Hi
I have watched some basic videos from two universities on a disk (the vids are on disk). Both were doing basic acid base titration but they did it
with a slight difference in technique, so I am assuming one is wrong but which?
One says that near the end point you wash down the Burrete tip with DI water in order to make sure it has all gone into the solution.
However the other one warns against doing this saying it will give an inaccurate result, what makes it harder is both advise that the flask walls be
washed down with DI water near the end point to make sure all acid/base solution is in the solution.
So which technique is correct? do you wash down the tip of the burrette after each drop once near the end point or not?DutchChemistryBox - 27-12-2014 at 09:57
I would say that washing down with water will give you a lower result. You will wash an amount of titer from the inside of the burette tip down and
replace some of the titer in the tip with water. I've never learned at school to wash it down.
Maybe it depends on what kind of burette you use. In case of a glass burette I think I will always say no.
I know that some automatic titrators (e.g. Metrohm titrando) have tips which can be washed off, but those tips have to stay in the solution during the
titration (so washing of would be useless).
I do recommend to wash off the sides of your titration vessel.
Edit:
It may be a nice idea to just try it. Titrate a known amount of reference standard for let's say 5 times, with each method. Then you can compare the
repeatability and accuracy of the two methods.
[Edited on 27-12-2014 by DutchChemistryBox]Marvin - 27-12-2014 at 10:58
The usual confusion is with volumetric pipettes and if you blow out the tip, which depends on the type of pipette - to contain or to deliver.
Unless solution is splattered I don't see why you would want to rinse the burette. The important thing is that the difference is true, from start of
titration to the end. That means you want the tip to have the same amount of adherent liquid at the end of the titration as at the start.DraconicAcid - 27-12-2014 at 12:53
If you have a half-drop hanging from the tip of the burette, touch it to the inside of the flask and wash it down from there. Don't rinse off the tip
itself, because you'll then have water hanging off of the tip, which will diffuse into the titrant that is in the tip.diddi - 27-12-2014 at 15:37
I never ever washed the burette tip. the water migrates up the inside of the tip by capillary action, diluting the solution and thus overdelivering at
the wash point. this introduces a random error. before starting I run 1cc or so thru and touch the tip to side of waste vessel. this is then the
systematic startup state. at near end point, touch the side of conical flask to return the burette to startup state, and wash down the flask thus
whatever is in the flask has been measured. titrating on a stirrer is also helpful when you are just starting titrations.
and of course, you are titrating for concordant results here. 3 minimum you should be able to get to +-.05cc or betterblogfast25 - 28-12-2014 at 07:24
No 'tip washing', you hear!
No, I don't do it either, I do it like DA.
But the error due to tip washing is very small. I'd challenge anyone to actually empirically demonstrate the bias it actually introduces...
and of course, you are titrating for concordant results here. 3 minimum you should be able to get to +-.05cc or better
+/- 0.05 ml 'or better'? With a normal burette? You wish...
[Edited on 28-12-2014 by blogfast25]unionised - 28-12-2014 at 07:31
Well, I always wash down the tip of the burette.
Over two decades working in an analytical lab tells me that, if there's a bias, from washing stuff from inside the tip, its immeasurably small.
Also, it's quite common for stuff to get spattered onto the bottom of the burette, and if you don't was that down, you will certainly introduce a
bias.blogfast25 - 28-12-2014 at 08:38
Well, I always wash down the tip of the burette.
Over two decades working in an analytical lab tells me that, if there's a bias, from washing stuff from inside the tip, its immeasurably small.
Some robotitrators inject the titrant directly into the analyte, with the dispensing tip submersed into the analyte. Seems to cause no bias either.diddi - 28-12-2014 at 15:02
a range of .1cc in results would be considered quite sloppy even in undergrad circles, I would have thought. a standard 50cc burette will deliver
quite accurately and repeatably as long as the operator is consistent with their technique and is patient when it comes to not overrunning the
endpoint.
I guess if one has the technique of washing the tip, and they always do it including before the run, then it is probably as accurate. I was never
taught the method, however.
