Enzyme electrode for urea with amperometric indication: Part I--Basic principle

A novel method for amperometric determination of substrates of hydrolytic enzymes has been developed. As an example the pH dependence of electrochemical oxidation of hydrazine in the Tafel region was combined with the urease catalysed splitting of urea to construct an amperometric membrane electrode for urea. The characteristic features of this sensor are: a linear dependence of the current of hydrazine oxidation on hydrogen ion concentration (as opposed to the logarithmic response of potentiometric sensors), linear urea concentration-signal relationship between 0.8 and 35 mmol/litre sample concentration, a response time of about 20 s, a relative standard deviation of 1% and measuring frequency up to 40 samples/h with an electrode operational stability of 2 weeks. Calibration curves for aqueous urea solutions are given as functions of starting pH, urease and hydrazine concentration and the potential dependence of the signal was determined.