(8.4) Acid-Base Titration

What is Titration? Demo Time! The precise addition of a solution in a burette into a measured volume of a sample solution. This allows us to determine the concentration of a specific chemical. The titrant is the solution in a burette during a titration, while the sample is the solution being analyzed.

Primary Standard is… Primary standard is… – a chemical available in a pure and stable form, for which an accurate concentration can be prepared. - then used in a titration to determine the precise concentration of a titrant.

The Equivalence Point  The (stoichiometric point) of a titration is the measured quantity of titrant recorded at the point at which chemically equivalent amounts have reacted.  The equivalence point (stoichiometric point) of a titration is the measured quantity of titrant recorded at the point at which chemically equivalent amounts have reacted. n H + = n OH - Recall: C = n/V or n = CV

The Endpoint  The endpoint of a titration is a point in which a sharp change in a measurable and characteristic property occurs (ie. colour change, pH change)  The appropriate indicator is selected so that its colour change occurs in the pH range of the equivalence point  (see indicator list p609 &p804)

How To Perform a Titration 1. Standardize titrant by using primary standard. (Often done by teacher!) 2. Measure exact volume of sample and add a couple of drops of indicator. 3. Add the titrant until a colour change occurs. 4. Record amount of titrant added to solution to reach endpoint.

pH Indicators-p609

Figure 1 (p595)

Titration Curves  We plot our experimental data on a graph, with the pH of the solution on the y-axis, and the volume of titrant (ex: NaOH) added on the x-axis.

Titration Curves: Strong Acid & Strong Base (Figure 2 p599)

Titration Curves  The midpoint is the sharp increase in pH, which is where the equivalence point is located.  For strong acids and bases the equivalence point will occur at a pH of 7.  The equivalence point represents the autoionization of water since the conjugate acid of a strong base is weak, and the conjugate base of a strong acid is weak.  Neither a strong base or strong acid hydrolyze water.

Titration Curves: Strong Base & Weak Acid (Figure 4 p607)

Titration Curves: Weak Base & Strong Acid (Figure 5 p608)

A Summary :Types of Titration At equivalence point pH = 7  strong acid (SA) – strong base (SB) pH > 7  weak acid (WA) – strong base (SB) pH < 7 strong acid (SA) – weak base (WB)

Titration Curves: Polyprotic Acids (Figure 8 p611)  Polyprotic acids have more than one vertical line because the first proton given away has an equivalence point, and so does every other proton that is donated.

Sample Problem #1 In a titration, mL of an KOH (aq) is required to neutralize 50.0 mL of mol/L HBr (aq). a) What is the molar concentration of KOH (aq) b) What is the pH at the equivalence point? Ans: mol/L, pH=7

Sample Problem #2 In a titration, mL of mol/L HCl (aq) is titrated with standardized mol/L NaOH (aq). What is the amount of unreacted HCl (aq) and the pH of the solution after the following volumes of NaOH (aq) have been added? i)0 mL2.00 mmol, pH=0.699 ii)2.00 mL 1.50 mmol, pH=0.903 iii)8.00 mL 0 mmol, pH=7