DEVELOPMENT OF SOME POTENTIOMETRIC SENSORS FOR THE DETERMINATION OF TOXIC IONS

Abstract

Various instrumental techniques such as atomic absorption spectrophotometry (AAS), inductively coupled plasma-atomic emission spectrophotometry (ICP-AES), fiuorimetry, gas chromatography (GC) and high performance liquid chromatography (HPLC) and so forth are available to analytical chemists. These methods provide sensitive and reproducible results, but often requirement of chemical manipulation of the sample and expensive infrastructure back up makes them not very appropriate for routine analysis of large number of environmental samples in field. Besides this, the use of these techniques can be problematic and erroneous in coloured and/or turbid solutions. A technique which provides rapid, accurate, low cost and on-line method of analysis is the ideal choice for chemists and such a situation is met to a great extent by ion selective sensors (ISSs). Ion selective electrodes are also commonly known as potentiometric chemical sensors. The important requirement for a selective ion sensor is a membrane which allows transfer of a particular ion more as compared to others. Various materials such as solid electrolytes, metal chelates, macrocycles, crown ethers, calixarenes, porphyrines cryptands and inorganic and organic ion exchangers etc have been used as a active component responsible for the selective transport of the ions. These electroactive materials are often referred as ionophore. Some good ion selective sensors (ISSs) have been made available for H+, NH/, Na+, K+, Ca2+, Ag+, CN", NO3", S2", SO42", halide ions etc. Besides this, sensors for many other heavy metals have also been reported but they suffer from poor sensitivity and selectivity, long response time and limited pH range. (i) Therefore efforts are needed to prepare selective and sensitive sensors for many heavy toxic metal ions. It is obvious that the development of ion sensor requires an ionophore which has high affinity for a particular ion and poor for others. Such materials are not easily available. However, newer materials such as dendrimeric porphyrins, Schiffs bases, neutral carriers and derivatized calixarens are being continually synthesized and some of them have shown high affinity for a particular ion and therefore, possess the potential to provide selective membrane sensor for them. Thus, we have explored some newly synthesized and selective dendrimeric porphyrins, Schiffs bases, neutral carriers and calixarens for preparing Cu2+, Ni2+, Fe3+, Mn2+, Zn2+, Al3+, Pb2+ and Cd2+ selective sensors. The work carried out reported in this thesis. The porphyrins are unique class of naturally occurring macrocyclic compounds. They have attracted attention as good ionophores for ions due to their electronic and structural characteristics and some of them selectivity for specific ions. Keeping in this mind we have investigated newly synthesized porphyrins viz. meso-tetrakis-[4-(allyl dimethyl silyl)phenyl] prophyrin (I) and meso-tetrakis-[4-(diallyl methyl silyl)phenyl] prophyrin (II) as Cu2+ selective sensors and meso-tetrakis-{4-[tris-(4-allyl dimethylsilylphenyl)- silyl]-phenyl}porphyrin (III) as Ni2+ selective sensor. Homogeneous membranes of these porphyrins (I, II and III) could not be studied due to their fragile nature. Therefore, PVC based membranes of porphyrin were prepared with and without plasticizers. The results shows that membrane of porphyrin (I) without plasticizer exhibits linear response to Cu2+ over a working activity range of 1.0 x 10"5 to 1.0 * 10"1 M with a slope of 30.6 mV/decade of activity. It was found that the addition of plasticizers to the PVC based membranes improves their performance characteristics. (ii) ' Thus, in order to improve performance characteristics of PVC based membranes of (I) the addition of various plasticizers viz., dioctyl phthalate (DOP), tri-n-butylphosphate (TBP), 1-chloro naphthalene (CN) and dibutyl(butyl) phosphonate (DBBP), was tried and their potential response investigated. It is seen the addition of plasticizers to the membranes of (I) does not significantly affect the performance of membrane. However, amongst the four plasticizers added, DOP improves performance of the membrane of (I) as it exhibits maximum working activity range of 6.3 x 10"6to 1.0 x 10"1 Mwith almost Nernstian slope of 29.4 mV/decade of activity. Similar studies were also carried out with the membranes of porphyrin (II) and the results show that in this case also, the addition of DOP improves the performance of the membrane of (II) which shows the widest working activity range 4.4 x 10'6 to 1.0 x 10'1 M and a Nernstian slope of 29.3 mV/decade of activity. The membranes having composition of (I)(5):PVC(150):NaTPB(2):DOP(150) (w/w; mg) and of (II)(5):PVC(150):NaTPB(2):DOP(150) (w/w; mg) performed best and showed maximum working activity ranges. Of the two porphyrins , the sensor based on porphyrins (II) is found to be more selective and exhibits widest working activity range, minimum response time and maximum shelf life. This sensor is comparable in most respects to good reported sensors but superior to them in terms of selectivity over Hg2+, Ag+ and Fe +ions. As the sensor based on porphyrin (II) shows better selectivity than (I) it was applied for the estimation of copper in real samples and could be used to determine Cu2+ in foliar and swimming pool water samples successfully. Similarly, the membranes of meso-tetrakis-{4-[tris-(4-allyl dimethylsilyl-phenyl)-silyl]-phenyl}porphyrin (III) were investigated as Ni selective sensor. The optimum composition of the membrane of (iii) porphyrine (III) performing best was found to be (III) (5):PVC (150): NaTPB (5) (w/w; mg). It was found that the addition of plasticizers to the membrane (III) does not affect the performance of membranes as in the case of porphyrins (I) and (II). Thus, the membrane of (III) without plasticizer exhibits widestworking activity range of 2.5 x 10" to 1.0 x 10"1 M with Nernstian slope of 29.5 mV/decade of activity and shows high selectivity towards Ni2+ over a large number of interfering ions and therefore, could be employed for Ni2+ determination at trace level in some Indian brands of chocolates and also as an indicator electrode in potentiometric titrations. Besides dendrimeric porphyrins, neutral carriers have also been used to prepare ion selective sensors (ISSs). It was found that neutral carriers N,N',N",N'"-l,5,8,12- tetraazadodecane-bis(salicylaldiminato) (IV), 2-phenyl-l,3-bis[3'aza-4'-(2'- hydroxyphenyl)-prop-4-en-1'-yl]-1,3-imidazolidine (V), N,N'-bis(2-hydroxy-1 - napthalene)-2,6-pyridiamine (VI), iV^-bis[2-(salicylideneamino)ethyl]ethane-1,2- diamine (VII), dithizone (VIII) and morin(IX) were found to be good ionophores. Thus their PVC based membranes of optimum composition, (IV) (10): PVC (150): NPOE (150): NaTPB (10) (w/w; mg), (V) (10): PVC (150): CN (200): NaTPB (9) (w/w; mg), (VI) (3): PVC (120): NPOE (150): NaTPB (2) (w/w; mg), (VII) (5): PVC (150): CN (150): NaTPB (5) (w/w; mg), (VIII) (5): PVC (150): CN (150): NaTPB (2) (w/w; mg) and (IX) (5): PVC(150): TBP (150): NaTPB (5) (w/w; mg) showed selective response to Mn2+, Fe3+, Pb2+, Ni2+, Zn2+ and Al3+, respectively in the activity range 5.0 x 10"6 to 1.0 x 10"1, 6.3 xl0"6tol.0x 10"1, 3.2 x 10"* to 1.0 x 10"1, 5.0 x 10"6to 1.0 x 10"1, 5.1 x 10"6to 1.0 x 10"1 and 5.0 x 10"7 to 1.0 x 10'1 M. The slope of these sensors was found to be Nernstian. The values of selectivity coefficient showed that these six sensors could be (iv) used in presence of large number of metal ions. All the six sensors exhibit fast response time, generate reproducible potential and have sufficient life time (2.5 months for manganese (II), 2 months for iron (III) and aluminum (III), 6 months for lead (II) and zinc (II) and 4 months for nickel (II)). They all also work satisfactorily in partially non aqueous medium. Recent studies have shown that in addition to dendrimeres and neutral carriers, calixarenes can also be the potential source for use as electroactive component in the membrane ion selective sensors (ISSs) because of their unique property of selective complexation with metals. A survey of literature reveals that on varying the lipophilicity of the thiacalix[4]arene, a higher extraction efficiency was observed for the less hydrophobic complexing moiety. Therefore, the membranes of t-butyl thiacalix[4]arene (X) and thiacalix[4]arene (XI) were explored as Cd selective ion sensors. Thus, several membranes were prepared with and without plasticizers and investigated. It was found that the membrane having composition (X) (5): PVC (120): NPOE (150): NaTPB (3) (w/w;mg) and (XI) (5): PVC (120): NPOE (150): NaTPB (3) (w/w;mg) performed best and showed working activity range 7.1 x 10"6 to 1.0 x 10"1 and 3.2 x 10"6 to 1.0 x 10"1 M Cd , respectively. These sensors showed Nernstain response and also worked satisfactorily in partially non-aqueous medium. The selectivity studies for the two sensors under consideration showed that the sensor based on thiacalix[4]arene (XI) is more selective for Cd2+ over a number of ions as compared to sensor based on t-butyl thiacalixarene (X). Not only this, the sensor of (XI) found to be better than reported sensors in term of high selectivity towards Cd2+ over many reported sensors. This sensor (v) was used for the estimation of cadmium in water samples and zinc plating mud, successfully. Thus, the present investigations on the membranes of some porphyrins, neutral carriers and calixarenes resulted in the development of selective and sensitive sensors for Cu2+, Ni2+, Mn2+, Fe3+, Pb2+, Zn2+, Al3+ and Cd2+ cations. All these sensors are mechanically as well as chemically stable, have long shelf life of about two & half months to six months, low response time of about 8-15s, reasonably wide working activity range with Nernstian slope and exhibit good selectivity. A comparison of their performance with reported sensors show that the ion sensors developed for Cu +, Pb and Cd2+ are comparable to the reported sensors. However, the prepared sensors for Ni + are better in terms of selectivity compared to reported sensors as Cu2+, Ni2+ and Ag+, Hg2+ do not interfere in their functioning, respectively whereas reported electrodes show interference to these ions. The sensors for Mn2+, Fe3+ and Al3+ have also been found to be better than reported sensors in terms of sensors characteristics, e.g., widest working activity range, low response time and less interference to different metal ions. Thus, the present work on ion sensor has resulted in the availability of some improved ion sensors which can be used successfully for analytical purposes.