A Nessler cylinder is a laboratory instrument used for the determination of the total amount of ammonia in a sample of water. It is a type of glass cylindrical container with a graduation mark on its side and an open top. The Nessler cylinder is filled with a solution of potassium hydroxide (KOH) which reacts with ammonia in the sample to form a yellowish coloration.

The intensity of the yellow color is proportional to the amount of ammonia in the sample, and the volume of the sample can be determined by comparing the intensity of the color with a calibrated scale.

Nessler cylinders are commonly used in water quality testing and environmental monitoring to measure the level of ammonia in water samples. They are also used in industrial applications for monitoring the level of ammonia in wastewater and process streams.