1. 11.9 Buffers
A buffer solution maintains the pH by neutralizing small amounts of added acid or base.
An acid must be present to react with any OH− added, and a base must be present to react with any H3O+ added.
Learning Goal Describe the role of buffers in maintaining
the pH of a solution; calculate the pH of a buffer.

2. Buffers
When an acid or a base is added to water, the pH changes drastically.
In a buffer solution, the pH is maintained; pH does not change when acids or bases are added.

3. How Buffers Work Buffers work because
they resist changes in pH from the addition of an acid or a base.
in the body, they absorb H3O+ or OH− from foods and cellular processes to maintain pH.
they are important in the proper functioning of cells and blood.
they maintain a pH close to 7.4 in blood.
A change in the pH of the blood affects the uptake of oxygen and cellular processes.

4. Components of a Buffer A buffer solution
contains a combination of acid–base conjugate pairs, a weak acid and a salt of its conjugate base, such as
HC2H3O2(aq) and C2H3O2−(aq)
has equal concentrations of a weak acid and its salt.

5. How Buffers Work
In the buffer with acetic acid (HC2H3O2) and sodium acetate (NaC2H3O2),
the salt produces acetate ions and sodium ions.
NaC2H3O2(aq) C2H3O2−(aq) + Na+(aq)
the salt is added to provide a higher concentration of the conjugate base C2H3O2− than from the weak acid alone.
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)
Large amount Large amount

6. Function of a Weak Acid in a Buffer
If a small amount of base is added to this same buffer solution, it is neutralized by the acetic acid, HC2H3O2, which shifts the equilibrium in the direction of the products acetate ion and water.
HC2H3O2(aq) + OH−(aq) C2H3O2−(aq) + H2O(l)
Equilibrium shifts in the direction of the products.

7. Function of Conjugate Base in a Buffer
When a small amount of acid is added, the additional H3O+ combines with the acetate ion, C2H3O2−, causing the equilibrium to shift in the direction of the reactants, acetic acid and water.
The acetic acid produced contributes to the available weak acid.
HC2H3O2(aq) + H2O(l) C2H3O2− (aq) + H3O+(aq) Equilibrium shifts in the direction of the reactants.

8. Working Buffers
The buffer described here consists of about equal concentrations of acetic acid (HC2H3O2) and its conjugate base, acetate ion (C2H3O2−).
Adding H3O+ to the buffer reacts with the salt, C2H3O2−, whereas adding OH− neutralizes the acid HC2H3O2.
The pH of the solution is maintained as long as the added amounts of acid or base are small compared to the concentrations of the buffer components.

9. Calculating the pH of a Buffer
By rearranging the Ka expression to give [H3O+], we can obtain the ratio of the acetic acid/acetate buffer and calculate the pH.
Solving for H3O+ gives
Weak acid
Conjugate base
Core Chemistry Skill Calculating the pH of a Buffer

10. Study Check
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)

11. Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)
STEP 1 State the given and needed quantities.
ANALYZE Given Need
THE [HC2H3O2] = 1.0 M pH of solution
PROBLEM [C2H3O2−] = 1.0 M
Equation
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)

12. Solution
The Ka for acetic acid, HC2H3O2, is 1.8 × 10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)
STEP 2 Write the Ka expression and rearrange for [H3O+].

13. Solution
The Ka for acetic acid, HC2H3O2, is 1.8  10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)
STEP 3 Substitute [HA] and [A−] into the Ka expression.

14. Solution
The Ka for acetic acid, HC2H3O2, is 1.8  10–5. What is the pH of a buffer prepared with 1.0 M HC2H3O2 and 1.0 M C2H3O2−?
HC2H3O2(aq) + H2O(l) C2H3O2−(aq) + H3O+(aq)
STEP 4 Use [H3O+] to calculate pH.

15. Calculating the pH of a Buffer
Because Ka is a constant at a given temperature,
the [H3O+] is determined by the [HC2H3O2]/[C2H3O2−] ratio.
the addition of small amounts of either acid or base changes the ratio of [HC2H3O2]/[C2H3O2−] only slightly.
the changes in [H3O+] will be small and the pH will be maintained.
the addition of a large amount of acid or base may exceed the buffering capacity of the system.

16. Buffers and pH Changes
Buffers can be prepared from conjugate acid–base pairs such as H2PO4−/HPO42− and HPO42−/PO43−, HCO3−/CO32−, or NH4+/NH3.
The pH of the buffer solution will depend on the conjugate acid–base pair chosen.

17. Buffers and pH Changes
Using a common phosphate buffer for biological specimens, we can look at the effect of using different ratios of [H2PO4−/HPO42−] on the [H3O+] and pH. The Ka of H2PO4− is 6.2 × 10−8.
The equation and the [H3O+] are written as follows:
H2PO4−(aq) + H2O(l) H3O+(aq) HPO42−(aq)

18. Buffers and pH Changes

19. Chemistry Link to Health: Buffers in Blood Plasma
The arterial blood plasma has a normal pH of 7.35 to If changes in H3O+ lower the pH below 6.8 or raise it above 8.0, cells cannot function properly and death may result.
In our cells, CO2
is continually produced as an end product of cellular metabolism.
is carried to the lungs for elimination, and the rest dissolves in body fluids such as plasma and saliva, forming carbonic acid, H2CO3.
As a weak acid, carbonic acid dissociates to give bicarbonate, HCO3−, and H3O+.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

20. Chemistry Link to Health: Buffers in Blood Plasma
Kidneys also supply more of the bicarbonate anion, HCO3−, setting up an important buffer system in the body fluid:
CO2(g) + H2O(l) H2CO3(aq) HCO3−(aq) + H3O+(aq)
Excess H3O+ entering the body fluids reacts with the HCO3−, and excess OH− reacts with the carbonic acid.
H2CO3(aq) + H2O(l) HCO3−(aq) + H3O+(aq)
Equilibrium shifts in the direction of the reactants.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

21. Chemistry Link to Health: Buffers in Blood Plasma
Kidneys also supply more of the bicarbonate anion, HCO3−, setting up an important buffer system in the body fluid:
CO2(g) + H2O(l) H2CO3(aq) HCO3−(aq) + H3O+(aq)
Excess H3O+ entering the body fluids reacts with the HCO3−:
H2CO3(aq) + H2O(l) HCO3−(aq) + H3O+(aq)
Equilibrium shifts in the direction of the reactants.
Excess OH– entering the body fluids reacts with the H2CO3:
H2CO3(aq) + OH−(aq) H2O(l) + HCO3−(aq)
Equilibrium shifts in the direction of the products.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

22. Chemistry Link to Health: Buffers in Blood Plasma
To maintain the normal blood plasma pH (7.35 to 7.45),
the ratio of [H2CO3]/[HCO3−] needs to be about 1 to 10.
concentrations of M H2CO3 and M HCO3− work to maintain that pH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

23. Chemistry Link to Health: Buffers in Blood Plasma
In the body, the concentration of carbonic acid is closely associated with the partial pressure of CO2, PCO2.
If the CO2 level rises, increasing H2CO3, the equilibrium shifts to produce more H3O+, which lowers the pH. This condition is called acidosis.
A lowering of the CO2 level leads to a high blood pH, a condition called alkalosis.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

24. Chemistry Link to Health: Buffers in Blood Plasma
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

25. Concept Map