Hi Paul,
Difficult to be sure of painful effect, of course, but erring on the side of caution I would buffer it (equal amount of sodium bicarbonate to that of MS 222 usually ok, although some places say twice as much sodium bicarb as MS 222).
Sorry to be so pedantic - I know it's easy (I've done it myself) to forget that other countries (or even people in your country) may routinely think of a different preparation to the one you usually use. (I'd make some mutterings about "American" English at this point but probably best not

).
Hope this helps,
Bruce.