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Possible side reactions of nitric acid and methyl red

I am titrating a twice distilled very concentrated solution of nitric acid. When I add a few drops of methyl red ⇦this on eBay & Amazon [affil links] indicator, the solution turns a pale color then turns orange to yellowish color and orange colored gas starts coming off. I have even tried titrating ultrapure nitric acid with the same result. What could possibly cause this to occur and what reaction could be taking place?

Samantha Morrison
student - Los Angeles, California, USA
May 14, 2009

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First of three simultaneous responses --

Your concentrated nitric acid is a very powerful oxidising agent, and is used in many bio-analytical methods to totally destroy all organic matter in a sample. Methyl red ⇦this on eBay & Amazon [affil links] is, of course, an organic substance so is oxidised to water, carbon dioxide, one of the nitrogen oxides, etc.

If your intent is to measure the concentration of the concentrated acid, you need to take a small sample and dilute it largely before titrating. Remember that the concentrated acid is in fact primarily an oxidising agent and not an acid - it needs to be diluted in water in order to ionise to H+ and NO3-. Only then can it behave as an acid.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
May 18, 2009

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Second of three simultaneous responses --

Samantha, You can not titrate pure HNO3 as it is an extremely strong oxidizing agent and your methyl red ⇦this on eBay & Amazon [affil links] is an organic indicator. Take a 1 ml sample of your acid and dilute it with 100 mL of distilled water and titrate that. You just need an appropriate fudge factor for the 100 fold dilution. Chemists do it all of the time.

James Watts
- Navarre, Florida
May 18, 2009

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Third of three simultaneous responses -- May 19, 2009

Samantha,

At very high concentrations nitirc acid is a very effective oxidising agent. Methyl red ⇦this on eBay & Amazon [affil links] is an organic dye that just happens to be pH sensitive.

Essentially, when you add your indicator to the acid the nitric is oxidising it and producing the relevant breakdown products. If you are seeing a brown gas this is most likely nitrogen dioxide. Other gases may include carbon dioxide and carbon monoxide.

If you are doing this in an open lab then stop now, do it in a fume cabinet. If you don't know what the gases are you do not know what hazards you may be exposing yourself to. It is fortunate that you do not add a lot of indicator to the solution so you will be producing only tiny amounts of the gases.

Brian Terry
Aerospace - Yeovil, Somerset, UK