7(m)2015, pH changes during acid/base titrations (and titration curves) #2

The University of California, Davis, (UC Davis) has a very good page on titration curves / titrations. There’s also some calculations at the end that employ the useful ICE method – which will be extremely similar to the methods you used for quantitative (mathematical) discussion of Kc and Kp.
Please note, your syllabus says

7(f)2015:

” calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations) “

so don’t worry about the “complicated” cases where quadratics come up, but hey, we do maths yes? we’ve seen this at SPM/iGCSE etc. It shouldn’t be ‘difficult’ for us, plus there’s no harm in applying our maths techniques to the real world physical sciences.o

Here’s the website:

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titration/Titration_of_a_Weak_Base_With_a_Strong_Acid.

And here’s some videos demonstrating use of the ICE method.

pH of a Weak Acid (0.1 M Acetic Acid) EXAMPLE  – chemistNATE

Find the pH of a Buffer Solution  – chemistNATE

 

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3 thoughts on “7(m)2015, pH changes during acid/base titrations (and titration curves) #2

  1. Solubility product- Upto what pH must a solution containing a precipitate of Cr(OH)3 be adjusted so that all of precipitate dissolves
    (When Cr3+ = 0.1 mol/l , Ksp = 6×10^-31)

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    • It is quite tricky to deduce this from just Ksp equations as changing the pH changes the complex which have different solubilities. We could make [Cr(H2O)4(OH)2]+ at lower pH’s and [Cr(H2O)2(OH)4]- at high pH’s.
      Before when we used Ksp we only ever had ions and it’s salt so the situation, so things were a LOT simpler.

      Obviously however, there will be some pH where all the hydroxide ppt dissolves. One would have to consider the stepwise-equilibrium constants and consider the [OH]- concentrations. When the value of the equilibrium constant reaches a value of about 100 (or 1000) then it would effectively be dissolved. I have never done such a calculation however and it would take me some time to figure out a correct expression and calculate it.

      Alternatively we could get a pH probe and measure it in the lab, which I’ll try and do. (After which it would probably be easier to do the calculation)

      I have heard the value of about pH10 but it has to be a strong base. Ammonia base has the similar problem that new complexes form, but from the basic action alone I don’t think it can force redissolving (dissolution) of the ppt, same for carbonate bases, so only fairly concentrated hydroxide bases (out of the common range would do).

      Until I’ve time to check it out in more detail this page may introduce the relevant concepts to you

      1) http://www.chemguide.co.uk/inorganic/complexions/stabconst.html

      2) http://www.science.uwaterloo.ca/~cchieh/cact/c123/complex.html

      3) http://tinyurl.com/klfsepv (Lister and Renshaw. New Understanding Chemistry)

      Sorry, but that’s the best I can do at this time. {I’m still mindful of the other PbBr2 / PbI2 question also)

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    • Zhao Xuan.
      There’s an explanation here: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch18/complex.php, Problem 8. (but the actual calculation seems to be missing 😦
      I’ve e-mailed Prof Bodner about it. Hopefully he’ll see to it that the actual calculation page is made available.
      Anyway, the explanation – which gives a bit of the answer, gives a [H+(aq)] ion conc of about 1.9×10-10 (so pH is about 9.7 – which did correspond to the kind of almost pH=10 values I had stumbled across before) is what I was kinds of ). That pH is going from Cr(OH)3(s) to [Cr(OH)4]-(aq). I’m not sure about going the other way, i.e. making Cr(OH)3(s) dissolve by lowering the pH to make, say, [Cr(OH)2(H2O)4]+(aq) but I would predict a very similar treatment as to what’s presented could be used for going in the opposite direction, although I don’t have enough time right now to go into it in much detail.

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