This paper describes methods for determining the quantity of inorganic phosphate and total phosphorus in seawater by extracting the molybdophosphoric acid with isobutyl acetate and by then reducing the solution to heteropoly blue. Arsenic and silicate in amounts ordinarily encountered in seawater do not interfere.
Pakalns, Paul, and Blanka R. McAllister. 1972. "Determination of phosphate in seawater by an isobutyl-acetate-extraction procedure." Journal of Marine Research 30, (3). https://elischolar.library.yale.edu/journal_of_marine_research/1247