A system for monitoring an analyte concentration in liquid is provided. The system includes a coupon comprising an absorbent body with a window through the absorbent body wherein the liquid is maintained in said window by capillary action and surface tension. A reactant is in the absorbent body wherein the reactant is capable of diffusing into the window to react with an analyte in the liquid, or the reactant is able to react with the analyte within the coupon itself, with color-indicating by-products of the reaction diffusing into the window, wherein the analyte is present in an analyte concentration, to form a reactant with a color wherein the color has an intensity which correlates to the analyte concentration. A light source is provided which is capable of passing light into the window wherein the light is attenuated by the color proportional to the analyte concentration to form attenuated light. A detector is provided which is capable measuring an intensity of the attenuated light. Alternatively, the color change can be read by eye and compared to a color chart relating the color to an analyte concentration.