Acids and Bases Titration curves

Acid – base titrations The end point is reached when enough base has been added to react completely with the acid (or vice versa). The pH at the end point is not necessarily 7. For a strong acid – strong base titration, the end point has pH = 7 For strong acid – weak base titration the end point has pH < 7 why? For a weak acid – strong base titration the end point has pH > 7 why? A titration curve is a measurement of the recorded pH in a solution (on the y-axis) and volume of base added (on the x – axis) {or volume of acid added}. The pH remains virtually constant while the base is added and then the pH changed dramatically as equivalence is approached. The pH reaches a plateau in the xs alkaline region (for base added to acid). e.g. HCl + NH3 → NH4+ + Cl- weak acid e.g. CH3COOH + OH- → H2O + CH3COO- weak base

Common indicators: methyl orange, methyl red, phenolphthalein pH at end point = 7 Common indicators: methyl orange, methyl red, phenolphthalein Note: starting pH = 1 (base added to acid) starting pH = 13 (acid added to base) Vertical range pH = 3 – 11 Final pH = 13 final pH = 1

Common indicators: methyl orange, methyl red pH at end point = 5 – 6 why? Common indicators: methyl orange, methyl red Starting pH = 1 (base added to acid) starting pH = 11 (acid added to base) Vertical range pH = 3-7 Final pH = 11 (base added to acid) final pH = 1 (acid added to base) NH3 + HCl → NH4+ + Cl- NH4+ + H2O  NH3 + H3O+ i.e. NH4+ undergoes hydrolysis to increase the [H3O+]

Common indicator - phenolphthalein pH at end point = 8 - 9 why? Common indicator - phenolphthalein Starting pH = 3 (base added to acid) starting pH = 13 (acid added to base) Vertical range pH = 7-11 Final pH = 13 (base added to acid) final pH = 3 (acid added to base) CH3COOH + OH- → CH3COO- + H2O CH3COO- + H2O  CH3COOH + OH- i.e. the acetate ion undergoes hydrolysis to increase the [OH-]

Sketching titration curves Decide whether the acid and base are strong or weak Draw and label axes Calculate the volume that has to be added to reach the equivalence point (the point at which the base reacts exactly with the measured amount of acid). Note, indicators are chosen so that the end point (the point at which the indicator changes colour) coincides with the equivalence point. Estimate the pH - at the start - at the equivalence point - at the end Draw a smooth curve – make sure you know the range of the vertical section expected.

Explaining titration curves Consider the addition of a 0.10 mol dm-3 NaOH solution from a (burette) to 20.0 cm3 of a 0.10 mol dm-3 solution of HCl. The pH at various point during the titration can be calculated: before the addition of NaOH after the addition of 10.0 cm3 NaOH after the addition of 15.0 cm3 NaOH after the addition of 18.0 cm3 NaOH after the addition of 19.0 cm3 NaOH after the addition of 19.5 cm3 NaOH after the addition of 19.8 cm3 NaOH

after the addition of 20.0 cm3 NaOH So, the addition of a small amount of strong base can change the pH dramatically around the equivalence point

Calculation of pKa from titration curves In a titration between a strong base and a weak acid, the pH at the point when half the acid has been neutralised can be used to determine the pKa and hence Ka of the acid. HA  H+ + A- Ka = [H+][A-] and the value of Ka is constant [HA] At the halfway point: [HA]halfway = [A-] halfway ஃ Ka = [H+] halfway and pKa = pHhalfway See titration curve for weak acid – strong base

Indicators Indicators are weak acids in which the acid molecule, HInd, is a different colour from its conjugate base, Ind- HInd + H2O  Ind- + H3O+ eq1 colour A colour B In acid solution, the equilibrium will shift ______ producing more of the __________ ________, and therefore we see colour ___ If base is added, H3O+ + OH-  2H2O shifts to the ___________, thus the Ωm of eq 1 shifts _________ and we see colour ____

When does an indicator change colour? For the reaction : HInd + H2O  Ind- + H3O+ Kind = [Ind-]eq[H3O+]eq [HInd]eq At the end point, [HInd] = [Ind-] ஃ Kind = [H3O+]endpoint pKind = pHendpoint The noticeable colour change for an indicator usually occurs over a range of 1 pH unit. E.g. methyl orange (pKind = 3.7) is red below pH 2.7, gradually changes to orange at pH 3.7 and then to yellow at pH 4.7.

Choosing indicators Titration Vertical range Indicator pH range Strong acid/strong base 3.5 – 10.5 Methyl red Methyl orange Phenolphthalein 4.2-6.3 3.2-4.4 8.2-10.0 Strong acid/weak base 3.0 – 6.5 Weak acid/strong base 6.5 – 10.5 Colours – Acid indicator base red methyl orange yellow colourless phenolphthalein magenta red methyl red yellow The choice of indicator depends on the pH at the equivalence point in the titration. The point at which the indicator changes colour is the end point. The pH range over which an indicator changes colour must lie completely within the vertical part of the titration curve.

Choosing indicators Choose an indicator for … these titrations pH range Methyl red 4.2-6.3 Methyl orange 3.2-4.4 Phenolphthalein 8.2-10.0 Bromothymol blue 6.0-7.6 Bromocresol green 3.8-5.4 Hydrochloric acid with ammonia solution Ethanoic acid with ammonia solution Nitric acid with potassium hydroxide