The invention relates to a fluid sensor apparatus and a related sensor cardfordetermining and/or monitoring a pH and/or solute concentration in a fluid. Thefluid sensorapparatus can be used in any application requiring the detection of fluidcomponents orconcentrations of solutes, and in an exemplary embodiment, is used to detectammoniaconcentration and/or pH in dialysis fluid. The fluid sensor apparatus has acamera, a lightsource positioned opposite to the camera, and a receiving slot for positioninga removablesensor card between the camera and the light source such that both sides ofthe sensor cardare exposed to the sampled fluid.
