A cellulose acetate based optode membrane containing 4-dimethylamino-4
'-octylazobenzene as an acid-base indicator is described. Other essent
ial components of the membrane for a fast-responding and durable senso
r are diethyl phthalate, triethyleneglycol and potassium tetrakis(4-ch
lorophenyl)borate. Factors affecting the sensor behaviour are discusse
d and an application in a flow-cell is demonstrated.