Analyst, July 1994, Vol. 119 zyx 1613 z Application of a Picrolonate Ion-selective Electrode to the Assay of Calcium and Piperazine in Pharmaceuticals and Serum Panayiotis G. Veltsistas, Mamas I. Prodromidis and Miltiades I. Karayannis* Laboratory zyxwvutsrq of Analytical Chemistry, University zyxwvuts of Ioannina, University Campus, zyxwvutsrqp 451 I0 Ioannina, Greece The construction and some analytical applications of a liquid picrolonate (Picrl-) ion-selectiveelectrode based on the zyxwvu tris(phenanthrolinato)iron( zyxwvutsrqp 11) (ferroin) picrolonate salt, dissolved in 2-nitrotoluene, are described. The liquid membrane electrode exhibits a rapid and almost Nernstian response to picrolonate anions in the concentration range from 5 X 10-2 to 5 X 10-2 mol 1-1. The response is virtually unaffected by pH changes in the range S10. Major interferents are picrates, 4-nitrophenolates, 33- dinitrosalicylatesand 2,4-dinitrophenolates. In analytical applications direct potentiometric methods for the determination of picrolonates and indirect methods for the determination of calcium and piperazine are described. Calcium in the ranges 4-24 and 50-90 mg 1-1, under specified experimental conditions, can be determined with mean relative errors of 1.1 and 1.4%, respectively. The method is applicable to soluble samples, pharmaceuticals and industrial and real human serum. Piperazine in the ranges 1-10 and 1S20 mg can be determined with a mean relative error of 1.2% in soluble salts and pharmaceutical formulations, respectively. An accuracy of better than 1.5% and a reproducibility (relative standard deviation) of 0.1 YO were achieved in the analysis of pharmaceutical products. zyxwvut Keywords: Picrolonic acid; picrolonate liquid ion-selective electrode; calcium and piperazine determination; pharmaceuticals; serum Introduction A picrolonate (Picrl-) ion-selective electrode (ISE) using an ion exchanger consisting of the ferroin picrolonate salt as an electroactive material in 2-nitrotoluene as a solvent is described. The picrolonate-selective membrane electrode has been used successfully in direct potentiometry for the determination of picrolonic acid, calcium cations and pipera- zine cations. Picrolonic acid (PicrlH), C10H805N4, is a strong monoprotic organic acid due to its pyrazolonic hydrogen with pK zyxwvutsrqpo = 1.00, and it may exist in the enol or keto form according to the equilibrium given below. It is sparingly soluble in water (0.12% at 17 "C) and one of its soluble compounds is its salt with the triethanolammonium cation (TEA+). 02NFcH3 0 NMN L I - I zyxwvutsrqponmlkjihg Q Q * To whom correspondence should be addressed. The compound is used extensively in pharmaceutical,' clinical,2 biological ,3 biochemical4 and radiochemicals analy- sis. It is slightly soluble in ethanol, more soluble in warm methanol and freely soluble in dirnethyl sulfoxide and dimethylformamide . Picrolonic acid forms very stable com- pounds with transition metal and alkaline earth cations, usually insoluble in water. The sparingly soluble salts of Ca2+ , Pbzt-, Sr2+ , Th4+ and some organic bases and alkaloid cations6 have been suggested for their determination. The last are sparingly soluble in water and ethanol and exhibit characteris- tic melting-points. The acid was therefore successfully used also for the isolation and determination of many metals and organic ligands.7 The crude product must be purified by recrystallization from methanol, after filtering through Whatman No. 43 filter- paper. An efficient thin-layer chromatographic (TLC) method has been developed for purity control, using dioxane- acetonitrile-triethanolamine (5 + 1 + 0.5) as the solvent and alumina plates. The TLC plates are exposed in an iodine chamber and the substances appear as yellow-brown spots. Piperazine is a crystalline, hygroscopic, strong diprotic base usually available in the hexahydrato form. It is very soluble in water, with dissociation constants pK1 = 9.8 and pK2 = 5.7. From the above values it is evident that its diprotonated form predominates at pH <3 (>99.99%). Experimental Preparation of the Liquid Exchanger Various bulky insoluble picrolonate salts have been synthe- sized and tested as electroactive materials. Mercury(rr), triphenyltetrazolium, nitronium, papaverinium, Aliquat 336, ferroinium and hexadecylpyridinium picrolonate salts have been tested as ion exchangers, each in connection with 2- nitrotoluene, decan-1-01 or o-nitrobenzaldehyde as solvents. Combinations of triphenyltetrazolium picrolonate or hexa- decylpyridinium picrolonate with 2-nitrotoluene gave good ion exchangers, but they showed drawbacks such as limited lifetime, poor solubility in conventional solvents and photosensitivity. Procedure For the preparation of the liquid ion exchanger, 5 ml of a 0.025 mol I-' ferroin solution and 4 ml of a 0.05 mol I-' triethanolammonium picrolonate solution are mixed in a 100 ml separating funnel, then 40 ml of 2-nitrotoluene are introduced and, after strong shaking, the system is allowed to settle into two separate layers. The aqueous layer is discarded and the organic phase is washed 34 times with distilled water. After drying with anhydrous Na2S04 for 6 h, the system is filtered through Whatman No. 42 filter-paper into a well stoppered amber-coloured bottle.