Ammonium molybdate reacts with phosphate ions to form phosphomolybdic acid which is reduced to molybdenum blue. The tests with the designation DEV use the same reagents as the German Standard Methods. Larger amounts of oxidising compounds inhibit the formation of the blue colour complex, and have to be destroyed prior to the test. H2S interferes in concentrations above 2 mg/l, but can be expelled after acidification of the water sample.

Heavy metals in excess of 10 mg/l can slightly decrease the colour intensity, vanadium causes an increase in colour. Silica interferes and causes falsely high results. Arsenates react like phosphates. Citrate, oxalate, tartrate etc. reduce the sensitivity of the phosphate test.
(Reaction principle analogous to APHA 4500-P E, EPA 365,2+3, DIN EN ISO 6878-D11)

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