L5-6 - pH Curves

objectives

Understandings:

• The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.

• An acid–base indicator is a weak acid or a weak base where the components of the conjugate acid–base pair have different colours.

• The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pKa value.

• The buffer region on the pH curve represents the region where small additions of acid or base result in little or no change in pH.

Applications and skills:

• The general shapes of graphs of pH against volume for titrations involving strong and weak acids and bases with an explanation of their important features.

• Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.

• While the nature of the acid–base buffer always remains the same, buffer solutions can be prepared by either mixing a weak acid/base with a solution of a salt containing its conjugate, or by partial neutralization of a weak acid/base with a strong acid/base.

• Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.

Guidance:

• Only examples involving the transfer of one proton will be assessed.

Important features are:

– intercept with pH axis

– equivalence point

– buffer region

– points where pKa = pH or pKb = pOH.

• For an indicator which is a weak acid: – HIn(aq) H+(aq) + In-(aq) Colour A Colour B – The colour change can be considered to take place over a range of pKa ± 1.

• For an indicator which is a weak base: B+(aq) + OH- – BOH(aq) (aq) Colour A Colour B

• Examples of indicators are listed in the data booklet in section 22.

• Salts formed from the four possible combinations of strong and weak acids and bases should be considered. Calculations are not required.

• The acidity of hydrated transition metal ions is covered in topic 13. The treatment of other hydrated metal ions is not required.

Introduction to Indicators

General: Many indicators are weak acids and partially dissociate in aqueous solution

HIn_(aq) ⇋ H⁺_(aq) + In¯_(aq)

• The un-ionised form (HIn) is a different colour to the anionic form (In¯).
• Apply Le Chatelier’s Principle to predict any colour change

In acid - increase of [H⁺]

- equilibrium moves to the left to give red undissociated form

In alkali - increase of [OH¯]

- OH¯ ions remove H⁺ ions to form water; H⁺(aq) + OH¯(aq) → H₂O(l)

- equilibrium will move to the right to produce a blue colour

Characteristics of Indicators

• Must have an easily observed colour change.
• Must change immediately in the required pH range over the addition of ‘half’ a drop of reagent.

Basic Indicators

• For an indicator which is a weak base:

B+(aq) + OH-(aq) –> BOH(aq)

Colour A Colour B

You can again apply Le Chatelier's Principle in order to gain a colour change

ph curves

Features:

• Show how pH changes when differing amounts acids and bases are added to each other
• Starts at the pH of the solution in a conical flask - This is dependent on what that solution is and how much of it is there
• Ends at the pH of the solution in the burette as it is added in excess
• The Graph should show an asymptotic curve with a large vertical section
• The vertical section is called the equivalence point and this is where the amounts acids and bases in the mixture are exactly equal - this is the area we need to use an indicator in.

Types:

There are four types of acid-base titration; each has a characteristic curve.

• strong acid (HCl) v. strong base (NaOH)
• weak acid (CH₃COOH) v. strong alkali (NaOH)
• strong acid (HCl) v. weak base (NH₃)
• weak acid (CH₃COOH) v. weak base (NH₃)

In the following examples, alkali (0.1M) is added to 25cm³ of acid (0.1M)

End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

Strong Acid vs Strong Base Titration pH Curve

Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion

Strong Acid vs Weak Base Titration pH Curve

Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

Weak Acid vs Strong Base Titration pH Curve

Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion

Weak Acid vs Weak Base Titration pH Curve

There is no suitable indicator- none change in the ‘vertical’ portion.

The end point can be detected by plotting a curve using a pH meter.