[Catalist] Titrations

[Yarra Korczynskyj]

Nice one!

'tho the resulting discussion was fascinating ... continue ...

Yarra


On 5/4/2016 9:16 AM, jaclyn mcgregor wrote:
> Thank you to Lyndon, Leon, Peter and Mike,
> So the answer is it shouldn't matter whether you place the acid or the 
> base in the burette or not?
> Is it conventional then for the standard solution to always go in the 
> burette?
> Many thanks
> Jackie Drake
>
>
>
> ------------------------------------------------------------------------
> *From:* Leon Harris <leon at quoll.com>
> *To:* catalist at lists.stawa.net
> *Sent:* Sunday, 1 May 2016, 19:16
> *Subject:* Re: [Catalist] Titrations
>
> 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 <mailto: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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