halides, cyanide, nitrite, sulphite etc. among the anions. Inspite of large amount of work done in this field good ISEs are still not available for many heavy metal ions and therefore, further efforts are needed in this direction. The availability of highly selective materials such as crown ethers, porphyrines, calixarenes, newer ligands and their metal chelates has opened up the possibility of developing more electrodes which may turn out to be efficient and selective and better than those already reported. Therefore, the efforts were initiated by me using three groups of compounds namely crown ethers, porphyrines, ligands and their metal chelates to develop electrodes for heavy metals. The results obtained on the electrochemical performance of the membranes of these materials are briefly summarized as under. Estimation of Cd2+, Pb2+ and Cr3+ is important as they are toxic and occur in various effluents and waste waters. A number of ISEs have been reported in literature for their estimation. They have been found not very suitable as they generally show limited working concentration range, high response time and poor selectivity. Better electrodes are, therefore, required. To meet this aim; I have used crown ethers to prepare electrodes for these metals. Three crown ethers viz. dicyclohexano-24-crown-8 (I), N,N'-dibenzyl-l ,4,10,13-tetraoxa-7,16-diazacyclo- (ii) octadecane (II) and tri-o-thymotide (III) were found suitable. Homogenous membranes of these crown ethers could not be studied due to their fragile nature. Therefore, their PVC based membranes were investigated. It was found that PVC based membranes of (I), (II) and (III) responded selectively to cadmium, lead, and chromium ions. Further studies revealed that the performance of the membranes got changed on addition of plasticizers viz. di-butylphthalate (DBP), di-octylphthalate (DOP), dibutyl(butyl) phosphonate (DBBP), 1-chloronaphthalene (CN), tributylphosphate (TBP) and tris-(2-ethylhexyl)phosphate (TEP). The plasticizer which improved the performance most, was picked up. The membranes of various compositions were prepared and after trials, the optimum composition of the membrane giving widest working concentration range, low response time and reproducible results was determined. The PVC based membrane of dicyclohexano- 24-crown-8 having the composition of 10:1:100:100 (I:NaTPB:PVC:DBBP) (w/w) was found to give the best performance. The electrode of this membrane generates linear potential response within the concentration range 3.0xlO"5-1.0xlO"'M Cd2+. The potential developed is stable and is attained within 23s. The electrode works satisfactorily in the pH range 3.0-6.4 and was used over a period of 5 months without any significant drift in potentials. The selectivity of this cadmium selective electrode was assessed in terms of selectivity coefficient values determined by matched potential and fixed interference method. The results have revealed that the electrode is selective to Cd2+ ions compared to large number of metal ions. Only Na+ and Ca2+ ions caused some interference at high concentrations. However, it could be used in reduced working concentration ranges of 4.2x10" -l.OxlO"1, 9.6x10" -l.OxlO"1 and (iii) 3.1xlO"3-1.0xlO"1M in presence of l.OxlO"4, l.OxlO"3 and l.OxlO"2 MNa+ while the working concentration range reduced to 4.7xl0"5-1.0xl0"1, 2.3xl0"4-1.0xl0"1 and 6.5xlO'3-1.0xlO"1M in the presence of l.OxlO"4, l.OxlO"3 and l.OxlO"2 M Ca2+ respectively. The performance of the electrode was also evaluated in the presence of non-aqueous media and it was found that the electrode's response to Cd2+ remains largely unaffected in presence of 40% (v/v) non-aqueous content of methanol, ethanol and acetone. Similarly, the membranes of II and III and having the optimum compositions of 12:1:100:200 (II:NaTPB:DBP:PVC) and 10:2:150:200 (III:NaTPB:DBP:PVC) were found to respond to lead and chromium ions in the concentration ranges 8.2xlO~6-1.0xlO"' and l.OxlO'M.OxlO"1 M respectively. Pb(II) sensor was found moderately selective over most of the cations except Ag+ and Cd2+. However, the sensor could be used in reduced working concentration ranges of 9.8xl0"6-1.0xl0"1 and 7.3xl0"5-1.0xl0"1M in the presence of l.OxlO'4 and l.OxlO"3 Ag+ while the working concentration range reduced to l^xlO'M.OxlO'1 and 9.7xlO"5-1.0xlO"' M in the presence of l.OxlO"4 and l.OxlO'3 Cd2+ respectively. On the other hand Cr3+-selective electrode also exhibited selectivity over most cations but Na+ and Cd + were found to interfere when present at higher concentration level. As a result the working concentration range of electrode got reduced to 1.3x10"-l.OxlO'1, 7.9xlO'5-1.0xlO'' and I^xKTM.OxKT'M in presence of l.OxlO"4, l.OxlO"3 and l.OxlO'2 M Na+ and to 3.5xlO"5-1.0xlO"1, 9.0xl0"5-1.0xl0"1 and 4.0xlO"3-1.0xlO"1M in presence of l.OxlO"4, l.OxlO"3 and l.OxlO"2 M Cd2+. These sensors show fast (iv) response time, long life time and performed well in presence of 25 and 30% non aqueous content, respectively. Porphyrins were investigated by me to prepare ISEs. Investigations revealed that the membrane of 5,10,15,20-tetraphenyl porphyrin (IV) could be developed as Zn2+-selective electrode. The homogeneous membranes of the porphyrin were not found suitable due to its poor mechanical stability and membranes, therefore, were prepared using PVC as binder. A number of trials revealed that the membranes having composition 5:200:200:2 (IV:PVC:TBP:NaTPB) (w/w) was found to give the best performance with regard to working concentration range, response time and selectivity. The electrode prepared from the membrane of this composition responded to Zn2+ over a wide concentration range of 6.2xl0"6-1.0xl0'1 Mwith a slope of 29.0 mV/decade of activity. The electrode exhibits fast response time of 12s, generated reproducible potential (standard deviation of 0.3 mV) up to a period of 6 months and can function satisfactorily in partially non-aqueous media with non aqueous content 40%. Selectivity of this electrode is also found to be good as most ions do not interfere. The only ions which interfere at higher concentration level are Na+ and Mg2+. Therefore, in presence of these ions, the electrode functions over the reduced concentration range. The mixed run studies revealed that Zn + could be estimated in the reduced concentration ranges l.OxlO'M.OxlO'1, 3.2x10'-1.0x10' and 7.9xl0"4-1.0x10"' M in presence of l.OxlO"4, l.OxlO"3 and 1.0xlO"2M Na+ whereas similarly, the presence of l.OxlO"4, l.OxlO"3 and 1.0xlO"2M Mg2+ restricted determination of Zn2+ to the concentration range 1.6xlO"5-1.0xlO"1, 4.0xlO'5-1.0xlO"1 and 5.0x10"4-l.OxlO'1 M by this electrode.

Besides crown ethers and porphyrines, selective ligands and metal chelates have also been used to prepare ISEs. It was found that organic ligands methylene bis salicyl (o-aminobenzylalcohol) (V) and methylene bis salicyl (o-hydroxybenzylamine) (VI) and its copper chelate (VII) were found to be electroactive material and their PVC based membranes responded selectively to nickel, cobalt and copper ions respectively. The optimum composition of the membranes giving the best performance was found after a number of trials. The PVC based membranes of the optimum composition 10:2:200:200 (V:NaTPB:DOP:PVC), 10:150:2:200 (VI:DBP:NaTPB:PVC) and 5:1:75:100 (VII:NaTPB:DBBP:PVC) responded to nickel, cobalt and copper ions in the concentration ranges 9.1xl0"6-1.0xl0"', 1.7xl0"5-1.0xl0"' and 4.5xlO"6-1.0xlO"1M, respectively. The slope of all these three electrodes was found to be Nernstian. Selectivity coefficient determined by matched potential method and fixed interference method showed that these three electrodes could be used in presence of large number of metal ions. Na+ was found to be the only ion that interferes in the functioning of these electrodes. Thus, nickel, cobalt and copper could be determined over reduced concentrations ranges in presence of sodium ion. All the three electrodes exhibit fast response time, generated reproducible potentials and have sufficient life time (six months for Ni2+ and Cu2+ and three month for Co2+). They also work satisfactorily in partially non-aqueous medium. The seven electrodes prepared in this thesis performed satisfactorily over wide concentration range. The electrodes exhibit fast response time, generate reproducible potential and have life time of 3-6 months. The selectivity is good, (vi) potential response is Nernstian or near Nernstian and the electrodes can also function in partially non-aqueous media. Considering the overall performance of the electrodes, their preparation represents an improvement in some respects especially selectivity, response time and working life time over the existing electrodes.