Method for determining ammonium

A method for determining of a plausibility of a measurement result of a determination of an ammonium content of an aqueous sample, the method comprising:

• providing a blue indole dye mixture of the aqueous sample with an alkaline pH by,
• providing the aqueous sample;
• adding a base, a chlorinating agent, a phenol derivative, and nitroprusside, so as to obtain the blue indole dye mixture, whereas, the method further comprises:
• determining a first extinction value of the blue indole dye mixture in a first absorption region from 650 to 670 nm, the first extinction value being the measurement result;
• determining a second extinction value of the blue indole dye mixture in a second absorption region from 670 to 775 nm, wherein the second absorption region is distant from the first absorption region;
• dividing the first extinction value by the second extinction value so as to obtain a quotient;
• and
• determining the measurement result to not be plausible if the quotient ≥1.1.