Kjeldahl method is commonly used in the detection of protein: firstly, organic matter is treated with concentrated sulfuric acid to generate NH4+, and then concentrated alkali is added to evaporate NH3. After absorption, it can be determined by acid-base titration, that is, kjeldahl method.

Melamine chemical formula (molecular formula) C3H6N6

Both melamine and protein are digested with sulfuric acid to obtain NH4+, so melamine will be regarded as protein in protein test.

However, the N content of melamine with the same quality digested with sulfuric acid is several thousand times that of protein.

That's how criminals passed the test with melamine instead of protein.