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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jaira on October 26, 2015, 10:04:46 PM

Title: Test for Phosphate Ion using Nitric Acid and Ammonium Molybdate
Post by: jaira on October 26, 2015, 10:04:46 PM

Hey all,

I'm trying to determine the equation for the phosphate ion reacting with nitric acid and ammonium molybdate. I believe it produces an ammonium phosphomolybdate precipitate, but I'm not sure what else.

Based on the information from http://pubchem.ncbi.nlm.nih.gov/ I've come up with:

H₂₄Mo₇N₆O₂₄ + HNO₃ + PO₄³⁺ = H₁₇MoN₄O₇P + ? + ?

Any help would be much appreciated. I can balance it once I figure out what the products are, and if I even have the correct formula for ammonium molybdate and/or ammonium phosphomolybdate. Please, please, please :-)

Title: Re: Test for Phosphate Ion using Nitric Acid and Ammonium Molybdate
Post by: Borek on October 27, 2015, 03:39:29 AM

Nitric acid doesn't take part in the reaction as a whole, from what I remember it is there to keep the pH low and to keep redox potential in the solution high to not allow side reactions.

Try

H₃PO₄ + (NH₄)₂MoO₄ + H⁺ -> (NH₄)₃PO₄·12MoO₃·2H₂O + NH₄⁺ + H₂O

(this is one of many ways of expressing the phosphomolybdate formula).

Title: Re: Test for Phosphate Ion using Nitric Acid and Ammonium Molybdate
Post by: jaira on October 28, 2015, 09:49:18 AM

Thank you, Borek - leaving out the nitric acid does make sense since I've read the precipitate is insoluble in nitric acid so it should be viewed suspended there.

However, I'm uncertain about the phosphoric acid instead of the phosphate ions being part of the reactants. Is that what would be present in a sample of blood?

Thank you again for your *delete me* I really want to get this assignment right.

Title: Re: Test for Phosphate Ion using Nitric Acid and Ammonium Molybdate
Post by: Borek on October 28, 2015, 12:53:01 PM

Quote from: jaira on October 28, 2015, 09:49:18 AM

I'm uncertain about the phosphoric acid instead of the phosphate ions being part of the reactants.

What fraction of phosphates is completely protonated in the low pH produced by adding excess nitric acid?

Title: Re: Test for Phosphate Ion using Nitric Acid and Ammonium Molybdate
Post by: jaira on October 28, 2015, 03:44:58 PM

I haven't learned anything about pH levels yet, but I'm assuming this means the phosphate ions are binding with the hydrogen from the nitric acid, producing the phosphoric acid used in the equation.

If my assumption is correct, thank you so much because this definitely solves my problem :-)