The use of hydrazine to reduce 12-molybdophosphoric acid to phosphomolybden
um blue in continuous flow analysis of phosphate in natural water samples i
s characterized. Using hydrazine in gas-segmented continuous flow phosphate
analysis minimizes coating and silicate interference in comparison with us
ing ascorbic acid. The addition of Sb to the molybdate reagent increases se
nsitivity at temperatures greater than 50 degreesC but causes severe additi
onal coating. The degree of coating was found to be a function of pH. Minim
al coatings were achieved at a final solution pH of 0.5. Silicate interfere
nce was found to increase dramatically with color development temperature.
At room temperature no detectable silicate interference was found. We recom
mend hydrazine in preference to ascorbic acid for gas-segmented continuous
flow phosphate analysis with optimal reaction conditions of room temperatur
e color development and a final solution pH of 0.5.