Formaldehyde, a toxic substance, is used in the following applications: metal plating baths, textile treatments, biological specimen preservatives, and disinfectants of medical equipment. Commercial formaldehyde gas is readily soluble in water. The Purpald Method: Purpald® is subject to fewer interferences than Shiffs' reagent or chromotropic acid procedures. A purple colored complex is formed when Purpald in alkaline solution reacts with formaldehyde. Results are expressed as ppm (mg/L) CH2O. Shelf life of the Purpald Reagent: six months. We recommend stocking quantities that will be used within five months. Refrigeration will dramatically extend the shelf life of these products. The Acid Titrimetric Method: CHEMetrics offers a titrimetric method for formaldehyde, which uses sulfuric acid and sodium sulfite. The end point indicator, thymolphthalein, provides a sharp color change from colorless to bright blue. Results are expressed as percent (%) formaldehyde.