Kinds of TitrationsKinds of Titrations

Titration of a Strong Acid With A Base

As a strong base is titrated into a strong acid, there is just a slight increase in pH until the equivalence point (point at which the amount of H+ and OH- are equal). Near the equivalence point (pH 7) there is a sudden upswing of pH caused by just one drop of the base. If an indicator is chosen that has a transition point close to the equivalence point (pH 7) then a colour change will indicate that the equivalence point has occurred.

Features of Strong Acid/Base CurvesSince the salts of strong acids and bases are neutral (no

hydrolysis effects), the pH of the equivalence point is 7. For this kind of titration, an indicator whose transition point is near 7 should be chosen like phenolpthalein.

Titrating A Weak Acid With A Strong BaseWhen a weak acid is titrated

with a strong base, there is an initial upswing followed by a gradual rise and then a large upswing around the equivalence point. The pH of the equivalence point is alkaline because the anion salt of the weak acid (CH3COO-) hydrolyzes water, forming OH- ions.

CH3COO- + H2O CH3COOH + OH-

An indicator like phenolphthalein should be chosen because it changes colour at pH 9.1, near the equivalence point.

Calculating the [Weak Acid] From pH Data

For CH3COOH + OH- CH3COO- + H2O ,Consider a .100M solution of CH3COOH at the V½ point in the

titration. At this point the [CH3COOH] = [CH3COO-] = .050 . At this point, the simplified expression for Ka becomes:Ka = [H3O+][CH3COO-] / [CH3COOH] = [H3O+]Thus the pKa = pH½

Calculating the [Weak Acid] From pH Data

(Continued)

Given the pHstart = 2.667, the pH½V = 4.733, the pHend = 8.033,Then [H3O+]start = 10-2.667 = .00215278 and Ka = 10-4.733 = 1.849E-5 CH3COOH + H2O CH3COO- + H3O+

I [X]i -- 0 0C -2.153E-3 -2.153E-3 +2.153E-3 +2.153E-3 E [X - 2.153E-3]eq -- +2.153E-3 +2.153E-3 Ka = 1.849E-5 = [2.153E-3]2/(X – 2.153E-3), X – 2.153E-3 = [2.153E-3]2/1.849E-5 = .2506 MX = .2506 M + 2.153E-3 M = .2527999 M = .253 M

Sample Problem (Hebden p171)25.0 mL of a weak acid, furoic acid, is titrated with NaOH. It takes

28.8 mL of NaOH to reach equivalence. The pH of furoic acid is initially 2.021 and the pH½V is 3.170 when 14.4 mL of NaOH have been added. Find the concentrations of the furoic acid (initial) and NaOH.

The initial [H3O+] = 10-2.021 = 9.52796E-3The Ka = 10-3.170 = 6.7608E-4 acid cation + H3O+

I X [furoic]st. __ 0 0 C - 9.528E-3 __ +9.528E-3 +9.528E-3E X - 9.528E-3 __ +9.528E-3 +9.528E-3From Ka = 6.7608E-4 = [H3O+]2/[Furoic acidst – 9.528E-3]eq, Furoic acidst - 9.528E-3 = [9.52796E-3]2/ 6.7608E-4 = .13428 MSo [Furoic acidst] = .13428 M + 9.528E-3 = .14380 M = .144 MThe moles Furoic acid = 25.0 mL(.144 mmol/mL) = 3.60 mmol F.ASince the moles NaOH = moles Furoic acid = 3.60 mmol,[NaOH] = 3.60 mmol/28.8 mL = 0.125 M

Titration of a Weak Base With A Strong AcidWhen a weak base (NH4OH) is

titrated with a strong acid (HCl), there is an initial rapid downswing followed by a gradual lowering of pH. Near the equivalence point there is a rapid downswing with an acidic equivalence point due to the base cation hydrolyzing water. Ex:

NH4+ + H2O NH3 + H3O+

An indicator like methyl orange should be chosen because it’s colour transition point is in the acid range close to the equivalence point.

Concentrations From Titration pH Readings

As with a weak acid and strong base, the initial pH and pH½ V are useful in determining initial concentrations. The pKb = pOH½V

Convert pH½V to pOH½V by 14-pH.To determine the initial weak base

concentration,[w. base]EQ = [OH-]2/Kb (from pOH½V)The [OH-] is obtained by 10-(14-pH)

End of Presentation

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