One use of nitrous acid, or HONO, is the transformation of amines (i.e., R-NH2) into diazonium salts (i.e., R-N2+).
You learn about this transformation in organic chemistry as an undergrad. It finds its way into a number of reactions, but the most famous is no doubt the Sandmeyer, which sounds absolutely wonderful until you try it and realize it produces an absolute mess. If anyone has any luck with these, I want to hear it.
Its conjugate base is "nitrite," which is an important part of the nitrogen cycle in fish tanks - fish excrete ammonia, which bacteria oxidize to nitrite, which another set of bacteria oxidize to nitrate. Ammonia and nitrite are toxic, nitrate is (relatively) benign. You need a population of bacteria in your fish tank to oxidize ammonia and nitrite, otherwise you'll have sick-to-dead fish. This is why goldfish often die after a week (there also wasn't enough aeration or filtration) or why tanks get sick after the addition of too many fish - even healthy ones - at once.
Many aquarium types will "cycle" their tanks with sacrificial fish, essentially using them as ammonia factories to get the population of bacteria up and running. Some weirdoes just throw in food and let it decompose. You don't have to do anything so cruel or gross. Just add clear, non-sudsy ammonia solution to the tank, a few drops a day, and the bacteria will take hold in mere weeks.
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I think it actually transforms amines into diazonium salts (R-N2+).
oops, you're right, fixed, thanks.
I've used both NaNO2 and alkyl nitrites to produce diazonium salts, and both reactions were fairly straightforward and easy. And if I remember correctly, the NaNO2 reaction was a nice sapphire blue.
"Its conjugate acid is nitrite." What's its conjugate base then?
A bit of Tylenol in vinegar. Stir in a pinch of nitrite and agitate. Acetaminphen oxidizes to delightfully red and toxic N-acetylquinonimine, as it does in your liver by other means. Your liver is loaded with glutathione to grab and defuses aid metablite... unless it's the morning affer a drunken binge and you painfully ooze into the bathroom for some Tylenol.
I've done a ton of Sandmeyers (well, almost, I've made a bunch of iodo-arenes from diazonium salts mixed with KI) and never had too much trouble purifying them. The whole deal is done aqueously, so just extract with Ether and wash away everything else in your aqueous workup. I guess sometimes I've had to distill solids to get them clean, but nothing too bad...