PURPOSE: To continuously and automatically measure the concn. of ammonia, by providing an analytical specimen line, which consists of a pressure reducing valve, a cooler and a refractometer, so as to branch the same from an ammonia supply system and converting high temp. and high pressure aqueous ammonia to low temp. and low pressure aqueous ammonia.

CONSTITUTION: In ammonia recovery equipment, an ammonia analytical specimen sampling line is branched on the way of the piping for connecting an ammonical liquor tank 9 and a rectifying column 10. That is, a pressure reducing valve 1, a blocking valve 2, a cooler 3 and a refractometer 4 are connected to a branch pipe 2 in series and the exit side of the refractometer 4 is connected to a conductor 6 and a recorder 5 for recording a measured value is connected to the refractometer 4. The high temp. and high pressure aqueous ammonia flowed in the branch pipe 12 is reduced in pressure by the pressure reducing valve 1 and further cooled to low temp. by the cooler 3 to be passed through the refractometer 4. Noticing that the change in a refractive index per 1°C is 0.00173 in a liquid temp. range of 16.9W38.3°C, a conversion mechanism is provided from the relational formula of ammonia concn. and a refractive index to measure the concn.