Method 7C (Colorimetric) and 7D (Ion Chromatography) are used for the determination of nitrogen oxide emissions.

A sample is passed through a series of specially built impinger that contains an alkaline potassium permanganate solution. The impingers are of standard height with a narrow bottle, which improves the collection efficiency. The sample rate is between 400 and 500 cubic centimeters per minute. Normally, three one-hour samples are collected for the test.

The XC-623 is the recommended console for this application. The sample is analyzed colorimetrically for Method 7C and by ion chromatographically for Method 7D.