[Catalist] Titrations
Leon Harris
leon at quoll.com
Wed May 4 17:56:35 AEST 2016
Physics forums say:
Greetings, Just a quick question. I would like to know what order of
magnitude the etch rate will be for a solution with a pH of 14 on silica
glass (flat surface at room temp) - are we talking nm/hr? microns/hr?
mm/hr? Thanks, Claude.
Reference
https://www.physicsforums.com/threads/etching-glass-using-alkalis.227609/
Greetings, Just a quick question. I would like to know what order of
magnitude the etch rate will be for a solution with a pH of 14 on silica
glass (flat surface at room temp) - are we talking nm/hr? microns/hr?
mm/hr? Thanks, Claude.
Reference
https://www.physicsforums.com/threads/etching-glass-using-alkalis.227609/
Q: Greetings, Just a quick question. I would like to know what order of
magnitude the etch rate will be for a solution with a pH of 14 on silica
glass (flat surface at room temp) - are we talking nm/hr? microns/hr?
mm/hr? Thanks, Claude.
A: If those were my three choices, I would say it is microns/hr. Visible
etching is observed during that time at that concentration so the scale
must be larger than a wavelength of light (500 to 900 nm) but the rate
is insufficient to completely dissolve the bottom out of glass beakers
(thickness of 1-2 mm) containing pH 14 liquid.
So 83 days (assuming 500nm/h) to etch a mm at 1M NaOH. Cylinder surface
area = 2 pi r h + 2 pi r^2. Drop the (2 pi r^2) bit, as we don't have
end caps. So we have etched a 1mm deep by (2 x 3.14159 x 5 x 500mm=)
15708 mm^2 volume (assume a 50cm tall burette, radius 0.5cm), or 15.7ml.
Quite a bit larger than +/- 0.05ml -eh !
For sure, we use less concentrated solution in titrations, and for
shorter durations but certainly appreciable amounts. And it is probably
an equilibrium, so the rate the physicist reported in the answer is
probably an overestimate for most of those 83 days!
Glass etching is supposed to be highly non-linear with respect to alkali
concentration. Furthermore, the fact that glass bottles don't usually
have their bases fall out makes me think some kind of equilibrium is at
play. However, there should be enough of a warning here to suggest that
alkali in burettes over the long term may affect accuracy. There is a
known and plausible mechanism. I would say, just don't do it.
Interestingly enough, a quick scan fails to show any iso9000
requirements for burette replacement schedules. This is either an
oversight, or perhaps they need to be calibrated throughout their
working life. Or possibly because titration is not practised on any
sort of wide scale these days. (Although I was using it in the mid 2000s
industrially, to manage volatile fatty acid production in a
semi-experimental methane generating waste water treatment plant, but
that's another story and another life). And modern borosilicate glasses
have much better resistance than earlier versions.
Damn I wish Doug Clarke was still alive - he would have known. (He knew
everything!). I don't really know any old school industrial chemists who
I could ask!
Just a quick question. I would like to know what order of magnitude the
etch rate will be for a solution with a pH of 14 on silica glass (flat
surface at room temp) - are we talking nm/hr? microns/hr? mm/hr?
Reference
https://www.physicsforums.com/threads/etching-glass-using-alkalis.227609/
I wouldn't completely discount my mechanism yet. But clearly, there is a
lot of "lore" to a good titration. Certainly, weak alkali for short
periods and then cleaned out should not have much of an effect. Figuring
it out is a nice science project for a kid!
Cheers,
Leon
On 4/05/2016 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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