A system for determining solute concentration and/or pH of a fluid, the system comprising: a sensor card (101, 201, 1801, 2401, 2501, 2508, 2516, 2601, 2608, 2616), comprising: at least two fluid sensor membranes (301, 303) comprising a colorimetric material; at least one reference element with a surface comprising a reference sensing region having a solid colour; a front carrier overlaying a front side of the at least two fluid sensor membranes and at least one reference sensing region; at least two first sampling holes (308, 310) positioned on the front carrier aligned over the front side of the fluid sensor membranes, wherein the at least two first sampling holes are equidistant to an axis perpendicular to the sensor card; and a first reference sampling hole positioned on the front carrier aligned over the front side of the at least one reference sensing region, wherein the first reference sampling hole is positioned at the axis perpendicular to the sensor card; and a light emitting source configured to transmit light through the at least two fluid sensor membranes and the at least one reference element of the sensor card, wherein the at least one reference element with the reference sensing region having the solid colour is configured to transmit a light emitted from the light source. a sensor card (101, 201, 1801, 2401, 2501, 2508, 2516, 2601, 2608, 2616), comprising: at least two fluid sensor membranes (301, 303) comprising a colorimetric material; at least one reference element with a surface comprising a reference sensing region having a solid colour; a front carrier overlaying a front side of the at least two fluid sensor membranes and at least one reference sensing region; at least two first sampling holes (308, 310) positioned on the front carrier aligned over the front side of the fluid sensor membranes, wherein the at least two first sampling holes are equidistant to an axis perpendicular to the sensor card; and a first reference sampling hole positioned on the front carrier aligned over the front side of the at least one reference sensing region, wherein the first reference sampling hole is positioned at the axis perpendicular to the sensor card; and a light emitting source configured to transmit light through the at least two fluid sensor membranes and the at least one reference element of the sensor card, at least two fluid sensor membranes (301, 303) comprising a colorimetric material; at least one reference element with a surface comprising a reference sensing region having a solid colour; a front carrier overlaying a front side of the at least two fluid sensor membranes and at least one reference sensing region; at least two first sampling holes (308, 310) positioned on the front carrier aligned over the front side of the fluid sensor membranes, wherein the at least two first sampling holes are equidistant to an axis perpendicular to the sensor card; and a first reference sampling hole positioned on the front carrier aligned over the front side of the at least one reference sensing region, wherein the first reference sampling hole is positioned at the axis perpendicular to the sensor card; and a light emitting source configured to transmit light through the at least two fluid sensor membranes and the at least one reference element of the sensor card, wherein the at least one reference element with the reference sensing region having the solid colour is configured to transmit a light emitted from the light source.