21) Buffers and neutralisation Flashcards
Define buffer solution
a system that minimises pH changes on addition of small amounts of an acid or base
What are the 2 key components of a buffer solution and their roles?
weak acid HA - removes added alkali
conjugate base A- - removes added acid
When is a buffer solution’s buffeting ability lost?
as soon as 1 component is all used up
Give 2 ways of preparing a buffer solution
- a weak acid and a solution of its salt (salts of weak acids are ionic compounds which when added to water completely dissolve to provide the conjugate base)
- partial neutralisation of the weak acid (an aqueous solution of an alkali is added to an excess of weak acid, forming the conjugate base - some weak acid is unreacted and left over)
Describe the action of a buffer solution on addition of acid, H+(aq)
[H+] increases
H+ react with conjugate base A-
position of equilibrium shifts to the LHS, removing H+ ions
HA(aq) <=> H+(aq) + A-(aq)
Describe the action of a buffer solution on addition of alkali, OH-(aq)
[OH-] increases
small conc. of H+ reacts with OH- forming water
HA dissociates, position of equilibrium shifts to the RHS to restore H+ ions
HA(aq) <=> H+(aq) + A-(aq)
When is a buffer most effective at removing either acid or alkali?
when there are equal concentrations of the weak acid and its conjugate base
What is significant about when [HA] = [A-] for a buffer solution?
Ka = [H+] pKa = pH
What is the typical operating pH range of a buffer?
2 pH units centred at the pH of the pKa value
What does the pH of a buffer solution dependent upon?
pKa value of weak acid
ratio of [H+] and [A-]
Give the key equation for the pH of a buffer solution
[H+] = Ka x ( [HA] / [A-] )
For a buffer solution why is [H+] not equal to [A-]
because A-(aq) has been added as one of the components
What is the pH range blood plasma needs to maintained at?
7.35 - 7.45
What happens if the pH of blood plasma < 7.35?
acidosis
fatigue, shortness of breath, shock or death
What happens if the pH of blood plasma > 7.45?
alkalosis
muscle spasms, light-headedness, nausea
How is blood plasma pH controlled?
carbonic acid - hydrogencarbonate (H2CO3 / HCO3 -) buffer system
The body produces far more _ (acidic or alkaline?) products / materials which the _ converts to the _?
acidic
conjugate base HCO3 -
weak acid H2CO3
How does the body prevent the build-up of H2CO3?
by converting it to CO2 which is then exhaled by the lungs
What is a pH meter?
an electrode that is dipped into a solution and connected to a meter which displays the pH reading (typically 2dp - more accurate than indicator paper)
Give a 6 step method for using a pH meter and an alternative
- add a measured volume of acid to conical flask with a pipette
- place electrode in flask
- add base (aq) to burette and add to conical flask 1cm3 at a time, after each addition swirl contents and record both pH and total volume added
- when pH starts to change more rapidly, add base dropwise for each reading until pH changes less rapidly
- add base 1cm3 at a time again until an excess has been added and the pH has been basic, with little change for several additions
- plot a graph of pH against total vol. of base added
alternative: attach pH meter to data logger and use a magnetic stirrer in flask - the base would be added slowly and the pH titration curve could be plotted automatically
Give and describe the 3 key stages of a pH titration curve
excess of acid - pH increases slowly as basic sol. is added
vertical section - pH increases rapidly on addition of a very small amount of base
excess of base - pH increases slowly
What are the axis for a pH titration curve?
x axis = volume of base added
y axis = pH
Define equivalence point
the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
found halfway up the vertical section
When adding an acid to a base - how is the pH titration curve affected?
the curve is flipped - goes from a high pH to a low pH
