Buffers & Neutralisation
Why is there no indicator suitable for a weak acid weak base titration
because there is no vertical section and even at the steepest section, pH requires several cm3 to pass through a pH indicator range of 2 pH units.
how does the body prevent carbonic acid building up
body converts it to carbon dioxide gas, which is exhaled by the lungs.
What is this buffer controlled by
carbonic acid/ hydrogencarbonate buffer system.
When [HA] = [A-] (2)
(1) pH of buffer solution = pKa value of HA (2) the operating pH is typically over about 2 pH units, centred at the pH of the pKa value.
pH of the buffer solution depends upon...
(1) pKa value of the weak acid (2) ratio of the concs of weak acid and its conjugate base.
procedure for monitoring pH as an aqueous base is added to an acid solution?
(1) use pipette- add measured volume of acid to conical flask (2) place the electrode of the pH meter in the flask (3) add base to burette and add the base to the acid in the conical flask 1cm^3 at a time. (4) after each addition, swirl the contents, record the pH and the total vol of base added. (5) repeat steps 3-4 *until the pH starts to change more rapidly* then add the base dropwise for each reading until the pH changes less rapidly. (6) now can add base 1cm3 at a time again until an excess of the base has been added and the pH= basic with little change for several additions.
2 ways of preparing a buffer solution
(1) weak acid and an aq solution of one of its salts (2) excess weak acid and strong alkali
Acids & alkalis added to a buffer then...
- 2 components in the buffer solution react and will eventually be used up - as soon as 1 component has all reacted, the solution loses its buffering ability towards added acids/ alkalis - as the buffer works, pH changes but only by a small amount: don't assume that the pH stays constant.
Prep by strong alkali and excess acid
- add aq soln of an alkali to excess of the weak acid - weak acid partially neutralised by the strong alkali, forming its conjugate base - some of the weak acid is left over unreacted whereas all of the base has reacted. - resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid.
In the ethanoic acid example, CH3COOH and its base CH3COO- act as....
2 reservoirs to remove acid or alkali: act independently to remove any added acid and alkali which is achieved by shifting the buffer's eqm system either to the right or left.
Blood pH (healthy)?
7.40
Where is the eqm position in acidic or basic conditions
Acidic= towards weak acid Basic= towards base.
Examples of both
CH3COOH & CH3COONa CH3COOH & NaOH
Buffer equation for the carbonic acid/ hydrogencarbonate buffer system>
H2CO3 -----> H+ + HCO3-
Role of the weak acid & its conjugate base
Weak acid: removes added alkali Conjugate base: removes added acid.
How do you know which weak acid to use
a buffer is most effective at removing added acid or alkali when there are equal concentrations of the weak acid and its conjugate base.
Buffer solution
a system which minimises pH changes on addition of small amounts of acid or a base.
A buffer solution contains... (2)
a weak acid its conjugate base
What is an acid base indicator
a weak acid that has a distinctively different colour from its conjugate base.
Remember in calcs for partial neutralisation method:
acid is in excess, so work out the moles of acid remaining (ie moles of acid - moles of alkali added) worked example page 336
If pH falls below 7.35
acidosis= fatigue, shortness of breath, shock/ death.
if pH rises above 7.45
alkalosis- muscle spasms, nausea, light headaches.
pH meter consists of
an electrode that is dipped into a solution and connected to a meter that displays the pH reading (2dp_
Why does H+ not equal A in buffer solutions
because A- (aq) has been added as a component of the buffer
Why does a pH meter give more accurate measurements of pH during a titration?
because indicator paper is usually matched from colour charts to the nearest whole number whereas you get accuracy of 2dp with pH meter.
Equivalence point:
centre of vertical section the volume of one solution that reacts exactly with the volume of another solution. amounts match stoichiometry of the reaction.
Alternative automatic method
could attach the pH meter to a datalogger and use a magnetic stirrer in the flask. aq base would then be added from the burette to flask slowly and pH titration curve could be plotted automatically using datalogger or appropriate software on a computer.
Diff indicators have..
diff Ka values and change colour over diff pH ranges.
the pH titration curve will be a diff shape for...
diff combinations of acids and bases with different strengths
Diff parts of the body require...
diff pH values for effective functioning
Ideally...
end point & equivalence point would coincide but it may not be possible and the end point may give a slightly different volume from the equivalence point but v similar.
When CH3COO- ions added
eqm posn shifts to left, reducing the already small conc of H+ ions, leaves soln containing mainly the 2 components of acid and its base, so they both act as 2 reservoirs: act independently to remove any added acid and alkali which is achieved by shifting the buffer's eqm system either to the right or left.
Shape of the graph 3 sections
excess of acid/ base vertical section excess of acid/ base
Where is no indicator suitable
for a weak acid- weak base titration
What indicator must you choose?
has a colour change coinciding with the vertical section of the pH curve
What is the conjugate base
hydrogen carbonate ions.
What do acid base reactions use to monitor neutralisation reactions
indicators.
Body produces...
more acidic materials by alkaline= conjugate base converts to carbonic acid
What graph is then plotted
pH against total volume of aq base added.
example of a buffer solution in the body
plasma- pH 7.35-7.45 maintenance
Range over which most indicators change colour
range of about 2 pH units.
Diff weak acids...
result in buffer solutions that operate over different pH ranges
Enzymes are..
sensitive with each having an optimum pH
The position of the eqm in this example is...
shifted well towards the left (towards CH3COOH)
Sensitivity of an indicator depends on...
the indicator itself. eyesight
end point
the point in a titration when the indicator changes the colour: indicates when the reaction is complete.
What can then be adjusted to fine tune the pH of the buffer solution
the ratio of the concs of the weak acid and its conjugate base.
Explain how methyl orange works
the weak acid= red conjugate base= yellow at end point, equal concs of acid and conjugate base means the colour will be in between the 2 extreme colours. so end point colour is orange.
Percentage error????
theoretical value- experimental value divided by theoretical value x 100
Role of the conjugate acid base pair in a buffer solution
they control the pH of
What do the weak acid and its salt act as in the first buffer solution
weak acid: source of the weak acid component of the buffer solution salt: source of the conjugate base component of the buffer solution. (look at notes)
Diff in calculations using Ka expression for weak acids and buffers
weak acids: [H+] = [A-] buffers: [H+] DOES NOT EQUAL [A-]
