We use the tried and true ammonium molybdate / potassium antimonyl tartrate react in acid medium with reduction to an intensely colored molybdenum blue by ascorbic acid.
The resulting solution is supposed to be colorless, but no matter what we try, the solution always turns yellow. The problem appears to be the ascorbic acid. We tried refrigerating it, storing it under vacuum, and the campus chaplain tried to perform an exorcism. Nothing worked.
I did have some luck recrystalizing the ascorbic acid but is there a better way?
The EPA method mentions using a small amount of acetone as a preservative
Thank you
Kevin
The presence of phosphate ions is indicated by the formation of a bright yellow precipitate layer of ammonium phosphomolybdate. The appearance of the precipitate can be facilitated by gentle heating. This test is also used to detect arsenic, a yellow precipitate being formed. https://en.m.wikipedia.org/wiki/Phosphate_test
Kindly check https://www.epa.gov/sites/production/files/2015-08/documents/method_365-3_1978.pdf
Pease Kevin,
Are you mixing the ascorbic acid with the molybdate reagent?
Is the molybdate reagent turning yellow without any P being added??
What happens if you don't add the ascorbic acid to the molybdate reagent?
Please note that phosphomolybdic acid is pale yellow.
Ascorbic acid decomposes when it's in solution, so make fresh solutions when you need them.
Lastly, what acid(s) are present and at roughly what concentrations?
Paul,
First off, thank you for answering.
QUESTION: Are you mixing the ascorbic acid with the molybdate reagent?
Yes and no. We are using a flow injection analysis which means that the two reagents get pumped into a manifold where the color developing reaction occurs.
We never actually see the two reagents mixing.
Before we start our FIA, we mix the ascorbic acid and the molybdate in a small beaker. That is when we see the greenish yellow color developing. We then add a bit of phosphate solution and see the blue color.
Some years ago, when I was running this analysis manually, we used the Standard Method. Standard Methods calls for mixing the ascorbic and molybdate reagents and then adding this mixed reagent to our samples. Standard Methods also says that if the must be colorless before adding it to the samples. If not, try making it again.
QUESTION: Is the molybdate reagent turning yellow without any P being added??
Yes, either yellow or a yellow-ish green
QUESTION: What happens if you don't add the ascorbic acid to the molybdate reagent?
Nothing happens
Please note that phosphomolybdic acid is pale yellow.
Ascorbic acid decomposes when it's in solution, so make fresh solutions when you need them.
We make fresh ascorbic acid every time and no more than an hour or two before we start the flow injection analysis run
QUESTION: Lastly, what acid(s) are present and at roughly what concentrations?
Ascorbic Acid Reducing Solution
In a 1 L volumetric flask, add 700 mL DI water. Then add 70 g granular ascorbic acid. Bring to 1 L volume and invert to mix. Add 1 g sodium dodecyl sulfate,
Stock Ammonium Molybdate Solution
In a 1 L volumetric flask, add 800 mL DI water. Then add 40 g ammonium molybdate tetrahydrate. Bring to 1 L volume with DI water and stir on a stir plate for a minimum of 4 hours.
Stock Antimony Potassium Tartrate Solution
In a 1 L volumetric flask, add 800 mL DI water. Then add 3 g antimony potassium tartrate hemihydrate OR 3.22 g antimony potassium tartrate trihydrate. Bring to 1 L volume with DI water and invert to mix.
Molybdate Color Reagent
In a 1 L volumetric flask, add 500 mL DI water. Then in a fume hood, add 35 mL concentrated sulfuric acid. Swirl to mix and allow to cool to room temperature. Then add 213 mL Stock Ammonium Molybdate Solution (1) and 72 mL Stock Antimony Potassium Tartrate Solution (2). Bring to 1 L volume with DI water and invert to mix. The solution should be discarded if it turns blue, or if a yellow precipitate forms.
I am beginning to suspect that our ascorbic acid has already begun to degrade even before we dissolve it in water.
Any thoughts?
Thank you again
Kevin
You most probably using deinosed water requiring th replacement of cartidge. Alternatively, Use distilled water, if available. You will solve the problem.
Veli,
Thank you, that is a good idea.
We are using a Milllipore system and 18 mega-ohm water. Because we have state certification, we test the water monthly but I do remember that there are certain impurities that can sneak past a Millipore system.
Thank you again.
Kevin
I faced many time with the similar problem. Even if the deionised water have a 18megaohm or EC
We do a lot of Phosphate analysis, and this never happened. I would check the grade of the acid and the preperation. However, we always use double-distrilled water (Mili-Q). What we did find out that sometimes the ascorbic acid reduces the stability of the reaction and we reduced the protocol concentration to get better precision.
Mili q is deionised water. Late replacement of cartridge can cause such problem.
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