pH-based colorimetric detection of monofunctional aldehydes in liquid and gas phases

An ammonium chloride–Congo red (NH4Cl-CR) solution is presented as a potential sensing solution for aldehydes. Monofunctional aldehydes such as formaldehyde, acetaldehyde, benzaldehyde, and isobutyraldehyde caused changes in the pH of the sensing solution, producing a color transition from red-orange to violet to blue. This distinguished them from the other compounds, thereby acting as a qualitative test for the functional group. The NH4Cl-CR solution was also employed in making filter paper-based and silica gel-based sensors for formaldehyde and acetaldehyde vapors. These sensors responded positively towards the aldehyde gases through a color transition from pink to blue. The NH4Cl-CR mixture provides a simple and easy-to-handle reagent for the detection of both liquid and gaseous aldehydes which has potential applications in environmental monitoring.