[Catalist] Titrations

[Lyndon Smith]

Hi Leon,

 

May I disagree with you a little?

 

Mostly when titrating we use dilute solutions of sodium hydroxide and while
I agree that SiO2 is acidic I cannot see the burette volume being changed in
a dilute solution in a short time.

 

When I began titrations many years ago there were two kinds of burette. An
acid burette with a unique handmade ground glass tap and a base burette with
a rubber tap, and I think this is the key. 

 

When using a ground glass burette, improperly washed after using NaOH
solution, the remaining solution would evaporate down and become super
concentrated and freeze the ground glass tap. Thus for bases we used rubber
ended burettes with squeeze valves.

 

The issue was the ground glass tap not the burette.

 

Today we have Teflon taps and so the debate about NaOH and glass taps no
longer counts. I would suggest using either acid or base will not damage the
Teflon tap nor change the volume of the burette.

 

Ps as the taps now rotate in the burette we no longer have burettes for left
handed people and the case for placing the operator's hand around the tap
(originally to stop the ground glass tap falling apart) probably has ceased
to be meaningful too!

 

Lyndon

From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Leon
Harris
Sent: Sunday, 1 May 2016 1:08 PM
To: catalist at lists.stawa.net
Subject: Re: [Catalist] Titrations

 

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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