Ion-Selective Coated-Wire Electrodes CWE

In coated-wire electrodes (CWE) the membrane is placed directly on the electrode. The electrode can be a wire of noble metal, graphite wire, silver paste or Ag/AgCl electrode, which is the most popular solution. The role of the inner solution of ISE-s can be played in CWE-s by poli(hydroxyethyl methacrylate), polyvinyl alcohol or a hydrogel saturated with NaCl solution, placed between the electrode and the membrane. The membrane stability can be improved through addition of silver complexes to the membrane. The advantage of CWE-s over ISE-s is the possibility of cheap, mass production. The disadvantage of CWE-s is that the membrane can easily unstick from the electrode. Moreover, due to the high contact surface between the membrane and the solution, the membrane contents, i.e., the plasticizer and the ionophore, can be easily washed out into the membrane, which deteriorates the characteristics of the sensor in time. 

An example of CWE with a coated silver wire can be a sensor of benzalko-nium chloride [47] containing in the polymer matrix benzalkonium-phosphomolyb-date (Bz-PM) as the active complex. Compared to a classic sensor, this electrode shows a wider range of linearity from 2.0 x 10 to 1.0 x 10 mol L with a Nernstian slope of 60 0.3 mV/decade and lower detection limit of 2.0 x 10 mol L. The electrode shows good selectivity toward inorganic and organic ions, sugars and some common drug excipients. It can be used for determination of benzalkonium chloride in pharmaceutical preparations in the form of eye, nose and ear drops. 

Based on the coated silver and copper wire design, an electrode for determination of quininium cation was developed [48], The ion-sensitive compounds dissolved in the polymer membrane were reineckate (Rn ), phosphotungstate (PT ) and phosphomolybdate (PM ) ions. The electrodes were tested in terms of deviation, life span, pH effect and the effect of the interfering ions. The electrodes were used in practice for determination of an active ingredient in pharmaceutical preparations. Studies show that the Ag-coated electrode with phosphotungstate as the anti-ion shows different properties in terms of precision of determination and the preferences of the silver electrode as the internal reference. 

An example of electrodes of different types, e.g with a coated graphite wire, used for determination of pharamaceutical substances is verapamil electrode [50], The polymer membrane contains 3.6% of verapamil-phosphomolybdate complex and 62.0% of dioc-tylphthalate DOP. The electrode has the slope of 62.6 mV decade of the response curve in the range of 6 x 10 to 1 x lO mol L the detection limit (LDL) is 1.9 x 10 mol L. The electrode was stored for approx. 1 year, after which determination of the active ingredient in the verapamil preparation was performed, showing 73.5% of the active ingredient. 

A new coated electrode for determination of an antibacterial agent, gatifloxadn, was constructed based on polymer membranes [51]. Different plasticizers from the group of phtalates were use in the electrodes, which led to obtaining optimal parameters linearity range of 1.0 x 10 to 1.0 x 10 mol L at 25°C over the pH range 2-6 and deviation of 59.12 0.08 mv and 51.50 0.14 mV decade. The electrodes provide short response time of 5-10 s for 15 days. The presented sensors are an accurate (recoveries 99.00%) and precise (RSD less than 1.6%) alternative to other methods of determination of gatifloxacin in pharmaceutical preparations. 

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