Ion-selective electrodes are now commercially available for about 20 common anions and cations and have found applications in many branches of science An ion of activity a A and charge z A solicits a potential response E from an ion-selective electrode according to 

$$E=conts\pm \frac{2303RT}{{{z}_{A}}F}\log {{a}_{A}}$$

(1)

and in the presence of an interferent of activity a B and charge z B to 

$$E=const\pm \frac{2303RT}{{{z}_{A}}F}\log \left[ {{a}_{A}}+k_{AB}^{pot}({{a}_{B}}) \right]$$

(2)

where k AB pot is the selectivity coefficient

Applications of Ion-Selective Electrodes

The multiple addition method has been used in combination with Gran plots for the determination of 2×10−4M to 1×10−6M (28 ppm to 14 ppb) of ammonia nitrogen The ammonia was determined with an ammonia gas-sensing electrode A computer-controlled titration system was used for the titration and for dataprocessing Results within ±2% of the added amount are obtained down to ∼15×10−6M (20 ppb) of ammonia nitrogen

Microdetermination of ammonia using an ammonia gas-sensing electrode and a minicomputer

A flow-through system for the measurement of urea concentrations is described, using soluble urease and consecutive determination of liberated ammonium ions by a selective disc-electrode. The active component of the electrode membrane was the carrier-antibiotic nonactin which was incorporated in a polyvinylchloride matrix.

/pdf/a-nh-4-selective-enzymatic-flow-through-system-a-method-for-4ohjkk4dkn.pdf

[A NH(+4)-selective-enzymatic flow-through system. A method for the continuous enzymatic, electrochemical determination of urea, II (author's transl)].

Chapter 11 – ENZYME ELECTRODES

Publisher Summary
This chapter discusses the chemical formula, chemical name, and electrodes sensitive to various diuretic drugs, such as mercury compounds, sulphonamides, and urea. Urea is an osmotic diuretic with a low renal threshold; it has been used intravenously in the treatment of acute increase in intracranial pressure because of cerebral oedema. Extensive research has been made on the construction and the use of electrodes sensitive to urea. Most of the sensors are based on enzymatically active membranes bound to an electrode, such as a cationic glass electrode or a gas electrode. Such arrangements are sensitive either to the substrate or to reaction products. Guilbault and Montalvo found that urease immobilized in poly-acrylamide gel has a high activity and they described the construction and the analytical use of several types of urea transducers applicable to the continuous determination of urea substrate. The electrode was made by immobilizing urease in a layer of acrylamide gel on the surface of a Beckman glass electrode sensitive to ammonium ions.

CHAPTER 12 – Diuretic Drugs

This report describes a method for the determination of ammonia and urea in raw sewage samples using an Orion Ammonia Gas-Sensing Electrode. Urea is determined by potentiometrically measuring the ammonia produced following exhaustive hydrolysis by urease and subtracting the amount of ammonia present before enzymatic hydrolysis. The recovery of urea from raw sewage samples using the procedure herein described is shown to be nearly Quantitative.

Darius S. Rogers

Papers

Analysis of Urea Nitrogen in the Presence of Ammonia Using an Ammonia Gas Sensitive Electrode