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Offline Mania28

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determining the acidity constant (Ka)
« on: October 19, 2006, 03:19:18 AM »
Hi,

i want to determine the acidity constant (Ka) of a colloid suspension. This suspension has a pH of 6,5 and a colloid concentration of 0,05 g/l. The colloids are carboxylated latex microspheres. How can I acquire the Ka and respectivly the pKa of this suspension? I guess through titration, but I don´t know exactly what to do. I would be thankful if someone could describe me what I exactly have to do.

Please help me, its urgent!!  :-[

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #1 on: October 19, 2006, 03:43:32 AM »
No idea if it'll work for colloidal suspension - but why not? Standard procedure will be titration with pH titration curve registration. At 50% pH=pKa.
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Offline Mania28

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Re: determining the acidity constant (Ka)
« Reply #2 on: October 19, 2006, 06:23:56 AM »
what does that mean 50% pH ? 50 % of what? how does a titration curve look like? what would be the exact approach? i would titrate with 0,01 M HCl or 0,01 M NaOH

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #3 on: October 19, 2006, 06:50:37 AM »
It is not 50% of pH, but 50% of substance titrated :)

When the substance is titrated in 50% - half is neutralized and half is not. At this point pH=pKa. Take a look at Henderson-Hasselbalch equation to see why.

Due to technical reasons I can't show you any titration curve ATM. Please download BATE and see how they look alike for yourself. Or take a look at this BATE titration curve page - acetic acid titration curve is more or less what you may expect.
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Offline Mania28

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Re: determining the acidity constant (Ka)
« Reply #4 on: October 20, 2006, 04:18:09 AM »
Ok, now I know what you mean. How is this procedure called in english? "Half-titration"? when I look at the typical titration curves, i see on the y-axis pH values from 2 to 14. My question is, since my suspension has a ph of 6,5 if i have to acidify my suspension? and with which substance would i titrate? i would suggest 0,01 M NaOH?? I think its important if one uses strong or weak acids/bases???
and how does one get exactly the point of equivalence (sorry, i dont know the right word)?

Thanks for your *delete me*

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #5 on: October 20, 2006, 05:24:56 AM »
Sorry, somehow I didn't noticed pH mentioned in your first post. If pH is so close to neutral, you have either extremally weak acid (pKa in the range of 10 or more), very low concentration of acid (like 10-5M or something), or it is already partially neutralized. That means that titration won't be as simple as I thought ot first (it may be even impossible at all). Not knowing anything about the system it is hard to say what is going on.
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Offline lavoisier

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Re: determining the acidity constant (Ka)
« Reply #6 on: October 20, 2006, 04:21:18 PM »
Mania28,
as far as I know the pKa (=-log10(Ka)) has a very specific, thermodynamic meaning. It is the equilibrium constant of the reaction where an acid HA protonates water to give the anion A- and H3O+.

Here we have a polymer with carboxyl groups, therefore:
1. you haven't got a single substance, but a mixture of substances with different molecular weights, more or less widely distributed;
2. it is not specified how many carboxyl groups each molecule has got on, and if they are too far apart they will react indipendently;
3. the susbtances are not 'clearly' soluble in the medium, which means precipitation equilibria may interfere;
4. the concentration (if we can call it so, as it's not a proper solution) is only 0.05 g/L, pretty small even for a 'standard' organic acid with MW, say, 100 (it would be 5*10-4 M).

In these conditions my guess is that you can't determine the pKa by titration.

However, if you want to try and see what happens, the procedure is simply the following:
1. if you can, start with a more concentrated 'solution';
2. add enough strong acid (HCl?) to bring the pH to about 1-2;
3. titrate with NaOH 0.01 M or 0.001 M (remember to calibrate the pHmeter);
4. draw the titration curve.

The pKa is the pH at the point (on the titration curve) where half of your (weak) acid has been titrated, i.e. where the titration curve has a horizontal inflexion point, i.e. where its 1st derivative has a relative minimum.

Having added the strong acid, you will have a first equivalence point (where the titration curve 'jumps up'). Let's say it's at 5 mL NaOH. At that point you have your free acid ready to be titrated. Going on with the addition of NaOH, you will have another 'jump up' of the curve. Let's say it's at 15 mL NaOH. That is the endpoint, i.e. where all of your acid is converted to sodium salt. The half point is of course the mean of these two (5+15 = 20, 20/2 = 10). So you go to the x axis at 10 mL NaOH, look for the corresponding point on the curve, go from this to the value on the y axis, and that's your pKa.

As I said before, though, this method could work for 0.01 M acetic acid, but in your case you may expect hundreds of complications.

I remember reading a paper about the determination of pKa by some special spectrometric method. Maybe some physical chemist out there knows more.

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #7 on: October 20, 2006, 04:42:18 PM »
Having added the strong acid, you will have a first equivalence point (where the titration curve 'jumps up').

While you are OK listing problems present, you are slightly wrong here. Whether the endpoints will be separated or not will depend on the pKa you want to determine - and if the acid is too strong (which is almost the case for acetic acid), first endpoint is very difficult to spot and localize - it is just a wrinkle on the titration curve, that can be easily overlooked.
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Offline lavoisier

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Re: determining the acidity constant (Ka)
« Reply #8 on: October 21, 2006, 12:48:55 PM »
Borek,
sorry but I DID carry out a potentiometric titration of a mixture of HCl and acetic acid, and I could perfectly identify both endpoints. It was actually part of an exam of analytical chemistry I.

While I see your point, especially in this case where we have a very diluted solution, I wonder whether it makes sense to start with that solution as such. The pKa would be bound to be equal or higher than 6.5, which is unrealistic I think.

Again, my guess is that it's not a simple problem, but if anyone has any 'magical' solution...

Offline Mania28

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Re: determining the acidity constant (Ka)
« Reply #9 on: October 21, 2006, 03:30:20 PM »
Hi and thanks for your answers,

I´ve got a little information on the polysterene particles: they have a PI < 0,1 (Polydispersity index)and so they arent widely distributed and so the molecular weights shouldnt be widely distributed. The density of COOH groups is 120 nmol/mg. The stock solution of the colloid suspension has a concentration of 10 g/l; i just work with a concentration of 0,05 g/l. What a concentration for the titration would be wise?

Another question: does someone know if these carboxyl groups are only ON the surface of the polysterene particles or of they are IN it too? and how are they fabricated?

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #10 on: October 21, 2006, 04:00:29 PM »
sorry but I DID carry out a potentiometric titration of a mixture of HCl and acetic acid, and I could perfectly identify both endpoints. It was actually part of an exam of analytical chemistry I.

No need to be sorry :)

Attached please find theoretical curve for this titration (0.1M/0.1M/0.1M). I agree that first endpoint is visible, whether it can be perfectly identified (in terms of the measurement precision) is doubtfull for me.
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Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #11 on: October 21, 2006, 04:10:13 PM »
The density of COOH groups is 120 nmol/mg. The stock solution of the colloid suspension has a concentration of 10 g/l

That gives concentration in the range of 1 mmol/L (assuming they are on the surface, I have no idea if they are or not). Not much, but it can be doable. Probably no dilution.
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Offline lavoisier

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Re: determining the acidity constant (Ka)
« Reply #12 on: October 22, 2006, 11:56:49 AM »
Borek,
try and plot the first derivative of the titration curve.
The equivalence points correspond to relative maxima (as it can be shown theoretically, too), which are much easier to detect.

It's not a brainiac theory of mine: a commercial automatic titrator I used where I worked previously actually had the option of printing the 1st derivative. It must be for a reason, don't you think?

Offline Borek

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Re: determining the acidity constant (Ka)
« Reply #13 on: October 22, 2006, 12:26:42 PM »
Calculating derivative will not improve accuracy. The lower the endpoint slope, the broader the peak, the lower the accuracy. If you use 2nd derivative endpoint detection is even simpler, but not more accurate.

Note: I am not telling it can't be done. I am aiming just at the fact, that accuracy of such determination is low. For some applications this accuracy will be OK, for others it will be too low. As I have stated earlier

Quote
if the acid is too strong (which is almost the case for acetic acid), first endpoint is very difficult to spot

See additional curves of the stronger acids - first has pKa 4, second pKa 3. For pKa 4 endpoint is barely visible, for pKa 3 it is not visible at all. Acetic acid is in the range where accuracy can be acceptable if you don't ask for too much - that's the "almost" part :)
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