Sketching titration curves

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The sketching of titration curves is a skill that needs some practice. In order to get the curve correct in a particular case, you have to have in your mind the following points:

  • the starting pH

  • the finishing pH

  • the end-point volume, i.e. the volume at which the vertical section of the curve is found

  • the extent of the vertical range.

Each of these is dependent on the nature of the acid and the base, whether strong or weak.

The starting pH will often have been the subject of a calculation in a previous part of the question, or it will have been given in the question itself. You would be expected to start at this value!

The final pH can usually be calculated fairly easily since either the acid or the base is in excess by the time the curve has levelled out.

Consider the titration of 25.0 cm3 of 0.1 mol dm - 3 HCl solution with
0.1 mol dm - 3 NaOH solution, where the curve is taken as far as the addition of 50 cm3 of sodium hydroxide.

  • The starting pH = - log 0.1 = 1. Perhaps it's worth commenting here that the definition of pH should be

pH = - log {[H+]/mol dm - 3}

          since you can only take the logarithm of a number, not of a physical quantity.

  • The endpoint volume will be 25.0 cm3 since the solutions are of the same concentration and the reaction ratios are 1:1.

  • The endpoint pH is 7 since neither the sodium ion nor the chloride ion interact with water.

  • The final pH is going to be somewhat less than the pH of the sodium hydroxide solution alone since it is diluted by the sodium chloride solution to which it is being added. In this case:

  Final volume = 75.0 cm3 and endpoint volume =  50 cm3

Volume of sodium hydroxide added in excess is therefore 25.0 cm3
Amount of sodium hydroxide added in excess = 0.025 dm3 x 0.1 mol dm - 3 = 0.0025 mol

Thus the concentration of NaOH = 0.0025 mol / 0.075 dm3 = 0.0333 mol dm - 3
Thus pOH = - log 0.0333 = 1.48

The pH is therefore 14  - 1.48 = 12.5(2).

In the absence of any dilution the pH would have been 13.

  • It is important to remember such increases of volume when calculating pH during a titration.

Similar arguments can be made for titration using weak acids or weak bases; remember that a weak acid/strong base titration will have an endpoint pH around 8, and a strong acid/weak base titration around pH 5.



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