A Fluidic Colorimetric Sensor Design for Water Hardness Detection

A new sensor design for detecting water hardness using complexometric and colorimetric concepts is presented in this paper. Calmagite, a metal ion indicator, changes color of a solution to wine red in presence of calcium (Ca²⁺) and magnesium (Mg²⁺) ions at pH 10. Ethylenediaminetetraacetic acid (EDTA) displaces the indicator from the metal-indicator complex after it is added to a solution and turns it blue. A red solution is transparent to a red light whereas nontransparent to a blue light and vice versa. A photodiode placed across the solution will give out a reading of voltage (V) variation across the diode depending on the amount of red or blue light passing through the solution and striking the photodiode. A sensor prototype is 3D printed with a channel embedded in it to hold water sample. Water hardness is detected based on visual inspection of color change of the sample and voltage change occurring when calmagite and EDTA are mixed to the sample. For a first-generation prototype, voltage variation of $\sim 30\%$ and 13% for blue light and 6% and 74% in average for red light in comparison to the baseline, i.e., when neither calmagite nor EDTA is mixed to the water sample, is observed due to color change. Then, a second-generation prototype is printed, which shows improvement in voltage readings compared with the first generation. Transmittance and absorbance of the solution are computed using the voltage variation data recorded across the photodiodes. A calibration curve is then compiled by measuring absorbance of various concentrations of standard CaCO₃ solution, which is then used to predict the water hardness type of the water sample.