Replacement o-rings for your SQN series pumps. A full rebuild requires two o-rings per pump, but these are available individually. If your pump is leaking from the head, it's probably a bad o-ring. If it's leaking from behind the head, through one of the weep holes, it's more likely to be a mechanical seal or a stationary seal.

Nitrile is the most common compound, and usually recommended unless you have exotic chemical compatibility requirements. In that case, use Viton.