Planet with acid ice caps I remember reading an old astronomy book mentioning a certain planet and the main point of interest was that this planet has or may have polar ice caps but of acid. I can't remember the name though, what planet or planets fit this description? 

Now there some very strange worlds out there though I'm unsure whether this one may or may not be all that strange. Is this something of an oddity or is a planet containing frozen acid not that uncommon?
Of the planets, two have ice caps: Earth and Mars.  The Earth's ice caps are water ice, and Mars has ice caps of Carbon dioxide, and some Water ice. The CO2 on Mars would give a weakly acidic solution if melted but it would not be a caustic acid.

Titan has hydrocarbon lakes at its poles, and other planets are too hot, or are uniformly frozen.