(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.

(8.4) Acid-Base Titration(8.4) Acid-Base Titration

What is Titration? What is Titration? Demo Time!Demo Time!

The precise addition of a solution in a The precise addition of a solution in a burette into a measured volume of a burette into a measured volume of a sample solution.sample solution.

This allows us to determine the This allows us to determine the concentration of a specific chemical.concentration of a specific chemical.

The The titrant titrant is the solution in a burette during is the solution in a burette during a titration, while the a titration, while the sample sample is the solution is the solution being analyzed.being analyzed.

Primary StandardPrimary Standard

Primary standard is… is…

– – a chemical available in a pure and stable a chemical available in a pure and stable form, for which an accurate concentration form, for which an accurate concentration can be prepared. can be prepared.

- then used in a titration to determine the - then used in a titration to determine the precise concentration of a titrant.precise concentration of a titrant.

The Equivalence PointThe Equivalence Point

The The equivalence point (stoichiometric (stoichiometric point) of a titration is the measured point) of a titration is the measured quantity of titrant recorded at the point at quantity of titrant recorded at the point at which chemically equivalent amounts have which chemically equivalent amounts have reacted.reacted.

nnHH++ = n = nOHOH

--

Recall: C = n/VRecall: C = n/V or or n = CVn = CV

The EndpointThe Endpoint

The endpoint of a titration is a point in The endpoint of a titration is a point in which a sharp change in a measurable which a sharp change in a measurable and characteristic property occurs (ie. and characteristic property occurs (ie. colour change, pH change)colour change, pH change)

The appropriate indicator is selected so The appropriate indicator is selected so that its colour change occurs in the pH that its colour change occurs in the pH range of the equivalence point range of the equivalence point

(see indicator list p609 &p804)(see indicator list p609 &p804)

How To Perform a TitrationHow To Perform a Titration

1.1. Standardize titrant by using primary Standardize titrant by using primary standard. (Often done by teacher!)standard. (Often done by teacher!)

2.2. Measure exact volume of sample and Measure exact volume of sample and add a couple of drops of indicator.add a couple of drops of indicator.

3.3. Add the titrant until a colour change Add the titrant until a colour change occurs.occurs.

4.4. Record amount of titrant added to Record amount of titrant added to solution to reach endpoint.solution to reach endpoint.

pH Indicators-p609pH Indicators-p609

Figure 1 (p595)Figure 1 (p595)

Titration CurvesTitration Curves

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

Titration Curves: Titration Curves: Strong Acid & Strong BaseStrong Acid & Strong Base

(Figure 2 p599)(Figure 2 p599)

Titration CurvesTitration Curves

The midpoint is the sharp increase in pH, which The midpoint is the sharp increase in pH, which is where the equivalence point is located.is where the equivalence point is located.

For strong acids and bases the equivalence For strong acids and bases the equivalence point will occur at a pH of 7.point will occur at a pH of 7. The equivalence point represents the autoionization The equivalence point represents the autoionization

of water since the conjugate acid of a strong base is of water since the conjugate acid of a strong base is weak, and the conjugate base of a strong acid is weak, and the conjugate base of a strong acid is weak. weak.

Neither a strong base or strong acid hydrolyze water.Neither a strong base or strong acid hydrolyze water.

Titration Curves:Titration Curves: Strong Base & Weak Acid Strong Base & Weak Acid

(Figure 4 p607)(Figure 4 p607)

Titration Curves:Titration Curves: Weak Base & Strong Acid Weak Base & Strong Acid

(Figure 5 p608)(Figure 5 p608)

A Summary :A Summary :Types of TitrationTypes of Titration

At equivalence point At equivalence point

pH = 7pH = 7 strong acid (SA) – strong base (SB) strong acid (SA) – strong base (SB)

pH > 7pH > 7 weak acid (WA) – strong base (SB)weak acid (WA) – strong base (SB)

pH < 7pH < 7

strong acid (SA) – weak base (WB) strong acid (SA) – weak base (WB)

Titration Curves: Polyprotic AcidsTitration Curves: Polyprotic Acids (Figure 8 p611) (Figure 8 p611)

Polyprotic acids have more than one Polyprotic acids have more than one vertical line because the first proton given vertical line because the first proton given away has an equivalence point, and so away has an equivalence point, and so does every other proton that is donated.does every other proton that is donated.

Sample Problem #1Sample Problem #1

In a titration, 25.00 mL of an KOHIn a titration, 25.00 mL of an KOH(aq)(aq) is is

required to neutralize 50.0 mL of required to neutralize 50.0 mL of 0.010 mol/L HBr0.010 mol/L HBr(aq)(aq). .

a)a)What is the molar concentration of KOHWhat is the molar concentration of KOH(aq)(aq)

b)b)What is the pH at the equivalence point?What is the pH at the equivalence point?

Ans: 0.020 mol/L, pH=7Ans: 0.020 mol/L, pH=7

Sample Problem #2Sample Problem #2

In a titration, 10.00 mL of 0.200 mol/L HClIn a titration, 10.00 mL of 0.200 mol/L HCl(aq)(aq)

is titrated with standardized 0.250 mol/L is titrated with standardized 0.250 mol/L NaOHNaOH(aq)(aq) . .

What is the amount of What is the amount of unreactedunreacted HCl HCl(aq)(aq)and and

the pH of the solution after the following the pH of the solution after the following volumes of NaOHvolumes of NaOH(aq)(aq) have been added? have been added?

i)i) 0 mL0 mL 2.00 mmol, pH=0.6992.00 mmol, pH=0.699

ii)ii) 2.00 mL2.00 mL 1.50 mmol, pH=0.9031.50 mmol, pH=0.903

iii)iii) 8.00 mL8.00 mL 0 mmol, pH=70 mmol, pH=7