Buffers

What is a buffer solution?

resists changes in pH (keep the pH almost constant) when small amounts of acids or alkalis are added

A buffer can consists of ____

weak acid - conjugate base

or

weak base - conjugate acid

Ethanoic acid & sodium ethanoate as a buffer

• Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions

  

• Sodium ethanoate is a salt which fully ionises in solution

• There are reserve supplies of the acid (CH₃COOH) and its conjugate base (CH₃COO^-)

  • The buffer solution contains relatively high concentrations of CH₃COOH (due to partial ionisation of ethanoic acid) and CH₃COO^- (due to full ionisation of sodium ethanoate)

• In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions

When H⁺ ions are added:

• The equilibrium position shifts to the left as H⁺ ions react with CH₃COO^- ions to form more CH₃COOH until equilibrium is re-established

• As there is a large reserve supply of CH₃COO^- the concentration of CH₃COO^- in solution doesn’t change much as it reacts with the added H⁺ ions

• As there is a large reserve supply of CH₃COOH the concentration of CH₃COOH in solution doesn’t change much as CH₃COOH is formed from the reaction of CH₃COO^- with H⁺

• As a result, the pH remains reasonably constant

When OH^- ions are added:

• The OH^- reacts with H⁺ to form water

OH^- (aq) + H⁺  (aq) → H₂O (l)

• The H⁺ concentration decreases

  • The equilibrium position shifts to the right and more CH₃COOH molecules ionise to form more H⁺and CH₃COO^- until equilibrium is re-established

CH₃COOH (aq) → H⁺ (aq) + CH₃COO^- (aq)

• As there is a large reserve supply of CH₃COOH the concentration of CH₃COOH in solution doesn’t change much when CH₃COOH dissociates to form more H⁺ ions

  • As there is a large reserve supply of CH₃COO^- the concentration of CH₃COO^- in solution doesn’t change much

  • As a result, the pH remains reasonably constant

    

The pH of a buffer solution can be calculated using:

• The K_a of the weak acid

• The equilibrium concentration of the weak acid and its conjugate base (salt)

• To determine the pH, the concentration of hydrogen ions is needed which can be found using the equilibrium expression

• Since -log₁₀ [H⁺] = pH, the expression can also be rewritten as:

Controlling the pH of blood

• In humans, HCO₃^- ions act as a buffer to keep the blood pH between 7.35 and 7.45

• Body cells produce CO₂ during aerobic respiration

• This CO₂ will combine with water in blood to form a solution containing H⁺ ions

CO₂ (g) + H₂O (l) ⇌ H⁺ (aq) + HCO₃^- (aq)

• This equilibrium between CO₂ and HCO₃^- is extremely important

• If the concentration of H⁺ ions is not regulated, the blood pH would drop and cause ‘acidosis’

  • Acidosis refers to a condition in which there is too much acid in the body fluids such as blood

  • This could cause body malfunctioning and eventually lead to coma

If there is an increase in H⁺ ions

• The equilibrium position shifts to the left until equilibrium is restored

CO₂ (g) + H₂O (l) ⇌ H⁺ (aq) + HCO₃^- (aq)

• This reduces the concentration of H⁺ and keeps the pH of the blood constant

• If there is a decrease in H⁺ ions

  • The equilibrium position shifts to the right until equilibrium is restored

CO₂ (g) + H₂O (l) ⇌ H⁺ (aq) + HCO₃^- (aq)

• This increases the concentration of H⁺ and keeps the pH of the blood constant

what is a buffer a mix of?

weak acid (HA) and one its salts (A-)

what the two ways of making buffers?

ethanoic acid

soidum ethanoate / NaOH

RULE FOR BUFFERS

(H+)≄ (A-)

EQUATION for weak acid + conjugate base:

CH3COOH → ← CH3COO- + H+

CH3COONa —> CH3COO- + NA+

equation of weak acid + strong base:

CH3COOH + NaOH —> CH3COO-NA+ + H20

use ice

buffers work best when

PHs equal to value of weak acids pKa. This is when fraction of weak acids conc divided by salt conc is 1

pH titration curve:

1. Calibrate pH metre

2. Measure acid: stir acid with magnetic stirrer

3. Fill burette with base and record inital

4. Add base slowly 1cm of base to acid and record new Ph

5. Continue adding base

6. Add base until pH stops changing

7. Plot curve of Ph against volume of base added

From the curves you can:

• Determine the pH of the acid by looking where the curve starts on the y-axis

• Find the pH at the equivalence point

• Find volume of base at the equivalence point

• Obtain the range of pH at the vertical section of the curve

strong acid + strong base graph”

1. Initial pH change is small

2. pH rises faster as more base is added

3. sharp rise in pH at equivalence point (equivalence point)

4. solution is alkaline after steep rise (strong base)

strong acid - weak base:

1. intial pH rise is small

2. steep rise in pH during neutralise

3. equivalence below 7 pH

4. solution is alkaline after steep ris

5. steep tise im pH seen at end (weak base)

weak acid- strong base:

1. starting pH is higher

2. intially pH rises fast (creates buffer)

3. steep rise in pH during neutralisatoon

4. equivlaence point above pH 7

5. solution is alkaline after steep rise (strong base)

weak acid- weak base:

1. no clear equivalence point

how do indicator work?

weak acids and the undissociated and dissociated form in equilibrium as a conjugate acid base pair

two forms of indicator molecule are different colours or one is coloured and one is colourless

equation of indicator

HIn (aq)  + H₂O (l)  ⇌ H₃O⁺ (aq) + In^- (aq)

colour 1                                          colour 2

• HIn and its conjugate base In^- are different colours

• The colour of the solution depends on the relative concentrations of the two species

• If the solution is acidic, the above equilibrium will be shifted to the left and more HIn will be present

  • Colour 1 will thus dominate

• If the solution is alkaline, the above equilibrium will shift to the right and more In^- will be present

  • Colour 2 will thus dominate

• The colour of the indicator thus depends on the pH of the solution

• The colour will not change suddenly at a certain pH, but will change gradually over a pH range

• The colour of the indicator depends on the ratio of [HIn] to [In^-]

K_a  of the indicator

endpoint of the reaction is where there is a balance between [HIn] and [In^-]. At this point these two concentrations are equal

Taking negative logs of both sides:

 pK_a  = pH

Choosing a suitable indicator

• Around the equivalence point of a titration, the pH changes very rapidly

• Indicators change colour over a narrow pH range approximately centred around the pK_a of the indicator

• An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration

Strong acid-strong base

• In strong acid - strong base titrations, the pH changes from 4 to 10 at the end-point so a suitable indicator must change colour within this range

  • Methyl red and phenolphthalein are suitable indicators for these titrations

  • Methyl orange is not ideal but it shows a significant enough colour change at the end point so is widely used

Weak acid-strong base

• In weak acid - strong base titrations, the pH changes from 7 to 10 at the end-point so a suitable indicator must change colour within this range

  • Phenolphthalein is the only suitable indicator for weak acid - strong base titrations that is widely available

Strong acid-weak base

• In strong acid - weak base titrations, the pH changes from 4 to 7 at the end-point so a suitable indicator must change colour within this range

  • Methyl red is the most suitable indicator for these titrations

  • However methyl orange is often used since it shows a significant enough colour change at the end-point and is more widely available than methyl red

Weak acid-weak base

• In weak acid -weak alkali titrations, there is no sudden pH change at the end-point and thus there are no suitable indicators for these titrations

• The end-points of these titrations cannot be easily determined

phenolphthalein

pH at equivalence: 7-11

weak acid

strong base

methyl orange

pH equivalence: 3-7

acid strong

weak base

end point (when indicator changes colour) occurs in middle when conc. of two species is equal