The Ammonia Leak Detection Cloth is treated with a dilute ethanol solution of bromothymol blue and phenolphthalein (10:1 ratio) and air dried. When dry, the cloth is a yellow-orange color. When exposed to ammonia vapors, the indicator dyes turn the cloth blue/green due to the alkaline nature of the ammonia.

In air, the cloth reverses back to the yellow-orange color as it absorbs CO2 from the air. The carbonic acid lowers the pH and thus the indicator dyes revert back to their acidic color. This reversal can be accelerated if the cloth is placed in an acidic environment (say a vinegar/acetic acid atmosphere.)