BuffersBuffers

A buffer is a solution that is highly A buffer is a solution that is highly resistant to changes in pH when a resistant to changes in pH when a strong acid or base is added.strong acid or base is added.

A buffer solution also has a pH close A buffer solution also has a pH close to the pKa of the weak acid in the to the pKa of the weak acid in the buffer systembuffer system

Buffer SystemsBuffer Systems Buffers are used to maintain a constant pH Buffers are used to maintain a constant pH

and are widely used in commercial products and are widely used in commercial products and a variety of lab procedures.and a variety of lab procedures.

Buffers are made by mixing a weak acid with Buffers are made by mixing a weak acid with a solution containing its conjugate base.a solution containing its conjugate base.

When a strong acid is added the H+ ions are When a strong acid is added the H+ ions are consumed by the conjugate base in the consumed by the conjugate base in the buffer.buffer.

When a strong base is added the OH- ions When a strong base is added the OH- ions are consumed by the conjugate acid in the are consumed by the conjugate acid in the buffer. buffer.

How It WorksHow It Works

Without A BufferWithout 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 BufferWith 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-HasselbalchHenderson-Hasselbalch

This equation is used to calculate This equation is used to calculate the pH of a buffer:the pH of a buffer:

Or the following equation can be Or the following equation can be used to calculate the [Hused to calculate the [H++]:]:

[H[H++] = K] = Kaa x x nnHBHB

nnB-B-

Calculating pHCalculating pH

Lactic acid, CLactic acid, C22HH55OCOOH, is a weak OCOOH, is a weak organic acid present in both sour organic acid present in both sour milk and buttermilk. A buffer is milk and buttermilk. A buffer is prepared by dissolving 1.00 mol of prepared by dissolving 1.00 mol of lactic acid (Klactic acid (Kaa = 1.4 x 10 = 1.4 x 10-4-4) and 1.00 ) and 1.00 mol of sodium lactate in enough mol of sodium lactate in enough water to form 550mL of solution. water to form 550mL of solution. Calculate the pH of the buffer.Calculate the pH of the buffer. Ans: 3.85Ans: 3.85

Calculating pHCalculating pH Calculate the pH:Calculate the pH:

(a)(a) Of a buffer solution consisting of 0.50 Of a buffer solution consisting of 0.50 MM CH3COOH and 0.50 CH3COOH and 0.50 MM CH3COONa. CH3COONa.

Ans: 4.74Ans: 4.74(b)(b) After adding 0.020 mol of solid NaOH After adding 0.020 mol of solid NaOH to 1.0 L of the buffer solution in part (a).to 1.0 L of the buffer solution in part (a).

Ans: 4.77Ans: 4.77(c)(c) After adding 0.020 mol of HCl to 1.0 After adding 0.020 mol of HCl to 1.0 L of the buffer solution in part (a).L of the buffer solution in part (a).

Ans: 4.70Ans: 4.70

Buffer CapacityBuffer Capacity It follows that It follows that a buffer has the a buffer has the

highest capacity when the highest capacity when the component concentrations are equalcomponent concentrations are equal, , that is, when [A−]/[HA] = 1:that is, when [A−]/[HA] = 1:

For a given concentration, For a given concentration, a buffer a buffer whose pH is equal to or near thewhose pH is equal to or near the p pKKa a of its acid component has the of its acid component has the highest buffer capacityhighest buffer capacity::

Preparing a Buffer Preparing a Buffer SolutionSolution

1. 1. Choose the conjugate acid-base pair:Choose the conjugate acid-base pair: First, First, decide on the chemical composition of the decide on the chemical composition of the buffer, that is, the conjugate acid-base pair. buffer, that is, the conjugate acid-base pair. This choice is determined to a large extent This choice is determined to a large extent by the desired pH. Remember that a buffer is by the desired pH. Remember that a buffer is most effective when the ratio of its most effective when the ratio of its component concentrations is close to 1, in component concentrations is close to 1, in which case the pH ≈ pwhich case the pH ≈ pKKa of the acid.a of the acid. Reference Table14.1 – page 382Reference Table14.1 – page 382 *know the acetic acid and ammonia system vlaues*know the acetic acid and ammonia system vlaues

Preparing a Buffer Preparing a Buffer (cont.)(cont.)

2. 2. Calculate the ratio of buffer Calculate the ratio of buffer component concentrationscomponent concentrations: Next, : Next, find the ratio of [A−]/[HA] that gives find the ratio of [A−]/[HA] that gives the desired pH. the desired pH.

Preparing a Buffer Preparing a Buffer (cont.)(cont.)

3. 3. Determine the buffer concentrationDetermine the buffer concentration: : Next, decide how concentrated the Next, decide how concentrated the buffer should be. Remember that the buffer should be. Remember that the higher the concentrations of higher the concentrations of components, the greater the buffer components, the greater the buffer capacity. For most laboratory-scale capacity. For most laboratory-scale applications, concentrations of about applications, concentrations of about 0.5 0.5 MM are suitable, but the decision is are suitable, but the decision is often based on availability of stock often based on availability of stock solutions.solutions.

Preparing a Buffer Preparing a Buffer (cont.)(cont.)

4. 4. Mix the solution and adjust the pH:Mix the solution and adjust the pH: Thoroughly dissolve the calculated amount of Thoroughly dissolve the calculated amount of salt containing the moles of conjugate base salt containing the moles of conjugate base desired in the desired concentration of the weak desired in the desired concentration of the weak acid to a final volume of 1 L. Finally, note that acid to a final volume of 1 L. Finally, note that because of the behavior of nonideal solutions, a because of the behavior of nonideal solutions, a buffer prepared in this way may vary from the buffer prepared in this way may vary from the desired pH by a few tenths of a pH unit. desired pH by a few tenths of a pH unit. Therefore, after making up the solution, Therefore, after making up the solution, adjust adjust the buffer pHthe buffer pH to the desired value by adding to the desired value by adding strong acid or strong base, while monitoring the strong acid or strong base, while monitoring the solution with a pH meter.solution with a pH meter.

Preparing a BufferPreparing a Buffer Suppose you need to prepare a buffer Suppose you need to prepare a buffer

with a pH of 9.00.with a pH of 9.00. Which of the following buffer systems would Which of the following buffer systems would

you choose? (hint: choose the acid with a you choose? (hint: choose the acid with a pKa=pH you want)pKa=pH you want)

Ans: ammonium-ammonia systemAns: ammonium-ammonia system What should be the ratio of the concentration What should be the ratio of the concentration

of weak acid, HB, to its conjugate base, B- ?of weak acid, HB, to its conjugate base, B- ? Ans: 1.8Ans: 1.8

What mass in grams of B- should be added to 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?100.0 mL of 1.00 M HB to give a pH of 9.00?

Ans: 0.95 g NH3Ans: 0.95 g NH3

Indicators in Acid-Base Indicators in Acid-Base TitrationsTitrations

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 BaseStrong Acid-Strong Base

Weak Acid-Strong BaseWeak Acid-Strong Base

Weak Base-Strong AcidWeak Base-Strong Acid

FRQ Practice #1FRQ Practice #1

Hypochlorous acid, HOCl, is a weak Hypochlorous acid, HOCl, is a weak acid commonly used as a bleaching acid commonly used as a bleaching agent. The acid–dissociation constant, agent. The acid–dissociation constant, KKaa, , for the reaction represented below for the reaction represented below is 3.2×10is 3.2×10–8–8. .

HOCl ↔ OClHOCl ↔ OCl–– + H + H++

(a)(a) Calculate the [HCalculate the [H++] of a 0.14–molar ] of a 0.14–molar solution of HOCl.solution of HOCl.

FRQ Practice #1 FRQ Practice #1 (continued)(continued)

(b)(b) Write the correctly balanced net ionic Write the correctly balanced net ionic equation for the reaction that occurs when NaOCl equation for the reaction that occurs when NaOCl is dissolved in water and calculate the numerical is dissolved in water and calculate the numerical value of the equilibrium constant for the reaction.value of the equilibrium constant for the reaction.

(c)(c) Calculate the pH of a solution made by Calculate the pH of a solution made by combining 40.0 milliliters of 0.14–molar HOCl combining 40.0 milliliters of 0.14–molar HOCl and 10.0 milliliters of 0.56–molar NaOH. And and 10.0 milliliters of 0.56–molar NaOH. And explain why Ms. Andre is so cool.explain why Ms. Andre is so cool.

(d)(d) How many millimoles of solid NaOH must be How many millimoles of solid NaOH must be added to 50.0 milliliters of 0.20–molar HOCl to added to 50.0 milliliters of 0.20–molar HOCl to obtain a buffer solution that has a pH of 7.49? obtain a buffer solution that has a pH of 7.49? Assume that the addition of the solid NaOH Assume that the addition of the solid NaOH results in a negligible change in volume.results in a negligible change in volume.

FRQ Practice #2FRQ Practice #2

Methylamine, CHMethylamine, CH33NHNH22, is a weak base that , is a weak base that reacts according to the equation below. The reacts according to the equation below. The value of the ionization constant, Kvalue of the ionization constant, Kbb, is , is 5.255.251010–4–4. Methylamine forms salts such as . Methylamine forms salts such as methylammonium nitrate, (CHmethylammonium nitrate, (CH33NHNH33

++)(NO)(NO33––). ).

CHCH33NHNH22 + H + H22O ↔ CHO ↔ CH33NHNH33++ + OH + OH––

(a)(a) Calculate the hydroxide ion Calculate the hydroxide ion concentration, [OHconcentration, [OH––], of a 0.225–molar ], of a 0.225–molar aqueous solution of methylamine.aqueous solution of methylamine.

FRQ Practice #2 FRQ Practice #2 (continued)(continued)

(b)(b) Calculate the pH of a solution made by Calculate the pH of a solution made by adding 0.0100 mole of solid methylammonium adding 0.0100 mole of solid methylammonium nitrate to 120.0 milliliters of a 0.225–molar nitrate to 120.0 milliliters of a 0.225–molar solution of methylamine. Assume no volume solution of methylamine. Assume no volume change occurs.change occurs.

(c)(c) How many moles of either NaOH or HCl How many moles of either NaOH or HCl (state clearly which you choose) should be (state clearly which you choose) should be added to the solution in (b) to produce a added to the solution in (b) to produce a solution that has a pH of 11.00? Assume that solution that has a pH of 11.00? Assume that no volume change occurs.no volume change occurs.

(d)(d) A volume of 100. milliliters of distilled A volume of 100. milliliters of distilled water is added to the solution in (c). How is the water is added to the solution in (c). How is the pH of the solution affected? Explain.pH of the solution affected? Explain.