The invention relates to a fluid sensor apparatus and a related sensor card for determining and/or monitoring a pH and/or solute concentration in a fluid. The fluid sensor apparatus can be used in anyapplication requiring the detection of fluid components or concentrations of solutes, and in an exemplary embodiment, is used to detect ammonia concentration and/or pH in dialysis fluid. The fluid sensor apparatus has a camera, a light source positioned opposite to the camera, and a receiving slot for positioning a removable sensor card between the camera and the light source such that both sidesof the sensor card are exposed to the sampled fluid.