A sensor and method for measuring the concentration of one or more chemical substances within a solution. The sensor including a body of saturated hydrogel held within a sensor body. A window to allow flow through of the chemical substance to be measured. The hydrogel is held under a predetermined compressive force within the sensor body such that the hydrogel exerts a base pressure. A pressure sensor such as a strain gauge senses the pressure exerted by the hydrogel within the sensor body. The sensed pressure is stored or displayed. The hydrogel is reactive to the one or more chemical substances so that contact causes the hydrogel to either increase the pressure above the base pressure under which the hydrogel is held or to decrease the pressure relative to the base pressure of the hydrogel.