An electrode for use in a electrochemical sensor comprises carbon modified with a chemically sensitive redox-active compound, excluding an electrode based on carbon having derivatized thereon two redox-active species wherein at least one of said species is selected from anthraquinone, phenanthrenequinone and N,N′-diphenyl-p-phenylenediamine (DPPD). The invention further provides a pH sensor comprising: a working electrode comprising carbon modified with a chemically sensitive redox active material; and a counter electrode, wherein the ratio of the surface area of the working electrode to the surface area of the counter electrode is from 1:10 to 10:1. Also provided is a pH sensor comprising: a working electrode comprising carbon modified with a chemically sensitive redox active material, and a counter electrode, wherein the area of the working electrode is from 500 μm2 to 0.1 m2. The uses of these electrodes and sensors are also described.