7. pH Curves and Titration

Titrant/Standard Solution

reactant (acid or base) solution that the concentration is known

Analyte

solution with unknown concentration

Equivalence Point

• titrant is added to an unknown concentration sample until all the reactant is consumed (equal moles)

• Visible by using an indicator

• Halfway point on ‘jump’ on pH curve

Titration

process of analyzing characteristics of a solution (ie. concentration and pH) by the reaction of a solution with a standard solution of acid or base

Steps of a Titration

1. sample place in flask

2. drops of titrant are slowly added

3. continues until equivalence

How is titration progress measured?

• pH meter

• data logger

  • records pH changes

pH data is plotted against…

volume of standard solution, producing a pH curve

What must be known about both substances?

• both volumes

• only one concentration

What do pH Curves depend on?

1. strengths and concentrations of the acid/base

2. addition order

Buffer Region

when both components of weak conjugate acid-base pair are present

Scenario 1: Strong Acid titrated with Strong Base

1. strong acid (HCl) will have a low pH (y-intercept at approx. 1)

2. strong base will raise the pH slowly at first (excess acid at first)

3. pH rises sharply at equivalence point to pH of 7 (no hydrolysis)

4. continues to rise quickly after pH 7 (unneutralized NaOH makes solution basic)

Scenario 2: Strong Base titrated with Strong Acid

1. strong base (NaOH) will have a high pH (y-intercept at approx. 14)

2. strong acid will drop the pH slowly at first (excess base at first)

3. pH drops sharply at equivalence point to pH of 7 (no hydrolysis)

4. continues to drop quickly after pH 7 (unneutralized HCL makes the solution acidic)

Scenario 3: Weak Acid and Strong Base (pH > 7 equivalence)

1. weak acid will have a higher pH at y-intercept (compared to strong acid)

2. pH rises slowly until equivalence (“buffer region”- both components of weak conjugate acid-base pair present)

3. pH rises sharply at equivalence (not as dramatic as scenario 1)

4. continues to rise (pH>7)

Scenario 4: Strong Acid and Weak Base (pH < 7 equivalence)

1. strong acid will have a lower pH

2. pH rises slowly until equivalence (“buffer region”)

3. pH rises sharply at equivalence (not as dramatic)

4. continues to rise but flattens at a fairly low pH

Scenario 5: Weak Acid and Weak Base (pH undefined)

1. initial pH fairly high (if acid)

2. pH rises slowly until equivalence (“buffer region”)

3. change in pH at equivalence is not sharp

4. continues to rise but plateaus at low pH

Acid-Base Indicators

• weak acids that show one colour as an acid and another colour as their conjugate base form.

• We can make the equivalence or endpoint visible by adding an indicator (change colour when pH= +/- 1 of the indicator’s pKa value)

• better if endpoint & equivalence point coincide with each other so must pick appropriate one

What are the Steps to Choose an Indicator?

1. Determine what combination of weak and strong acid are reacting together

2. Deduce the pH of salt solution at equivalence from the nature of the parent acid and base

3. Choose an indicator with an endpoint in the range of the equivalence point

What indicators should be used for a Strong Acid and a Strong Base?

Substance Type	Indicator	pKa	End-point range
Acid	Phenolphtalein	9.50	8.2-10.0
Base	Methyl Orange	3.46	3.2-4.4

What indicators should be used for a Weak Acid and a Strong Base?

Substance Type	Indicator	pKa	End-point range
Base	Phenolphtalein	9.50	8.2-10.0; colourless to pink
Base	Phenol Red	8.00	6.6-8.0; yellow → red

What indicators should be used for a Strong Acid and a Weak Base?

Substance Type	Indicator	pKa	End-point range
Base	Methyl Orange	3.46	8.2-10.0; red → yellow
Base	Bromophenol blue	4.10	3.0-4.6: yellow → blue

What indicators should be used for a Weak Acid and Base?

1. remember indicators indicate when pH = -/+ 1 of the indicator’s pK_a value

2. since weak acids and bases barely change pH, indicators would not pick up the changes