Ammonium molybdate reacts with phospat 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)

The phosphate content in a surface water has direct consequences for its ability to support the growth of certain organisms. Since ever increasing quantities of phosphates are being fed through domestic waste waters into rivers and lakes, these waters have a tendency towards eutrophication. Exact data about the phosphate content are important for boiler water and feed water. Precise dosing of phosphates in these waters can inhibit formation of boiler scale. Pyro-, meta- and polyphosphates are not determined with VISOCOLOR^®test kits. The determination of total phosphate requires a decomposition prior to the test.