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I have dilutions of different samples, which are made with 0.1 M $\ce{HNO3}$. Only I left them on the lab table for a few days and one turned yellow.

So my question is why would one turn yellow and not the other ones? They all have the same concentration of sample. The matrix in all the samples are the same. The difference is the concentration of $\ce{Cl-}$ in the samples. The one with the lowest concentration of $\ce{Cl-}$ turned yellow.

Any ideas why?

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Pure nitric acid is colourless, the brown colour is caused by dissolved nitrogen dioxide, $\ce{NO2}$.

Nitrogen dioxide is formed by the slow decomposition of nitric acid, according to $$\ce{4 HNO3 -> 4 NO2 + 2 H2O + O2}$$

Nitrogen dioxide is a good oxidant and it does so - supposed that it finds a suitable partner.

This would be chloride. So, the more chloride in solution the more redox chemistry furnishing colourless products.

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I don't understand the answer. First, the potentials don't work out; dilute $\ce{HNO3}$ cannot oxidize Cl$^-$ to Cl$_2$. Second, the OP says "the one with the lowest concentration of Cl$^-$ turned yellow". – Silvio Levy Aug 1 '14 at 20:46

Probably one of the solutions had some contaminant, or there was some other difference. Please tell us more about the "matrix" and the preparation of the solution -- in this sort of puzzle you never know what clues will turn out to be helpful.

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