SYNTHESIS OF SOME NEW lONOPHORES AND INVESTIGATION ON THEIR POTENTIALITY IN ION SENSING

Abstract

The quantitative determination of a target species is an area of paramount importance and research. An analytical technique employed must be accurate, sensitive, simple and convenient to operate. Numbers of techniques are at present available for the quantitative analysis of cations as well as anions. Amongst these UV-Visible and Infrared Spectrometry, Atomic Absorption Spectrometry (AAS), Flame Photometry, Fluorometry, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) are playing a key role. Though these techniques are highly sensitive and give reproducible results but are quite expensive and also very often require chemical manipulation of the sample. Ion-selective electrodes (ISEs) are an alternative to these expensive techniques and the field is being widely explored these days. The ion-selective electrodes (ISEs) have certain general advantages over conventional instruments for quantitative ion analysis such as, in portability of the device and in sample non-destruction. Besides this these are quite inexpensive and laboratory made. Fabrication and investigations using ISEs in the laboratory are fairly easy and have potential to be commercialized soon after their development. Considerable research has been carried out in the area of ion-selective electrodes in the last two decades, which is evident from the large and rapidlyexpanding volume of literature on this subject. Various types of ISEs have been developed and the materials tried for the purpose of fabrication of membranes include solid electrolytes, macrocycles, inorganic and organic ion exchangers, (i) crown ethers, metal chelates, cryptands, calixarenes etc. As a result of extensive research in the area, ISEs are now commercially available for hydrogen, ammonium, alkali, alkaline earth metal ions, halides, cyanide, nitrate, nitrite, sulphide, sulphate and a few heavy metal ions. However a survey of literature indicated that still there is a need of developing newer/better ion-selective electrodes for the determination of cations and anions. Therefore, an effort has been made to synthesize some new ionophores, and these have been used to fabricate ISE membranes. The selectivity of these ISEs and their other characteristics are the subject matter of this thesis. The results of the investigation are briefly summarized as under: The tetramefhyldibenzocyclam derivatives are synthesized, characterized and have been used for fabrication of potentiometric sensors for Ni2+ and Cu2+. [Ni{Me4Bzo2[14]aneN4}]Cl2 (5,7,8,14-tetramethyldibenzo[b,i]-l,4,8,11-tetrazacyclotetradecanenickel( II)chloride), (II) was obtained in good yield as an orange red salt by Fe/HCl reduction of the corresponding tetraazaannulene complex [Ni(Me4taa)J, (I). Complex (II), upon treatment with an excess of aqueous NaCN, yielded the metal-free macrocycle, (III). These compounds have been characterized using a combination of IR, UV-Visible, 'rlNMR and mass spectroscopic data. [Ni{Me4Bzo2[14]aneN4}]Cl2 (II) was tried for developing a Ni +-selective electrode and the metal-free dibenzocyclam Me4Bzo2[14]ane N4 (5,7,8,14-tetramethyldibenzo[b,i]-l,4,8,l 1-tetrazacyclotetradecane), (HI) was chosen for developing a Cu2+-selective electrode. (ii) The optimum composition ofthe membrane was determined by successive repeated trials. The effect ofaddition ofdifferent plasticizers e.g. dioctylphthalate (DOP), dibutyl butylphosphonate (DBBP), tri-n-butyl phosphate (TBP), tris-2- (ethylhexyl)-phosphate (TEP) and chloronaphthalene (CN) was studied. The concentration ofthe equilibrating solution and time ofcontact were also optimized so that the membrane, which imparts good stability as well as better electroanalytical properties, is generated. Thus the membrane exhibiting the best performance with regard to working concentration range, slope and response time was chosen for further studies. Membrane having LNi{Me4Bzo2[14]aneN4}]Cl2 (II) as electroactive material, sodium tetraphenylborate (NaTPB) as an anion discriminator, dibutyl butylphosphonate (DBBP) as plasticizer in PVC matrix in the percentage ratio 5:2.5:100:100 (II:NaTPB:DBBP:PVC)(w/w) exhibits a linear response to Ni2+ in the concentration range 7.0xl0"6- I.OxKT'M with a slope of 29.8±0.2 mV/decade of activity and afast response time of 12s. The sensor works well in the pH range 2.0-7.6 and can be satisfactorily used in presence of40% (v/v) methanol, ethanol or acetone. The sensor is selective for Ni2+ over a large number of cations with slight interference from sodium and copper only (>1.0xl0"5M). This membrane gives standard deviation of 1.4 mV in the observed values of potentials in the working concentration range from the least square fit line, and excellent consistency in slope with 90% confidence limit lying with in ±0.2 mV/decade of activity. For Cu +, the best performance was shown by the PVC based membrane

of macrocycle Me4Bzo2[14]aneN4 (III) having the composition 5:3:100:150 (III:NaTPB:DBBP:PVC). The sensor exhibits a wide concentration range 2.5xl0"5 - 1.0x10"'M with a slope of 30.2+0.3 mV/decade of activity, a fast response time of 13s and a lifetime of 5 months. The working pH range of this sensor is 2.6-5.5 and it shows excellent selectivity for Cu +over other mono-, bi- and trivalent cations, which are reported to cause interference in the working of other Cu sensors. Its performance in partially non-aqueous medium has been found to be satisfactory with a 40%(v/v) methanol, ethanol and acetone. The sensor has been successfully used as indicator electrode in the potentiometric titration of Cu (II) ions against EDTA. These electrodes can be used to determine the concentration of Ni + and Cu"+ in real samples. This membrane gives standard deviation of 2.15 mV in observed values of potentials in the working concentration range from the least square fit line. This membrane gives excellent consistency in slope with 90% confidence limit lying within ±0.3 mV/decade of activity. Twenty repeated measurements of potential on the same portion of the sample gave -99% reproducibility. Three metallotetraazaporphyrins [M(TAP)] viz., octabenzylthiotetraazaporphyrin complexes of Magnesium(II), Iron(III) and Cobalt(II) that bear eight benzylthio substituents at P-positions were synthesized. The magnesium(II)- tetraazaporphyrin (IV) being a literature known compound has been demetallated using standard procedure to obtain metal-free macrocycle (V). The metal-free macrocycle (V) was reacted with ferric acetate and cobaltous acetate to obtain (iv) corresponding complexes (VI) and (VII). The demetallation and remetallation processes were monitored spectrophotometrically. All these compounds have been characterized by metal determination, IR and electronic spectral data. They have been incorporated as ionophores into PVC membranes for the fabrication of magnesium, azide and nitrite selective electrodes respectively. For the [Mg(TAP)] complex (IV), membrane having IV:NaTPB:DOP:PVC in the ratio 10:2:133:200 (w/w) exhibits the best results for Mg2+ ions with the linear potential response in the concentration range 9.4xl0"6-1.0xl0"'M and aslope of 29.2±0.4 mV/decade of activity. The electrode works well in the pH range 3.5- 7.8 with a fast response time of 13s. It shows high selectivity for Mg2+ over other mono-, bi- and trivalent cations except K+ and Zn2+, which cause slight interference ifpresent at a level of >1.0x10-5 M. Its performance in partially non aqueous medium with up to 30%(v/v) methanol, ethanol and toluene has also been satisfactory. It can be used over a period of 5 months with good reproducibility (-1% error). The sensor has been successfully used as an indicator electrode in potentiometric titration of Mg2+ against EDTA. This membrane gives 3.86 mV standard deviation in observed values of potentials in the working concentration range from the least square fit line, and excellent consistency in slope with 90% confidence limit lying within ±0.4 mV/decade of activity. For [Fe(TAP)]+ complex (VI), membrane having the composition 6:100:200 (VI:DBP:PVC) (w/w) shows good properties with slope of 29.2±0.2 mV/decade of activity and a working concentration range of 8.9xl0"6-l .0x10"' M. The working pH range of this (v) electrode is 4.3-8.6. It displays a fast response time (15s) and gives reproducible results for about 5 months. The proposed electrode exhibits high selectivity for N3" over other monovalent and bivalent anions. Only SCN" and S2" (>1.0xlO'4M) cause slight interference. It also works satisfactorily in solvent mixtures having 40% (v/v) methanol, ethanol and toluene. This membrane gives 1.96 mV standard deviation in the observed values of potentials in the working concentration range from the least square fit line, and excellent consistency in slope with 90% confidence limit lying within ±0.2 mV/decade of activity. The [Co(TAP)] complex was used as an electroactive material for developing nitrite-selective electrode. The membrane with the composition in the ratio 10:152:200 (w/w) (VII :DOP:PVC) exhibits the best properties with a working concentration range of 1.1x10"5-1.0x10"'M and a slope of 30.0±0.2 mV/decade of activity. The working pH range of this electrode is 2.8-6.4 and it can tolerate a non-aqueous content of methanol, ethanol and acetone (40% (v/v)). It displays a fast response time of 13s and a lifetime of 5 months. This membrane gives 1.80 mV standard deviation in the observed values of potentials in the working concentration range from the least square fit line, and excellent consistency in slope with 90% confidence limit lying within ±0.2 mV/decade of activity. The copper(II)-ethambutol (VIII) complex has been synthesized and characterized using metal determination, IR and electronic spectral data. When this complex is incorporated in a PVC matrix its geometry gets frozen in the solid lattice, thereby, giving a Cu specific site in the membrane. The membrane having

Cu(II)-ethambutol complex (VIII) as electroactive material, along with sodium tetraphenylborate (NaTPB) as an anion discriminator, dioctylphthalate (DOP) as plasticizer in PVC matrix in the percentage ratio 6:2:190:200 (II:NaTPB:DOP:PVC)(w/w) exhibits a linear response to Cu2+ in the concentration range 7.9xl0"6- 1.0x10"' Mwith a slope of 29.9±0.2 mV/decade of activity and a fast response time of lis. The sensor works well in the acidic media in the pH range 2.1-6.3 and could be satisfactorily used in presence of 40% (v/v) methanol, ethanol and acetone. The sensor is selective for copper over a large number of cations with slight interference from Na+ and Co2+ (if present at a level >1.5xlO"5 and 6.5x10"5Mrespectively). It works well over aperiod ofsix months. It could be used as indicator electrode for the end point determination in the potentiometric titration of Cu2+ against EDTA and in the determination of Cu2+ in real samples. The maximum error (-1%) was observed by running twenty-five measurements. This membrane gives 1.60 mV standard deviation in the observed values of potentials in the working concentration range from the least square fit line, and excellent consistency in slope with 90% confidence limit lying within ±0 2 mV/decade of activity. It can be summarized that the present studies have helped in developing better sensors than the existing sensors for Ni2+, Cu2+, Mg2+, N3" and N02" in terms of working concentration range, pH range, slope, response time, life time and applicability in the real samples analysis. Thus, these are good addition to the available sensors for these ions.