Buffers & Buffer Capacity

Brown & LeMay17.2

Pages 664 - 671

Buffer Solutions (or Buffers)

• Can resist drastic changes in pH upon the addition of small amounts of strong acid or strong base

• Interesting example is Human Blood– Contains both

• An acidic species to neutralize OH-

• A basic species to neutralize H+

– The acidic and basic species must not consume one another.

How do you make a buffer solution?

• Buffers are usually composed of a weak acid-base conjugate pair.

• Example might be

HC2H3O2 / C2H3O2- made by adding

HC2H3O2 with NaC2H3O2 saltorNH4

+ / NH3 made by adding NH3 and NH4Cl salt

How do buffer solutions work?• Consider

HX (aq) H+ (aq) + X- (aq) • Ka = [H+] [X-] [HX]Rearranged to [H+] = Ka [HX] [X-]As long as the amounts of HX and X- in the buffer

are large compared to the amount of OH- added, the ratio of [HX] / [X-] doesn’t change much.

Therefore, [H+] and pH change is small.

Consider HCN and NaCN

• HCN = acidic component• CN - = basic component

• When acid is added, then • H + + CN- HCN• When base is added, then• OH - + HCN H2O + CN-

Consider NH3 Mixed With NH4Cl

• A. NH4+ = acidic component;

• NH3 = basic component

• When acid is added, then• H + + NH3 NH4

+

• When base is added, then• OH - + NH4

+ H2O + NH3

Buffers are most effective when the following two conditions are met:

• [weak acid] and [conj. base] are about equal

• Try to select a buffer whose acid form has a pKa close to the desired pH.

• Calculate the pH of a buffer prepared by dissolving 0.25 mol of acetic acid and 0.40 mole of sodium acetate in water and diluting to one liter.

• Use the ICE approach, where initial conc of HC2H3O2 and C2H3O2

- are given.

• HC2H3O2 (aq) H+(aq) + C2H3O2- (aq)

• I 0.25 0 0.40• C -x +x +x• E 0.25 –x x 0.40 + x

Ka= [H+] [C2H3O2-] [H+] = Ka [HC2H3O2]

[HC2H3O2] [C2H3O2] =1.8 x10-5 (0.40 / 0.25) = 1.1 x 10-5

Check to see if assumption is valid.pH = - log(1.1 x 10-5) = 4.96

Buffer solutions (a) When OH-is added to a buffer solution, some of the weak acid is neutralized and thus converted to the conjugate base. (b) When H3O+is added to a buffer solution, some of the conjugate base is neutralized and thus converted to the weak acid. However, as long as the concentration ratio [weak acid]/[conjugate base] stays close to its original value, [H3O+] and the pH won’t change very much.

Buffer Capacity and pH

• The amount of acid or base the buffer can neutralize before the pH begins to change appreciably.– Depends on the amount of acid & base from

which it was made.

• Henderson-Hasselbalch Equation• pH = pKa + log [base]

[acid]

• What is the pH of a buffer that is 0.12 M in lactic acid and 0.10 M in sodium lactate?

• Ka for lactic acid = 1.4 x 10-4

• Can use the Henderson-Hasselbalch equation to calculate pH directly:

• pH = pKa + log [base]

[acid]= -log(1.4 x10-4) + log (0.10 / 0.12)= 3.85 + (-0.08) = 3.77

• Calculate the pH of a buffer composed of 0.12 M benzoic acid and 0.20 M sodium benzoate. Ka for benzoic acid = 6.3 x 10-5

• pH = -log (6.3 x 10-5) +log (0.20 / 0.12)• = 4.20 + 0.22 = 4.42• Please read Page 669 – Chemistry

and Life: Blood as a Buffered Solution

Biological Acids And Their Salts that make Biological Buffers

• Citric acid and citrate• Fumaric acid and fumarate• Pyruvic acid and pyruvate• Lactic acid and lactate• Glutamic acid and glutamate