What is the difference between blue method and yellow methods for phosphates?

Document ID

Document ID CO201

Published Date

Published Date 17/05/2018
Question
What is the difference between blue method and yellow methods for phosphates?
Summary
Blue method vs yellow method for phosphate
Answer
Reactive phosphorous can be measured colorimetrically using a number of different chemistries.  First the phosphorous reacts with molybdate in an acidic solution to form a phosphomolybdate complex.  For low range measurements the phosphomolybdate complex is reduced with either an amino acid or ascorbic acid, creating a characteristic molybdenum blue species.  High range measurements can be made by reacting the phosphomolybdate complex with vanadium to create a yellow species.  Both the blue and yellow complexes are proportional to the concentration of phosphorous in the sample.