Chapter 21 - Buffers and Neutralisation Flashcards
What is a buffer solution?
A buffer solution is a solution that resists and minimises changes in pH when small amounts of acid or base are added.
What components do buffer solutions contain?
a. A Buffer solution contains two components to remove added acid or alkali – a weak acid (component 1) and its conjugate base (component 2).
i. The weak acid, HA, removes added alkali.
ii. The conjugate base, A-, removes added acid.
How does a buffer work?
a. When alkalis and acids are added to a buffer, the two components in the buffer solution will react and will eventually be used up. As soon as one component has all reacted, the solution loses its buffering ability towards added acid or alkalis.
b. As the buffer works, the pH does change but only by a small amount – you should not assume that the pH stays completely constant.
What are the two ways you can prepare a weak acid buffer solution?
a. Preparation from a weak acid and its salt.
i. Mix a solution of weak acid with a solution of one of its salts.
ii. When weak acid is added to water, it partially dissociates, and the amount of ethanoate ions in the solution is very small. Therefore, the weak acid is the source of the weak acid component in the buffer solution.
iii. Salts of weak acids are ionic compounds and when added to water, the salt completely dissolves. Dissociation into ions is complete and so the salt is the source of the conjugate base component of the buffer solution.
b. Preparation by partial neutralisation of the weak acid
i. Add an aqueous solution of an alkali, such as NaOH, to an excess of weak acid.
ii. The weak acid is partially neutralised by the alkali, forming the conjugate base. Some of the weak acid is left over unreacted as it is in excess. Therefore, the resulting solution contains a mixture of the salt of the weak acid (component 2) and any unreacted weak acid (component 1).
How does conjugate base remove any added acid?
a. On addition of an acid, H+(aq):
1. [H+] increases.
- H+ ions react with the conjugate base, A-.
- The equilibrium position shifts to the left, removing most of the H+ ions.
How does weak acid remove any added alkali?
a. On addition of an alkali, OH-(aq):
1. [OH-] increases.
- The small concentration of H+ ions reacts with the OH- ions:
a. H+ + OH- = H20 - HA dissociates, shifting the equilibrium position to the right to restore most of the H+ ions.
When is a buffer the most effective at removing either added acid or alkali?
a. When there are equal concentrations of the weak acid and its conjugate base (When [HA] = [A-]):
i. The pH of the buffer solution is the same as the pKa value of HA.
ii. The operating pH is typically over about two pH units, centred at the pH of the pKa value.
b. So, if the concentrations of HA and A- in a buffer are equal then the pH of the buffer is equal to the pKa value of the weak acid.
At what pH does blood plasma need to be maintained at and what is the most important buffer system used to control the pH.
a. Blood plasma needs to be maintained at a pH between 7.35 and 7.45.
b. The pH is controlled by a mixture of buffers, with the carbonic acid-hydrogen carbonate (H2CO3/HCO3-) buffer system being the most important. Normal healthy blood should have a pH of 7.40.
What happens if the pH slips outside this range?
a. If the pH falls below 7.35, people can develop a condition called acidosis, which can cause fatigue, shortness of breath, and in extreme cases, shock or death.
b. If the pH rises above 7.45, the condition is called alkalosis, which can cause muscle spasms, light-headedness, and nausea.
c. The body produces far more acidic materials than alkaline, which the conjugate base HCO3- converts to H2CO3. The body prevents H2CO3 building up by converting it to carbon dioxide gas, which is then exhaled by the lungs.
What is the Henderson-Hasselbalch equation?
pH = pKa + log(A-/HA)
How would you monitor the pH as an aqueous base is added to an acid solution?
a. Using a pipette, add a measured volume of acid to a conical flask.
b. Place the electrode of the pH meter in the flask.
c. Add the aqueous base to the burette and add to the acid in the conical flask, 1cm^3 at a time.
d. After each addition, swirl the contents. Record the pH and the total volume of the aqueous base added.
e. Repeat steps c and d until the pH starts to change more rapidly. Then 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.
What is an alternative method you could use to monitor the pH as an aqueous base is added to an acid solution?
You could attach the pH meter to a datalogger and use a magnetic stirrer in the flask. The aqueous base would then be added from the burette to the flask slowly, and the pH titration curve could be plotted automatically using the datalogger or an appropriate software on a computer.
Describe pH titration curves.
a. When the base is first added, the acid is in great excess and the pH increases very slightly. As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly.
b. Eventually, the pH increases rapidly during addition of a very small volume of base, producing the vertical section. Only drops of solution will be needed for the whole vertical section.
c. After the vertical section, the pH will rise very slightly as the base is now in great excess.
d. The equivalent point is the centre of the vertical section of the pH titration curve.
What is the equivalence point?
a. The equivalence point of the titration is the volume of one solution that exactly reacts with the volume of the other solution.
b. The solutions have then exactly reacted with one another and the amounts used to match the stoichiometry of the reaction.
What is an acid-base indicator?
a. An acid-base indicator is a weak acid, HA, that has a distinctively different colour from its conjugate base, A-, for example, for the common indicator methyl orange: the weak acid is red, and the conjugate base is yellow.
b. At the end point of a titration, the indicator contains equal concentrations of HA and A- and the colour will be in between the two extreme colours. For methyl orange, the colour at its end point is orange.
