I am working on phosphorus analysis in water using molybdenum blue method. I have several questions about it.
1. Sometimes the blank (distilled water or deionized water)becomes blue while it has no phosphorus.
2. Sometimes the color of standards and samples becomes more intensive one or two hours after reading of the absorbtions. Is there a problem?
3. When and why should we acidify the water samples and standards before adding a mixture of ammounium molybdate, antimony tartarate and ascorbic acid?
Thank you in advance.
Molybdate itself can be reduced to produce a blue chromophore unless the solution is first acidified. Read the paper by Murphy and Riley 1962, and the review by Nagul et al. 2015
Paul Milham
Omar Rady
Thank you so much Sirs for your answers. The question is that how much acid I should add to the standards and the samples. My standards( potassium dihydrogen phosphate solutions) are acidic i.e. they are colorless in the presence of phenolphthalein indicator. I added several drops of 5M nitric acid to standards and sample. However, the amount of absorptions was high and the colors of both standards and samples became quite strong blue after reading absorptions. Also,I read the paper by Murphy and Riley 1962. In this article the authors didnot mention the addition of acid to standards and samples. I would be grateful if you could answer and help me.
Thank you in advance.
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