[Catalist] Titrations

[Leon Harris]

You could well be right Lyndon.
I was taught that it had to do with etching and roughening glass. I 
always took this to mean in the tube. It could refer to etching the tap 
and making it stick. Certainly dilute hydroxide, reacting with CO2 in 
the air will both etch and freeze a joint.  (NaOH solubility, 1.1kg/l, 
Na2CO3 solubility is 500g/liter, so converting to a carbonate will make 
a solid that will freeze the joint, with a little evaporation perhaps)

Remember also that burettes were "bread and butter" instrumentation, in 
the day. So analysts could easily spend 4 hours a shift, 7 days a week 
doing titrations. That is a lot of time over a year, for a 75cm tube 
rated to +/- 0.05ml over that entire length. Mind you, you'd expect that 
the markings on the side would come off by then (probably faster in 
lye!- look at the effect on writing on glass ovens), and that would be a 
give away!

Cheers,
Leon


On 1/05/2016 4:03 PM, Lyndon Smith wrote:
>
> Hi Leon,
>
> May I disagree with you a little?
>
> Mostly when titrating we use dilute solutions of sodium hydroxide and 
> while I agree that SiO_2 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
>
>
>
>
>     _______________________________________________
>
>     Catalist mailing list
>
>     Catalist at lists.stawa.net <mailto:Catalist at lists.stawa.net>
>
>     http://lists.stawa.net/mailman/listinfo/catalist_lists.stawa.net
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.stawa.net/pipermail/catalist_lists.stawa.net/attachments/20160501/d3d44447/attachment.html>