Abstract: An ion selection electrode device for measuring concentrations of predetermined ions such as in biological samples in which measuring electrodes are disposed on a planar substrate with their active heads lying within a circle of small radius. A small sample of a liquid to be measured is dropped on and spreads over the circle contacting each electrode. In a preferred embodiment, two sets of electrodes are provided around adjacent circles on a single substrate, one for the sample and the other for a standard reference fluid. A dam separates the two liquids but electrical contact therebetween is provided by a porous bridge.