Cacodylic Acid trihydrate (>98.5% pure) is a good near physiologic buffer. Its pKa is 6.3 and it is either negatively charged or neutral, making it a good substitute for phosphate in applications where you want to avoid phosphates. Cacodylic Acid is useful for some DNA applications and is also popular in microscopy.

CAS Number: 6131-99-3
Molecular Formula: C₂H₆AsO₂Na•3H₂O
Molecular Weight: 214.03
Alternative Names: Cacodylic Acid Sodium Salt trihydrate; Dimethylarsinic acid; Dimethylarsonic acid; Hydroxydimethylarsine oxide; Sodium Cacodylate trihydrate