1
$\begingroup$

In our chemistry curriculum we are doing these lab experiments where we are supposed to test salts for certain anions,cations and other stuff.

I was told that if I dissolved the salt in concentrated $\ce{H2SO4}$ and then heated it for a while over a flame, the evolution of light brown vapours would indicate the possibility of $\ce{NO3-}$. To confirm its presence, however, I would have to repeat the test but this time also add some copper bits to the solution and check for dark brown vapours.

I figured the reason was that the copper reduced the $\ce{HNO3}$ gas to $\ce{NO2}$ which increased the amount of $\ce{NO2}$ gas produced, resulting in a darker shade of brown.

My question is that why copper, which is a very weak reducing agent, used here? Why not a real reducing agent like zinc which will give more $\ce{NO2}$ even with lesser salt?

$\endgroup$

1 Answer 1

3
$\begingroup$

This is a preliminary test, which is neither particularly selective nor sensitive. With nitrate, brown vapours of $\ce{NO2}$ are released. However, bromine or iodine could also release brownish or violet vapours. Copper (or silver) is used to remove bromide and iodide by precipitation.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.