7(m)2015, pH changes during acid/base titrations (and titration curves) #1

Titration curves plot the pH of a solution against the volume of acid (or base) added during a titration. The graph produced reveals many interesting and important aspects of acid-base reactions.

The syllabus doesn’t say you need to be able to draw titration curves, but it does say you have to be able to explain them. Being able to draw them, does actually help you to remember how to explain them.

7(m)2015

“describe the changes in pH during acid-base titrations and explain these changes in terms of the strengths of the acids and bases “

 

 

  1. Draw axes.
  2. draw a horizontal line (running parallel to the x-axis) at pH=7. This will produce an ‘acidic region’ (from pH7 downwards) and a ‘basic region’ from pH 7 upwards) each separated by the line.
  3. Determine which substance is in the conical flask (where the pH change will take place and is measured). For the sake of this example, we will take it that we have base in the flask. So the initial pH of the titration curve we are about to plot will begin in the basic region. For strong bases, you’ll probably be starting around about pH 13 to 14 (although you may be given data to calculate the initial pH!).
  4. Then, label the axes appropriately. As is usual with ‘scientific graphs’, we have a controlled quantity or input which we plot on the x-axis. The response or output to that controlled quantity is plotted on the y-axis.
    In this case our controlled quantity is the volume of acid added (from the burette) in cm3, so that’s the label for the x-axis. The y-axis label is simply ‘pH’ which has no units as it’s a log term.
  5. Consider the nature (or type) of acids and bases being used. Are you dealing with a strong acid (sA) and a strong base (sB)? or a sA and a weak base (wB)? Other possibilities are wA with a sB and finally, a wA with a wB.
    Strong species have a nearly horizontal section and a nearly vertical section. The horizontal sections (almost always) occur first, followed by the near vertical section later. Make vertical sections spans 4 pH units (or 3 at a very minimum). While weak components have NO VERTICAL SECTION, only displaying a curved region which flattens out in the centre (of the curved region).
  6. We now begin to sketch the curve from the initial pH remembering any horizontal or vertical considerations for the strong/weak nature of the acid or bases present. Start sketching from the correct acid or basic region. Date may be given to you so that you can calculate the initial pH but if it’s not then start at high pH’s, e.g. 14 or 13 for strong bases, about 11 (possibly 10) for weak bases. For strong acids start at about 1 or 2 and about 4 (possibly 5) for weak acids. Now is a good time to look at an example. (click to expand if necessary)drawing tutration curves #1
    Try and see all the features mentioned so far on the curves.
  7. The vertical section will be where you get the equivalence point (where the amount of acid added is sufficient to react with the base present). Read directly down from the vertical section to the x-axis to get the volume for the equivalence point. If you have no strong components, then the equivalence point can still be found. It will be the most vertical part of the curve, (mathematically it will be the ‘point of inflection’ – errrm, I think ;p)
  8. If any weak components are present, the most flat ‘plateau’ is an important point. It’s where the system operates as an efficient buffer. You can see that the pH is changing only slightly when small quantities of acid are added. This point is different from the almost horizontal part of the strong component. If you compare them carefully, you will see it continually becoming more curved, but the weak components ‘horizontal’ region went from curved to almost flat to curved again.
  9. Finish drawing your curve at a sensible pH, or if you are given data about the titration, you will be expected to finish drawing your pH curve at a specific calculated value. With no data, a sensible place to stop at for an acidic region with strong acid in the burette is about 1 or 2 and for a weak acid it will be about 4 (possibly 5). If base was in the burette and it was strong then stopping at about 13 or 14 would be appropriate, but if it was a weak base, about 11 (possibly 10) would be appropriate.

There is nothing ‘absolute’ about the way these steps. An individual is more than free to rearrange the steps and split or combine steps.

Try now to sketch some graphs (the other 3 possible combinations involving sA, wA, sB and wB) and compare them.

Try the question below, which requires a more accurately sketched titration curve based on the following data…

25cm3 of a 0.10 molar aliquot of NaOH was titrated against 0.080M ethanoic acid. A total volume of 48cm3 of acid was used. Plot the titration curve for this titration and show the equivalence point and indicate where the optimal buffering effect is observed.

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