ELSEVIER Sensors and Actuators 3 40 (1997) 15-20 Nickel( II) -selective sensors based on heterogeneous membranes of macrocyclic compounds AK. Jain *, V.K. Gupta, RD. Singh, U. Khurana, L.P. Singh Department of Chemistry, University of Roorkee, Roorkee 247 667, India Received 9 February 1996; revised 19 June 1996; accepted 7 August 1996 Abstract Membranes of 5,7,7,12,14,14-hexamethyl-1,4,&l I-tetraazacyclotetradeca-4,11-diene diperchlorate (I) and 3,5,7,7,10,12, 14,14-octame- thylYi,4,8,1I-tetraazacyclotetradeca-4,11-diene diperchlorate (II) in PVC andpolystyrene binders have been investigated as Ni’+-selective sensors. ThePVC membranes containing macrocycles, di-n-butylphthalate and dioctylphthalate as plasticizing solvent mediators and sodium tetraphenylborate as ananion excludershow near-Nernstian response in the concentration range= 10-5-10-’ moll-‘, while polystyrene- based membranes exhibit linearity in the concentration range = 10-6-10-’ mol 1-l. These membranes have response times as fast as15 s and canbe used over a period of six months. Theelectrodes work in thepH range1.7-5.4and also perform well in alcohol-water mixtures with alcoholic content up to 50%. The selectivity coefficient values reveal that the electrodes respond to Ni2+ ions selectively over a large number of cations. The membrane has also been used as an indicator electrode for the determination of the end point in the potentiometric titration of Ni2+ against oxine. Keywords: Ion-selective electrodes; Nickel sensors; Macrocycles 1. Introduction The development of selective chemical sensors has received widespread attention during the past two decades because of their possible use in environmental monitoring, as they provide a rapid, accurate and low-cost method of analysis. As a result of extensive research in this area, a number of ion-selective electrodes ( ISEs) mainly for alkali, alkaline earth metals and some anionsare now commercially available. However, relatively fewer reports are available on the development of ISEs for heavy metal cations. The deter- mination of nickel assumes importance in view of its toxicity and widespread occurrence in hydrogenated vegetable oils, chocolatesand various effluents. A literature survey reveals that the first nickel-selective electrode was reported by Pun- gor andcoworkers using anickel-dimethylglyoximecomplex [ 11. Later heterogeneous membranes of nickel phosphate in paraffin and silicone rubber [ 21, nickel bis-( 2-ethylhexyl) phosphate in PVC [3] and a nickel complex of 1,4,X,1 l- tetraazacyclotetradecane in Araldite [ 41 were usedfor pre- paring electrodes.In addition to thesesolid-membrane ISEs, liquid-membrane electrodes for nickel using 0,O’diisobutyl dithiophosphatonickel (II) in chlorobenzene [ 51, bis( tetra- * Correspondingauthor. Phone: +91 133272349.Fax: +91133273560. 0925-4005/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PIISO925-4005(96)02139-9 ethylammonium) bis( dithiobenzophenazine) nickelate in nitrobenzene [ 61 and nickel diethyldithiocarbamate in chlo- roform [7] have also been reported. These electrodesshow poor selectivity and reproducibility and exhibit non-Nerns- tian response.Thus, a good ISE for nickel is yet to be developed. Various materials, namely inorganic ion exchangers, organic ion exchangers,metal chelates,crown ethers, cryp- tands and macrocycles,may be employed assensor materials for the fabrication of membranes. A good electroactive mate- rial to be usedin membranes ought to show selective uptake/ exchange or extraction or binding. Crown ethers, cryptands and macrocycles are the recent materials of interest for their use in ISEs becauseof their selective complexation with metals. Their structure favours the formation of apolar cavity in which a specific ion can be enclosed because of ion-dipole interactions. Cryptands form strong complexes, whereas the complexes of crown ethers are weak and, therefore, armed macrocycleshaving intermediate stability constants can prof- itably be used as materials for the fabrication of selective membranes[8]. Keeping this in view, we have explored two macrocycles, 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra- azacyclotetradeca-4,11-diene diperchlorate (I) and 3,5, 7,7,10,12,14,14-octamethyl-1,4,&l l-tetraazacyclotetradeca- 4,l l-diene diperchlorate (II), that show selective complex-