The invention relates to a sensor card (101) for determining and/or monitoring solute concentration and/or pH of a fluid, e.g. liquid, based on an optically observable change of at least one sensing membrane (102, 103, 104) comprising colorimetric material, in the presence of ions in solution. The sensor cards can be placed in a fluid sensor apparatus and used to determine the solute concentration and/or pH of any fluid, such as dialysate.
