Formaldehyde is a highly toxic gas formed naturally by the incomplete combustion of many organic substances. Commercial formaldehyde gas is readily soluble in water. Formalin, a 37% by weight solution of formaldehyde with 10-15% methanol added to prevent polymerization, is the most readily obtainable form of formaldehyde.
Although toxic, formaldehyde has a variety of industrial uses. It is used as a raw material in the manufacture of particle board products, textiles and cosmetics. It is also used in tanning and photo developing processes and as a disinfectant.
The Colorimetric Method. The color-forming reagent Purpald¨was developed by chemists at Aldrich Chemical Co. 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. 5 months (Purpald reagent products only). We recommend stocking quantities that will be used within 4 months.
The 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.