pH titration curves

Question 1

What does a pH meter consist of?

Answer 1

An electrode, that is dipped into a solution, and connected to a meter that displays the pH reading.

Question 2

How can more accurate pH measurements be recorded?

Answer 2

By using indicator paper that you can match to colour charts to the nearest whole number.

Question 3

What is the practical for using a pH meter when monitoring the pH as an aqueous base is added to an acid solution?

Answer 3

1. Using a pipette, add a measured volume of acid to a conical flask.
2. Place the electrode of the pH meter in the flask.
3. Add the aqueous base to the burette and add to the acid in the conical flask, 1cm^3 at a time.
4. After each addition, swirl the contents. Record the pH and the total volume of the aqueous base added.
5. Repeat steps 3 and 4 until the pH starts to change more rapidly. Then add the aqueous base dropwise for each reading until the pH changes less rapidly.
6. Now, add the aqueous base 1cm^3 at a time again until an excess has been added and the pH has been basic, with little change, for several additions.

Question 4

How do you describe a pH titration curve?

Answer 4

1. When the base is first added, the acid is in great excess and the pH increases very slightly (slight slope).
2. As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly.
3. Eventually, the pH increases rapidly during addition of a very small volume of base, producing the vertical section. Only drops of solution will be needed for the whole vertical section.
4. After the vertical section, the pH will rise very slightly as the base is now in great excess (slight increase).

Question 5

What is the equivalence point of a titration?

Answer 5

The volume of one solution that exactly reacts with the volume of the other solution. The solutions have then exactly reacted with one another and the amounts used matching the stoichiometry of the reaction.

Question 6

What does the equivalence point look like on a pH titration curve?

Answer 6

The centre of the vertical section.

Question 7

How can a pH titration curve differ when adding acid to a base instead of adding base to an acid?

Answer 7

The shape is the same but the other way around, going from high pH to a low pH.

Question 8

What is an acid-base indicator?

Answer 8

A weak acid (HA) that has a distinctively different colour from its conjugate base (A-).

Question 9

How is methyl orange an example of an acid-base indicator? End point?

Answer 9

• The weak acid (HA) is red.
• The conjugate base (A-) is yellow.
• At the end point of a titration, the indicator contains equal concentrations of HA and A- and the colour will be in between = orange.

Question 10

What is an indicator?

Answer 10

A weak acid.

Question 11

What happens to the equilibrium position in the weak acid in acidic conditions?

Answer 11

It shifts towards the weak acid (HA) - changing the colour as it does so.

Question 12

What happens to the equilibrium in the weak acid in basic conditions?

Answer 12

It shifts towards the conjugate base (A-) - changing the colour as it does so.

Question 13

What indicator is usually used in a titration where a strong base is added to a strong acid? What happens?

Answer 13

• Methyl orange.
• The methyl orange is initially red as the presence of H+ ions forces the equilibrium position well to the left.

Question 14

What happens to the colour change of an indicator on addition of a basic solution containing OH- ions?

Answer 14

• OH- ions react with H+ in the indicator:
  H+ + OH- = H2O.
• The weak acid, HA, dissociates, shifting the equilibrium position to the right.
• The colour changes, first to orange at the end point and finally to yellow as the equilibrium position is shifted to the right.
  HA = A- + H+.
  —————->
  equilibrium

Question 15

What happens to the colour change of an indicator if methyl orange is added initially to a basic solution and acid is added?

Answer 15

• H+ ions react with the conjugate base (A-).
• The equilibrium position shifts to the left.
• The colour changes, first to orange at the end point and finally to red when the equilibrium position has shifted to the left.
  HA = A- + H+
  <—————
  equilibrium

Question 16

What is the pH value of the end point the same as?

Answer 16

• The pKa value of the weak acid (HA).
• Ka = [H+].

Question 17

What does the sensitivity of an indicator depend upon?

Answer 17

The indicator itself and eyesight; most indicators change colour over a range of about 2 pH units.

Question 18

In a titration, how do we know what indicator to use?

Answer 18

You use the indicator that has a colour change which coincides with the vertical section of the pH titration curve. Ideally the end point and the equivalence point would coincide (this may not be possible and the end point may give a volume that is slightly different from the equivalence point, but will be a very small difference).

Question 19

What combination of acid and base is suitable for an indicator? Why?

Answer 19

Weak acid-weak base: there is no vertical section, and even at its steepest, the pH requires several cm^3 to pass through a typical pH indicator range of 2 pH units.

Question 20

What does the pH titration curve look like for a strong acid (HCl) and a strong base (NaOH)? What indicators are suitable?

Answer 20

• Steep/slow increase to begin with and then straight and fast vertical increase at the equivalence point, then levels off from the vertical point.
• Methyl orange and phenolpthalein are both suitable because the colour changes line up on the equivalence point.

Question 21

What does the pH titration curve look like for a weak acid (CH3COOH) and a strong base (NaOH)? What indicators are suitable?

Answer 21

• Less steep increase than strong base to strong acid to begin with and then straight and fast vertical increase at the equivalence point which is shorter than strong acid and strong base, then levels off from the vertical point (longer than strong acid and strong base).
• Methyl orange is not suitable because equivalence point is not in the orange colour change.
• Phenolpthalein is suitable because equivalence point is in the purple colour change.

Question 22

What does the pH titration curve look like for a strong acid (HCl) and a weak base (NH3)? What indicators are suitable?

Answer 22

• Steep increase similar to the strong base to strong acid to begin with and then straight and fast vertical increase at the equivalence point which is shorter than strong acid and strong base, then levels off from the vertical point (longer than strong acid and strong base).
• Methyl orange is suitable because equivalence point is in the orange colour change.
• Phenolpthalein is not suitable because equivalence point is not in the purple colour change.

Question 23

What does the pH titration curve look like for a weak acid (CH3COOH) and a weak base (NH3)? What indicators are suitable?

Answer 23

• Less steep increase similar to the strong base to weak acid to begin with and then a slightly gradual vertical increase at the equivalence point which is shorter than strong acid and weak base, then levels off from the vertical point (longer than strong acid and weak base).
• Methyl orange is not suitable because equivalence point is not in the orange colour change.
• Phenolpthalein is not suitable because equivalence point is not in the purple colour change.

Question 24

What are the strong acids and bases, and the weak acids and bases we need to know for pH titration curves and the 2 acid-base indicators?

Answer 24

• Strong acid: HCl.
• Strong base: NaOH.
• Weak acid: CH3COOH.
• Weak base: NH3.
• Methyl orange: + orange.
• Phenolpthalein: + purple.