Microchim Acta 150, 47–54 (2005) DOI 10.1007/s00604-005-0323-x Original Paper Carbon Paste Electrode for the Potentiometric Flow Injection Analysis of Drotaverine Hydrochloride in Serum and Urine Yousry M. Issa, Hosny Ibrahim  , and Hazem M. Abu-Shawish Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt Received August 14, 2004; accepted January 18, 2005; published online March 31, 2005 # Springer-Verlag 2005 Abstract. A carbon paste electrode for drotaverine hydrochloride (DvCl) was prepared and fully charac- terized in terms of composition, life span, usable pH range, response time and temperature. The electrode was applied to the potentiometric determination of drota- verinium ions in pharmaceutical preparations and biolo- gical fluids in steady state and flow injection conditions (FIA). The electrode is based on a mixture of two ion exchangers, namely, drotaverinium-silicotungestate and drotaverinium-tetraphenylborate dissolved in tricresyl phosphate as pasting liquid. The modified electrode showed a near-Nernstian slope of 59.34  2 mV over the concentration range of 5.0  10 7 –1.0  10 2 M. The electrode exhibits good selectivity for DvCl with respect to a large number of inorganic cations, organic cations, sugars and amino acids. Potentiometric titrations of DvCl with several titrants have been monitored using this modified carbon paste electrode as an end-point indicator electrode. The proposed electrode offers the advantages of simplicity, accuracy, automation feasi- bility and applicability to turbid and colored samples. Key words: Drotaverine hydrochloride; carbon paste electrode; FIA; biological fluids; potentiometric determination. Chemically modified carbon paste electrodes (CMCPEs) have been successfully applied as potentiometric sen- sors for the determination of various species [1]. Most of these electrodes are based on the ion-exchange mechanism of the active component incorporated into the carbon paste matrix. These electrodes offer very attractive properties for the electrochemical investi- gation of various inorganic and organic species. In comparison with ion-selective electrodes based on polymeric membranes, CMCPEs possess the advan- tages of ease of preparation, ease of regeneration, and very stable response in addition to the very low Ohmic resistance [2, 3], probably due to the formation of a very thin film of pasting liquid coated onto small particles of carbon powder [4, 5]. Therefore, CMCPEs have found direct application in a variety of analytical situations, such as amperometry [6–8] and voltamme- try [9–11], in addition to potentiometry [12, 13]. Drotaverine [1-(3,4-diethoxybenzylidene)-6,7- diethoxy-1,2,3,4-tetrahydroiso-quinoline] [985-12-6], a hydrated derivative of papaverine, is an effective spasmolytic agent [14]. This drug is capable of re- lieving spasms of various organs, regardless of their function and innervation. Drotaverine has been proved to be superior in its efficiency to papaverine and it is also more reliably absorbed after oral administration [15, 16]. It is used in the symptomatic treatment of various conditions, e.g. gastrointestinal diseases, biliary dyskinesia, nephrolithiasis, gynaecological diseases and vasomotor diseases associated with smooth mus- cle spasms [17, 18]. It can also be used as an adjuvant to hypotensive agents in acute disturbances of blood  Author for correspondence. E-mail: dr_hosny@yahoo.com