| abstract |
A voltammetric pH sensor, especially for characterising wellbore fluids, comprises a plurality of electrodes with a redox active organic compound attached to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, wherein the compound has at least one substituent group which promotes hydrogen bonding at a said functional group and thereby increases the reaction rate of proton transfer. The substituent group may form an internal hydrogen bond with a redox-convertible group or may enhance polarity to promote electrostatic interaction with water molecules and reduce activation energy. Typical examples include alizarin or 1, 2 - dihydroxy- anthraquinone (RH=72 - 48 - 0 ), quinizarin or 1, 4 - dihydroxy- anthraquinone (RN=81 - 64 - 1), 2 - acetoxy-benzoquinone (RN=1125 - 55 - 9 ), chloranil or 2, 3, 4, 5 - tetrachloro -benzoquinone (RN=118 - 75 - 2 ) and 1, 4 - diamino - 2, 3 - dichloro - anthraquinone (RN=81 - 42 - 5 ) deposited on a glassy carbon electrode. In this way, anomalous measurements at low ionic strenght and low concentrations of pH buffering species can be overcome. |