The pure heart is a heart that Sandy shows with the scanner to SpongeBob. It briefly appears in the episode "It's a SpongeBob Christmas!" and SpongeBob Comics No. 46.
Role in episode
Sandy explains to SpongeBob that his tiny brain and his pure heart are making him immune to the Jerktonium, in which the slices of fruitcake that SpongeBob and the whole Bikini Bottomites ate included.