Buffers


A buffer is a solution that is highly resistant to
changes in pH when a strong acid or base is
added.
A buffer solution also has a pH close to the pKa
of the weak acid in the buffer system
Buffer Systems




Buffers are used to maintain a constant pH and
are widely used in commercial products and a
variety of lab procedures.
Buffers are made by mixing a weak acid with a
solution containing its conjugate base.
When a strong acid is added the H+ ions are
consumed by the conjugate base in the buffer.
When a strong base is added the OH- ions are
consumed by the conjugate acid in the buffer.
How It Works
Without A Buffer
A: A 100-mL sample of dilute HCl is adjusted to pH 5.00.
B: After the addition of 1 mL of 1 M HCl (left) or of 1 M NaOH (right), the pH
changes by several units.
With A Buffer
A: A 100-mL sample of a buffered solution, made by mixing 1 M CH3COOH
with 1 M CH3COONa, is adjusted to pH 5.00.
B: After the addition of 1 mL of 1 M HCl (left) or of 1 M NaOH (right), the pH
change is negligibly small.
Henderson-Hasselbalch

This equation is used to calculate the pH of a
buffer:

Or the following equation can be used to
calculate the [H+]:
[H+] = Ka x nHB
nB-
Calculating pH

Lactic acid, C2H5OCOOH, is a weak organic
acid present in both sour milk and buttermilk. A
buffer is prepared by dissolving 1.00 mol of
lactic acid (Ka = 1.4 x 10-4) and 1.00 mol of
sodium lactate in enough water to form 550mL
of solution. Calculate the pH of the buffer.

Ans: 3.85
Calculating pH

Calculate the pH:
(a) Of a buffer solution consisting of 0.50 M
CH3COOH and 0.50 M CH3COONa.
Ans: 4.74
(b) After adding 0.020 mol of solid NaOH to 1.0 L of
the buffer solution in part (a).
Ans: 4.77
(c) After adding 0.020 mol of HCl to 1.0 L of the
buffer solution in part (a).
Ans: 4.70
Buffer Capacity


It follows that a buffer has the highest capacity when
the component concentrations are equal, that is, when
[A−]/[HA] = 1:
For a given concentration, a buffer whose pH is
equal to or near the pKa of its acid component has the
highest buffer capacity:
Preparing a Buffer Solution

1. Choose the conjugate acid-base pair: First, decide on the
chemical composition of the buffer, that is, the
conjugate acid-base pair. This choice is determined to a
large extent by the desired pH. Remember that a buffer
is most effective when the ratio of its component
concentrations is close to 1, in which case the pH ≈
pKa of the acid.


Reference Table14.1 – page 382
*know the acetic acid and ammonia system vlaues
Preparing a Buffer (cont.)

2. Calculate the ratio of buffer component concentrations:
Next, find the ratio of [A−]/[HA] that gives the
desired pH.
Preparing a Buffer (cont.)

3. Determine the buffer concentration: Next, decide
how concentrated the buffer should be.
Remember that the higher the concentrations of
components, the greater the buffer capacity. For
most laboratory-scale applications,
concentrations of about 0.5 M are suitable, but
the decision is often based on availability of
stock solutions.
Preparing a Buffer (cont.)

4. Mix the solution and adjust the pH: Thoroughly dissolve
the calculated amount of salt containing the moles of
conjugate base desired in the desired concentration of
the weak acid to a final volume of 1 L. Finally, note that
because of the behavior of nonideal solutions, a buffer
prepared in this way may vary from the desired pH by a
few tenths of a pH unit. Therefore, after making up the
solution, adjust the buffer pH to the desired value by
adding strong acid or strong base, while monitoring the
solution with a pH meter.
Preparing a Buffer

Suppose you need to prepare a buffer with a pH of
9.00.

Which of the following buffer systems would you choose?
(hint: choose the acid with a pKa=pH you want)


What should be the ratio of the concentration of weak acid,
HB, to its conjugate base, B- ?


Ans: ammonium-ammonia system
Ans: 1.8
What mass in grams of B- should be added to 100.0 mL of
1.00 M HB to give a pH of 9.00?

Ans: 0.95 g NH3
Indicators in Acid-Base Titrations
Most have a range of about 2 pH units, in keeping with the useful buffer range of 2
pH units (pKa ± 1). (pH range depends to some extent on the solvent used to
prepare the indicator.)
The acidic form of bromthymol blue is yellow (left) and the basic form is blue
(right). Over the pH range in which the indicator is changing, both forms are
present, so the mixture appears greenish (center).
Strong Acid-Strong Base
Weak Acid-Strong Base
Weak Base-Strong Acid
FRQ Practice #1

Hypochlorous acid, HOCl, is a weak acid
commonly used as a bleaching agent. The acid–
dissociation constant, Ka, for the reaction
represented below is 3.2×10–8.
HOCl ↔ OCl– + H+

(a) Calculate the [H+] of a 0.14–molar solution
of HOCl.
FRQ Practice #1 (continued)



(b) Write the correctly balanced net ionic equation for
the reaction that occurs when NaOCl is dissolved in
water and calculate the numerical value of the
equilibrium constant for the reaction.
(c) Calculate the pH of a solution made by combining
40.0 milliliters of 0.14–molar HOCl and 10.0 milliliters
of 0.56–molar NaOH. And explain why Ms. Andre is
so cool.
(d) How many millimoles of solid NaOH must be
added to 50.0 milliliters of 0.20–molar HOCl to obtain
a buffer solution that has a pH of 7.49? Assume that
the addition of the solid NaOH results in a negligible
change in volume.
FRQ Practice #2


Methylamine, CH3NH2, is a weak base that
reacts according to the equation below. The
value of the ionization constant, Kb, is 5.2510–
4. Methylamine forms salts such as
methylammonium nitrate, (CH3NH3+)(NO3–).
CH3NH2 + H2O ↔ CH3NH3+ + OH–
(a) Calculate the hydroxide ion concentration,
[OH–], of a 0.225–molar aqueous solution of
methylamine.
FRQ Practice #2 (continued)



(b) Calculate the pH of a solution made by adding
0.0100 mole of solid methylammonium nitrate to 120.0
milliliters of a 0.225–molar solution of methylamine.
Assume no volume change occurs.
(c) How many moles of either NaOH or HCl (state
clearly which you choose) should be added to the
solution in (b) to produce a solution that has a pH of
11.00? Assume that no volume change occurs.
(d) A volume of 100. milliliters of distilled water is
added to the solution in (c). How is the pH of the
solution affected? Explain.