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I have quite a small quantity of concentrated nitric acid I had acquired about two years ago. This was given to me by a friend in a couple of Pyrex volumetric flasks with stopper and some in a borosilicate reagent bottles. The acid does have a whiskey brown color now.

Is there any simple method or visual cue to identify what concentration the nitric acid could be with a simple test? I want to see if the acid is 70% nitric acid. If it's the higher concentration of 90%, then I want to dilute it to 70%. I have attached a picture of one of the bottles for reference to see the color below.

Nitric Acid

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    $\begingroup$ Simply put, if you've had it for years, it can't be 90%. $\endgroup$ Commented Mar 11, 2021 at 21:34
  • $\begingroup$ For starters it would be fuming nitric acid. Why do you even think there could be sth stronger then usual conc. acid? $\endgroup$
    – Mithoron
    Commented Mar 11, 2021 at 22:24
  • $\begingroup$ @IvanNeretin I agree the concentration may have been lost a bit overtime. But it’s still from late 2018. $\endgroup$ Commented Mar 11, 2021 at 22:38
  • $\begingroup$ @Mithoron I actually wanted some 70% acid. The reason I wanted to to know if there is any visual way or simple test to check concentration is because I haven’t really dealt with concentration nitric much. Only sulfuric and HCl. Become of the change in color from clear I was a bit confused if this is the 70% or 90%+ conc. $\endgroup$ Commented Mar 11, 2021 at 22:41
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    $\begingroup$ $\ce{4 HNO3 -> 2 H2O + 4 NO2 + O2}$ If it's dark, there's plenty of nitrogen dioxide. Concentrated nitric acid should be clear. Btw. This does not look like a safe storage container. $\endgroup$ Commented Mar 11, 2021 at 22:46

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It looks like you have about 40 mL of the solution. You could measure the density by using a small volumetric flask (10 or 25 mL), or even a graduated cylinder (which you could calibrate with water). Add your solution to the mark, weigh reasonably accurately, and you will have your answer (which I predict will lie between 70 and 90%).

At 20C, the density of 70% acid is 1.41; the density of 90% acid is 1.48. The difference is large enough to determine easily, and if there is any deterioration, you should be able to determine the exact concentration fairly easily from density tables, e.g.: https://www.handymath.com/cgi-bin/nitrictble2.cgi?submit=Entry

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