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C21 - Buffers and Neutralisation - Coggle Diagram
C21 - Buffers and Neutralisation
Buffer Solutions
Action of the Buffer Solution
The control of pH can be explained in terms of shifts in the Eq position using le Chatelier's principle
Conjugate base removes added acid
[H+] increases
H+ ions react with the conjugate base A-
The Eq position shifts to the left, removing most of the H+ ions
The conjugate acid-base pair, HA/A-, in an acid buffer solution controls the pH
HA >< H+ + A-
Weak acid removes added alkali
[OH-] increases
The small concentration of H+ ions react with the OH- ions to make H2O
HA dissociates, shifting the Eq position to the right to restore most of H+ ions
Choosing the Components for a Buffer Solution
A buffer is most effective at removing either added acid or alkali when there are equal concentrations of the weak acid and its conjugate base
When [HA] = [A-]
The pH of the buffer solution is the same as the pKa value of HA
The operating pH is typically over about two pH units, centred at the pH of the pKa value
Different weak acids result in buffer solutions that operate over different pH ranges
The ratio of the concentrations of the weak acid and its conjugate base can then be adjusted to finetune the pH of the buffer solution
Preparing Weak Acid Buffer Solutions
Preparation by partial neutralisation of the weak acid
The weak acid is partially neutralised by the alkali, forming the conjugate base
Some of the weak acid is left over unreacted
A buffer solution can also be prepared by adding an aqueous solution of an alkali, such as NaOH, to an excess of the weak acid
The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid
Preparation from a weak acid and its salt
Salts of weak acids are ionic compounds and provide a convenient source of the conjugate base
When added to water, the salt completely dissolves
Ethanoic acid is the source of the weak acid component of the buffer solution
Dissociation into ions is complete and so the salt is the source of the conjugate base component of the bufffer solution
When ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small
A buffer solution can be prepared by mixing a solution of ethanoic acid with a solution of one of its salts such as sodium ethanoate
A buffer solution based on a weak acid needs a weak acid and its conjugate base
Two reservoirs to remove added acid and alkali
In the ethanoic acid equilibrium, the Eq position lies well towards the ethanoic acid
When CH3COO- ions are added to CH3COOH, the Eq position shifts even further to the left, reducing the already small conc. of H+ ions and leaving a solution containing mainly the two components, CH3COOH and CH3COO-
CH3COOH >< H+ + CH3COO-
CH3COOH and CH3COO- act as two reservoirs that are able to act independently to remove added acid and alkali
This is achieved by shifting the buffer's Eq system either to the right or the left
Calculating the pH of a Buffer Solution
To work out the pH of a buffer solution, rearrange the Ka expression
[H+] = Ka x [HA]/[A-]
When the concentration of Ha and A- are the same
[HA] = [A-]
Ka = [H+] and pKa = pH
When you calculate the pH of a weak acid, you assume that [H+] = [A-] but for a buffer solution, this isn't true as A- has been added as one of the components of the buffer
The Eq and Ka expression for a weak acid is
HA >< H+ + A-
Ka = [H+][A-]/[HA]
What is a Buffer Solution
When alkalis and acids are added to a buffer, the two components in the buffer solution react and will eventually be used up
As soon as one component has all reacted, the solution loses its buffer ability towards added acids or alkalis
Buffer solutions contain 2 components to remove added acid or alkali
Weak acid to remove added alkali
Conjugate base to remove added acid
As the buffer works, the pH does change but only by a small amount - should not assume that the pH stays completely constant
A buffer solution is a system that minimises pH changes when small amounts of an acid or a base are added
Buffer Solutions in the Body
Neutralisation
pH Titration Curves
Using a pH meter
Add the aqueous base to the burette and add to the acid in the conical flask, 1cm^3 at a time
After each addition, swirl the contents and record the pH and the total volume of aqueous base added
Place the electrode of the pH meter in the flask
Repeat until the pH starts to change more rapidly, then add the aqueous base dropwise for each reading until the pH changes less rapidly
Using a pipette, add a measured volume of acid to a conical flask
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
The pH titration curve
After the vertical section, the pH will rise very slightly as the base is now in great excess
The equivalence point of the titration is the volume of one solution that exactly reacts with the volume of the other solution
Only drops of solution will be needed for the whole vertical section
The solutions have then exactly reacted with one another and the amounts used matching the stoichiometry of the reaction
Eventually, the pH increases rapidly during addition of a very small volume of base, producing the vertical section
As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly
When the base is first added, the acid is in great excess and the pH increases very slightly
Acid-Base Indicators