[Catalist] Titrations

[Leon Harris]

Hi Jackie.
Acid is always in burette, unless unavoidable. This is because the 
silicon-oxygen bonds in glass are labile to OH- ions. Over time, glass 
is etched by OH-. As the volume accuracy of the burette is super 
important, convention has it to keep the base in the conical flask. 
(BTW, this is why when you are cutting glass tube, you spit on it after 
scoring it. The pH of your saliva (7.8) is alkaline enough to 
significantly weaken the newly-exposed Si-O bonds (before they reform 
again), and make the tube crack along the score mark.

The standard solution in the burette  is a combination of convention and 
not wanting poorly defined, (corrosive, prone to form precipitates) 
substances in your burette. Think "respect the burette" !
Note that the primary standard is not always in the burette, in a back 
titration, you react the standard with the sample in the conical flask, 
and then titrate with a secondary standard.

On 1/05/2016 10:56 AM, jaclyn mcgregor wrote:
> Hi all,
> I have had a Yr 12 student I tutor ask me what the protocol is for 
> titrations.
> Is the acid always in the burette? Why?
> Is the standard solution always in the burette? Why?
> The opinions of other Chemistry teachers on this matter would be 
> greatly appreciated.
> Many Thanks
> Jackie Drake
> Kalgoorlie
>
>
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