What is the buffering agent commonly added to local anesthetic to speed onset and reduce injection discomfort?

Sodium bicarbonate is used to buffer (alkalinize) local anesthetic solutions, which increases the proportion of anesthetic in its uncharged (base) form — speeding the onset of anesthesia and reducing the burning/stinging sensation typically caused by the acidic pH of standard LA formulations (especially those with vasoconstrictor, which are manufactured at a lower pH for stability).