A polyprotic acid is one which contains more than one hydrogen ion that it can donate. In other words, the acid can ionize more than once. Polyprotic acids are very commonly ternary acids as well, a situation which most easily allows multiple hydrogens that can be drawn away from the atom.

The ability to ionize multiple times makes a polyprotic acid extremely strong, some of the strongest acids naturally occurring. An example of one of this is the very well known sulfirc acid, H2SO4. To more easily see why it is both a polyprotic and ternary acid, the formula could be more clearly rewritten as (OH)2SO2. Thus the formula for sulfric acid:

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All those free electrons on the upper left corner of the molecule tug viciously at the oxygens on the lower right, making the bonds holding hydrogen to the two hydroxyl groups extremely weak, easy to pull at either of them.