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
What is an acid-base indicator?
a weak acid that has a distinctly different colour from its conjugate base
Define end point
the point in a titration where the indicator changes colour; indicates when the reaction is just complete
What will an indicator contain?
equal concentrations of HA and A-
What will the colour of an indicator at end point be?
in between two extreme colours
What varies between different indicators?
different Ka values
change colour over different pH ranges
What 3 pairs of values are equal at end point?
[HA] = [A-] Ka = [H+] pKa = pH
In a titration, you must use an indicator that has…?
a colour change which coincides with the vertical section of the pH titration curve
ideally end point and equivalence point would coincide
Is it true that there is no indicator suitable for a weak acid - weak base titration? Why?
yes
there is no vertical section and pH requires several cm3 to pass through a typical pH indicator range (2 pH units)
Give 2 examples of indicators
phenolphthalein
methyl orange
