A device including an ion-selective membrane arranged within an optical path of the device and coupled to a sample cell to interact with a fluid sample and thereby modify an optical response of the ion-selective membrane according to an ion concentration in the fluid sample, is provided. The device also includes an integrated computational element (ICE) arranged within the optical path, so that the illumination light optically interacts with the ICE and with the ion-selective membrane to provide a modified light that has a property indicative of the ion concentration in the fluid sample. A detector that receives the modified light provides an electrical signal proportional to the property of the modified light. A method and a system for using the above device are also provided.