A simple, rapid and low-cost method for the visual determination of formaldehyde in water based on the retention of two ion-associates on a membrane filter is proposed. Formaldehyde in water was reacted with 3-methyl-2-benzothiazolionone hydorazone (MBTH) to form an azine. The remaining MBTH was oxidized by ferric ion to form a light-yellow cation. The cation was reacted with the azine to form a deep-blue cation. Both blue and yellow cations were reacted with tetraphenylborate anion to form blue and yellow hydrophobic ion-associates, respectively. The ion-associates were retained on a membrane filter by filtration. The color of the filter was changed from yellow, green to blue, depending on the concentration of formaldehyde. The concentration was easily determined by comparing the color with those obtained from a series of standard solutions of formaldehyde. The formaldehyde from 0 to 0.07 ppm in water was determined with error less than ±0.01 ppm. Thirty minutes was enough for the entire analytical procedure. This method was successfully applied to tap-water samples spiked with formaldehyde. It will be useful for rapid judgment of a water sample to pass the quality standard for drinking water in Japan.
