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Collect:250 mL volumetric flask (1) 10 mL graduated pipet (2), pipet filler50 mL beaker (4)Dropper (2) (not broken)Magnetic stir bar (given out and collected by GTA)

Prepare: 10 and 50 mL graduated cylinders 250 mL beaker

Buffer Solutions(2015/11/18 revised)

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Objective & SkillsObjective Prepare buffer solutions of HOAc/NaOAc Determine the factors that affect buffer capacity [A-]/[HA] ratio A- and HA concentration

Skills Operation of pH-meter Preparation of solution Dilution of conc. acid Test the buffer capacity

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Buffer: solution that maintains a fairly constant pH value upon addition of a small amount of acid or base.

A buffer contains A weak acid to neutralize the added base

Its conjugated base to neutralize the added acid

To prepare a buffer solution, it could be: Higher concentrations of strong acids or strong bases

i.e. 1 M HCl, 1 M NaOH A mixture of a weak acid and its conjugate base

i.e. HF/NaF A mixture of a weak base and its conjugate acid

i.e. NH3/NH4Cl

Introduction I: Buffer Solution

A-(aq) + H+

(aq) HA(aq)Acid added

H A (aq)+ O H -(aq) H 2O (l) + A -

(aq)Base added

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Henderson-Hasselbalch equation

[HA]]][A[HKa

HA(aq) H+(aq) + A-

(aq)

][A[HA]K][H a

)][A

[HA]log(logK]log[HpH a

)[HA]

][Alog(pKpH a

Introduction II: pH of Buffer

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Introduction III: Buffer Capacity Definition: The number of moles of OH- or H+ that causes 1 L

of the buffer to undergo one unit change in pH

A buffer has the greatest capacity when the concentration ratio of the two components is 1 ([A-]/[HA] = 1)

A buffer with pKa - 1 ≤ pH ≤ pKa + 1, shows better buffer capacity

A buffer has higher buffer capacity when the concentrationof two components are higher

Calculation:

(mL)(mL/L)(eq/L)(mL/drop)(drop/pH)

pH)(meq/L

301000NV

slope1

Capacity Buffer

drop

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Introduction IV: pH MeterThree components of pH meter:pH electrode

Reference electrode (usually made of silver and silver chloride), the potential is a fixed value

Indicator electrode (usually made of glass), the potential changes when the concentration of H+

variesThermoprobe: measure the temperature of solutionVoltmeter: measure the potential difference between

the two electrodes

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Introduction V: pH Meter

Em = K − 2.3RT(pH)/nF Em = mT(pH) + K

Em: measured cell potential K: constant, determined by

the type of electrode used R: gas constant T: absolute temperature of

the solution pH: pH value of solution n: number of moles of

electrons transferred through the electrodes during a reaction

F: Faraday constantThe Relationship Between Measured Cell Potential and pH value

7 14

First standard solution

Second standard solution

0pH

+mV

- mV

Em

4

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Outline of Procedures

1. Calibrate pH-meter 2. Prepare solution 3. Calculate the volume of one drop of HCl/NaOH

4. Prepare test solutionsand record pH value

50 mL beaker

5 drops of acid

5 drops of base

Record allpH values

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Procedure 1. Calibrate pH-meter Push the “POWER”

button, warm up for 10 minutes

Remove the electrode cap by rotating it

Use washing bottle to clean the electrodes

Blot dry with a tissue

NT$ 4500 !!

Cap of electrode Control knobs Rearward of pH meter

pH Electrode

Thermoprobe

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Rinse thermoprobe and electrode with DI water

Immerse in pH 4.00 buffer solution Adjust Slope button until pH 4.00

Immerse thermoprobe and electrode into pH 7.00 buffer solution

Adjust Calib button until pH 7.00

pH 7.0

Calibbutton

Slope button

pH 4.0

(1) Collect standard buffer solution(2) Start calibrating pH meter

Procedure 1. Calibrate pH-meter

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Notice in Using pH Meter Use 50 mL beakers for testing Place the electrode on the holder When testing, both thermoprobe and

the electrode should be placed in soln The salt bridge of electrode should be

fully immersed in the test solution Position the electrode properly so that the

stirring bar will not strike the electrode Turn the magnetic stir on When changing the test solution, the

electrode should be rinsed with distilled water and blot dry with tissues

When the electrode is not in use, it needs to be immersed in clean distilled water

When the electrode is not in use for long period of time, it should be immersed in 3 M KCl solution

Salt bridge

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Odd groups:

Prepare 0.05 M HOAc

Use pipet to get 0.74 mL of conc. HOAc (17 M)

Transfer to volumetric flask that contains some D.I. water

Dilute to 250 mL

pH of soln is ~3

Even groups: Prepare 0.05 M NaOAc Obtain 1.70 g of

NaOAc·3H2O Dissolve in ca. 100 mL

D.I. water Transfer to volumetric

flask Rinse the beaker

several times Dilute to 250 mL pH of soln is ~ 8

Procedure 2. Prepare Solution

2 groups share one set of solution Label after solution is made in case of a mix up According to table 1, measure out each soln with 10 mL graduated pipet

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Testsolution

0.050 MHOAc (mL)

0.050 MNaOAc (mL)

DistilledWater (mL)

(a) 30 0 0

(b) 0 30 0

(c) 15 15 0

(d) 5 5 20

(e) 0 0 30

(f) 5 25 0

(g) 10 20 0

(h) 20 10 0

(i) 25 5 0

Preparation of Test Solutions

The pH change of water is drastic; test it last

(1) Concentration vs. buffer capacity

(2) [OAc-]/[HOAc]vs. buffer capacity

(Not done in this exp)

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Measure the volume of 50 drops of 1 M HCl or NaOH solution with a 10 mL graduated cylinder

Calculate the average volume of each drop

Droppers must be in good condition and not broken to prevent the volume of the drops that come out of the dropper to be vary too much

Procedure 3. Calculate the Average Volume of 1 Drop of HCl/NaOH

Vavg,HCl =_____ mL/drop

Vavg,NaOH =____ mL/drop

1.0 M HCl 1.0 M NaOH

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Procedure 4. Measure pH of Test Solution while Adding Acid/Base

Obtain 1.0 N of HCl with dropper

Add one drop into solution (a)

Mix thoroughly and record pH values

Repeat adding acid for 5 times

Repeat above steps with solution (b)~(e)

Prepare another beaker of solution (a)

Obtain 1.0 N of NaOH with dropper

Add drops progressively into solution and record change in pH

values

Set up apparatus Immerse the thermoprobe and

electrode in solution Position the electrode in the soln so

that the stirring bar will not strike the electrode

Stir and mix the solution continuously

Turn the stir on

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Experiment Completed

Clean and check pH electrodes Immerse the electrode into clean DI water Or place electrode in plastic-cap that containing

3 M KCl Turn the pH meter off Hand in magnetic stirring bar to TA Waste liquids (salts) can be discarded in sink

after neutralization

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Example of Data Analysis

No. of drop

a b c d e0.05 M HOAc

0.05 M NaOAc

0.05 M HOAc/NaOAc

0.017 M HOAc/NaOAc

Distilledwater

-5 2.39 5.60 4.47 4.15 11.49-4 2.47 5.72 4.51 4.26 11.40-3 2.59 5.85 4.55 4.36 11.27-2 2.71 6.07 4.58 4.46 11.09-1 2.82 6.37 4.61 4.56 10.790 3.00 7.71 4.65 4.64 5.49

1 3.15 10.78 4.67 4.73 3.01

2 3.33 10.93 4.70 4.80 2.71

3 3.48 11.22 4.73 4.89 2.55

4 3.59 11.39 4.76 4.99 2.43

5 3.65 11.51 4.80 5.09 2.35

Avg. pH of acid and base

Add acid

Add base

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Plots of pH vs. Drops of Acid/Base

1.00

3.00

5.00

7.00

9.00

11.00

-8 -6 -4 -2 0 2 4 6 8

pH v

alue

HCl / NaOH drops

pH change

0.05M HOAc

0.05M NaOAc

0.05M

HOAc/NaOAc

0.017M

HOAc/NaOAc

DI water

Graph with pH value as y-axis, and the drops as x-axis

Choose x-y scattering plot in Excel

Put plots of a, b, c, d, and e solutions all in one graph for comparison

The graph must be handed in along with the final lab report

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Calculation of Buffer Capacity

linear (c)

y = 0.03x + 4.642

R2 = 0.9912

linear (d)

y = 0.085x + 4.638

R2 = 0.9968

4.40

4.50

4.60

4.70

4.80

4.90

-3 -2 -1 0 1 2 3

pH va

lue

HCl/NaOH drops

Buffer Capacity(a) 0.05M (1:1)

(b) 0.017M(1:1)

linear (d)

Linear (c)

0.050 M (1:1) buffer capacity = 22 meq/L‧pH

0.017 M (1:1) buffer capacity = 7.8 meq/L‧pH

Use the central 5 points of solution (c) and (d) Show linear regression and R2 value

drop

Buffer Capacity (m eq/L pH)1 1000(m L/L)= (drop/pH) V (m L/drop) N(eq/L)

slope 30(m L)

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Additional Information

The pH values of the separately prepared solutions may be different at first, but if the pH does not vary too much, use the average value for calculation and graphing

If the values vary widely, then adding base and adding acid can be plotted separately

Label clearly the x and y-axis and the lines Use Microsoft Excel for graphing and finding the least

square regression lines