Dear colleagues,
I have been working on ortophosphate determination by Molybdenum blue method for more than an year and obtained excellent results. However, this summer I'm experiencing a problem with the color reagent.
We prepare the reagent by mixing Mo + Sb +H2SO4 - the solution A is stable. The ascorbic acid was added to a portion of the solution A just before measurement. Usually the mixed reagent become yellow. However, the last days I'm obtaining blue-green color of the freshly prepared color reagent. The blank absorbance as at 880 nm is very low to interfere with phosphate determination (we used 4 mL from the reagent for 25 mL of sample). However, it changed the reaction time for complete color development for higher concentrations (1.0 mg PO4/L): from 10 min up to 25 min.
Does any of you experienced the same problem? Any suggestions?
Dear Andriana,
We also used the Molybdenum blue method for the determination of ortophosphate for some decades, with excellent results, as you mentioned. But at one moment, something happens and we had problems, as you say. Our method is with 4 ml of H2SO4 10% added to 25 ml of sample in a 50 ml flask and about 10 ml of distilled water. We add 1 ml of ammonium molybdate 5%, we mix very well and wait 10 minutes before adding 1 ml of ascorbic acid 10%. We read the absorbance the next day, at 610 nm.
We thought there was a problem with the ascorbic acid, but the fact was that it was the molybdate, which was not in term any more. We bought another reagent (extra pur) and now we prepare the solutions each time we make determinations.
Your method prevent the alteration of the ammonium molybdate solution, as you have the H2SO4. But it is still possible that the problem be from molybdate.
That is my experience and suggestion.
Succes!
Carmen
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