scoob00, on Feb 26 2009, 11:42 AM, said:
Whilst I was pH-ing, one drop of HCl took the 1 litre of buffer from 8.3 to 4.5, when it had only been dropping a couple of 0.1's per drop before that. I was using concentrated HCl granted, but I've done this enough times to know this is really unusual. Out of interest, any ideas why it happened this time?
are you preparing the buffer at use concentration or a concentrated stock?
the pK of tris is about 8.3 (some say 8.1). when you were adjusting from above the tris was resisting change towards the pK. when you pass the pK the buffer won't resist change away from the pK as well especially with dilute buffer. hence the big change from a single drop of concentrated hcl.
we always prepare concentrated stocks to final pH then make small adjustments to the diluted stock, with dilute acid or base, as necessary.