Helen Johnson (asker) Jul 19, 2012:

Oh - beg pardon all if it wasn't - I simply thought that chemical reactions would be similar in whichever context (e.g. medical or scientific), but maybe the one I posted referred to chemicals that could be substituted. Maybe I'm getting even more confused here while continuously swapping a keyboard for a magnifying glass for minute text so I'll take your word for it. Thanks for explaining.

Christopher Schröder Jul 19, 2012:

Well in that case it's exchangeable. It wasn't exactly a "similar context"...

Helen Johnson (asker) Jul 19, 2012:

I had wondered whether it was something to do with precipitation or elimination. My document concerns haemodialysis and mentions "totalt udveksleligt natrium i organismen".