L30 - Acids and Bases

Definition of buffer solutions Finding the pH of a buffer Preparation of a buffer Example of natural a buffer

Buffer solutions

Solutions that resist changes in pH.

Acid-base dissociations

Governed by the equilibrium constant, denoted by the subscript 'a' for acid dissociation.

Autoionization of Water

An equilibrium in water where water reacts with itself, donating a proton to produce H3O+ and OH- with a special symbol Kw.

Ka times Kb

The equilibrium constant for the acid multiplied by the equilibrium constant for its conjugate base, and it's always equal to Kw.

Kw value

A constant in water, equal to 10 to the minus 14 at 25 degrees Celsius.

Relationship between pKa and pKb

pKa plus pKb will always be equal to 14 at 25 degrees Celsius.

Calculating pH

Use the ICE method to determine H3O+ or OH- concentration at equilibrium, and from that, calculate pH.

Conjugate base

The molecule minus a proton that can react with water.

Buffer solution

A solution that contains a significant amount of a weak acid and a significant amount of its conjugate base.

Buffers

Living organisms are full of these, as biological chemistry only works within a narrow pH range.

Buffered solutions

They contain a significant amount of both the weak acid and its conjugate base.

Components of Buffer solutions

Significant amount of both the weak acid and its conjugate base to absorb any H3O+ that's added or any OH- that's added up to some limit.

Solutions that resist changes in pH

A weak acid and its conjugate base

pH of Blood

Around 7.4 and cannot change by more than about 0.2 units to stay healthy.

Salt of the Acid

Consists of the conjugate base plus a sodium ion.

Making a buffer

Putting some acetic acid in solution and putting some sodium acetate in solution.

Making the salt of that weak acid

Reacting a weak acid with a strong base.

Salt of a Weak Acid or Base

A salt of a weak acid will be equivalent to adding the conjugate base of that acid. A salt of a weak base is equivalent to adding the conjugate acid of that base.

Components Needed to Resist Change in pH

You need both a significant amount of the acid and its conjugate base in order to resist changes in pH.

Adding Components of Buffer Solution

Add both the weak acid and its salt, which gives extra conjugate base.

Equal amounts of acid and conjugate base

pH equals pKa.

ICE Table Method

Used to work out the final pH after adding strong acid.

Designing Buffers

Have roughly equal amounts of the weak acid and its conjugate base in order to resist changes in pH.

Henderson-Hasselbalch equation

Helps when designing buffers to vary the ratio of the weak acid and its conjugate base to adjust the pH of that buffer solution to the value desired.

Relationship between pH and pKa

pH of your buffer solution will be equal to the pKa plus the log of this ratio.

Capacity of the Solution

Eventually reached in the start video where the buffer was able to absorb the acid or base quite a lot of it, but eventually the pH did start to change even in the buffer solution.

Initial Concentrations of Acid and Base

Determines how much a buffer can resist changes in pH; higher concentrations increase capacity.

Designing a Buffer

Want to look for an acid base combination where the pKa of the acid is close to the pH that we want at the end.