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About 2 years ago I purchased 2 liters of 98% concentrated sulfuric acid. It's in a standard poly bottle with leakproof lids, and originally was clear. Recently I noticed the acids have turned a dark gray; almost black color. They have both never been opened.

I suspect there was something in the lid that was eroded by the acid, but is there any other reason sulfuric acid would darken over time?

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The only reason I can think of that would cause the solutions would blacken over time is if the solutions reacted with organic substances. Sulfuric acid is an extremely strong oxidizing agent, and dehydrates many organic compounds to produce carbon in the form of graphite, and water. For example, the reaction of sulfuric acid with paper (cellulose) is:

$$(\ce{C6H10O5})_{n}\ce{(s)} \xrightarrow{\ce{H2SO4}} {6n~\ce{C(s)} + 5n~\ce{H2O(l)}}$$

and can be seen in the following photo:

Graphite Formation

If a considerable quantity of organic material was reacted in your solutions, the $[\ce{H+}]$ in your solutions would actually decrease, so I would recommend titrating one of your solutions to see if this has changed since you bought them.

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  • $\begingroup$ I don't think it would be dehydrated completely - to graphite. $\endgroup$ – Mithoron Oct 11 '16 at 0:29
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    $\begingroup$ Sulfuric acid is a moderately strong oxidizing agent at best. Dehydrating ability is another story. $\endgroup$ – Ivan Neretin Oct 11 '16 at 3:06

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