Orthophosphate calculated as hydrogen phosphate

Hydrolyzable phosphates and organic phosphorus compounds are not detected in the course of the determination of hydrogen phosphate ions under the conditions described here (no acid hydrolysis). In order to ensure complete differentiation of the individual phosphate and phosphorus compounds in the water sample, determination of the orthophosphate must be conducted at the latest 3 hours after taking the sample. Longer storage times may lead to shifts in the ratios between the individual forms of bonds as a result of precipitation or gradual hydrolysis. 

Spectrophotometric determination as a molybdophosphate complex (phosphorus molybdenum blue) 

Hydrogen phosphate ions react with ammonium molybdate to form ammonium molybdophosphate  

The ammonium molybdophosphate is extracted with a benzene/isobutyl alcohol mixture and reduced in the organic phase with acid tin (II) chloride solution to intensely blue-coloured phosphorus molybdenum blue  

This compound is measured photometrically at 623 nm. Its blue colour can probably be traced to the mixed oxide formation of the tetravalent and hexavalent molybdenum (Holleman-Wiberg). 

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