An improved electrode assembly of a modular configuration for sensing ion concentration includes a substrate mounted in a housing and being sandwiched by a first and second support layer of non-conductive material. A gelantinized solution is positioned in an opening between the first support layer and a substrate, while a second gelatinized solution is positioned in an opening between a second support layer and a substrate. Electric leads and electrodes are appropriately positioned on the substrate for operative contact with the respective gelatinized solutions. An ion-responsive member contacts one gelatinized solution, while an acqueous junction member extends through the respective support layers and substrates to contact another gelatinized solution.