What is the difference between blue method and yellow methods for phosphates?
Blue method vs yellow method for phosphate
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.