The 87.0000 is a case of Muriatic Acid 31.45%, that comes in 1 gallon, 4 per case. Muriatic Acid is a pool chemical that can be used to maintain pH balance and lower alkalinity in pool water. Muriatic Acid can also be used as a cleaning agent on pool surfaces and tiles. At the end of the pool season, use Muriatic Acid to clear out any mold or algae leftover in your pool.