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Idea Transcript
Potentiometric Titration of Acid-Base Collect
One 50 mL buret
One 100 mL volumetric flask
(2016/03/12 revised)
pH 7.00 and pH 4.00 standard buffer solution (shared by two groups) Two 125 mL Erlenmeyer flasks (check if broken)
One pipet filler (check for gas leak)
One magnetic stirring bar (from TA) 1
Objective & Skills I. Objective: To prepare and to standardize secondary-standard solutions To determine the equivalence point of titration by using the electric potential method To determine the dissociation constant of acetic acid, Ka II. Skills: Learn to weigh chemicals and prepare solutions To operate volumetric flask, graduated pipet, and burets To calibrate and operate pH-meter To determine the equivalence point by using titration curves
2
Standardization of Acid or Base Primary standard: larger molar mass and high purity Secondary standard: standardized acid or base Common primary standard acid: potassium hydrogen phthalate (KHP) Common primary standard base: sodium carbonate (Na2CO3) KHP is a monoprotic weak acid The neutralization with NaOH takes place in a 1:1 ratio HOOCC6H4COOK(aq) + NaOH(aq) C6H4(COO)22-(aq) + K+(aq) + Na+(aq) + H2O(l) 3
Determine the Equivalence Point •
The pH value of the reacting solution changes significantly near the equivalence point Base on the color change of the acid-base indicator or monitoring the change in pH values to determine the equivalence point 14.00
Equivalence point 12.00
Phenolphthalein
Indicator
Acid form
pH range
Basic form
Bromothymol blue
Methyl orange
Red
3~4
Orange
Methyl orange
Bromothymol blue
Yellow
6~7
Blue
Phenolphthalein
Colorless
8~10
Pink red
10.00
8.00
pH
•
6.00
4.00
2.00
0.00 0.00
10.00
20.00
30.00
40.00
50.00
60.00
NaOH滴定體積
Weak acid / strong base titration curve
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Acid-Base Indicator Acid-base indicator: a weak organic acid or base Weak acid (HIn) and its conjugate base (In－) with different colors
HIn + H2O
Acidic Color HIn
Color Change Range
pKa － 1
H3O+ + In－
Basic Color In－ pH increases
pKa + 1
Predict the pH range of the equivalence point Strong acid/weak base titration: pH < 7 Weak acid/strong base titration: pH > 7 Strong acid/strong base titration: pH = 7 Choose the appropriate indicator to match the end-point with the equivalence point
5
10
Equivalence Point pH
6 4
1. Acid-base titration curve The equivalence point is the point on the curve with the maximum slope
2 20
22
24
26
NaOH(aq) V (mL)
25
△pH/△V
Equivalence 20 point
2. First derivative of titration curve The maximum point is the equivalence point
15 10 5 0 20
150 100 △2pH/△V2
3. Second derivative of the titration curve 0 crossing is the equivalence point
Equivalence point
8
22
24 V1 (mL)
Equivalence point
26
A
50 0 -50 20
22
-100
24
26
B
-150 V2 (mL)
6
Acid Dissociation Constant of a Weak Acid
A H O
Ka
Weak acid – strong base titration curve
3
HA
At Half-Equivalence point
14.00
[HA] ＝ [A-] [H3O+]
12.00
= Ka
Therefore, pH = pKa
10.00
For example Equivalence volume = 37.50 mL
Half-equivalence volume = 18.75 mL
V = 18.00, pH = 4.60
V = 19.10, pH = 4.65
pH of the half-equivalence volume = 4.63
pH
pH = pKa
8.00
6.00
4.00
2.00
Half-equivalence volume 0.00 0.00
pKa = pH = 4.63
Ka = 2.3 × 10-5
10.00
20.00
30.00
40.00
50.00
60.00
V (NaOH, mL)
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pH Meter pH meter consists of three parts: 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+ varies Thermoprobe: used to measure the temperature of soln Voltmeter: used to measure the potential difference between the two electrodes
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Cell Potential and pH Value Em = K − 2.3RT(pH)/nF Em = mT(pH) + K Second standard solution
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 constant
First standard solution
The Relationship Between Measured Cell Potential and pH value
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Outline of Procedures I. Prepare NaOH(aq)
II. Standardization of NaOH with KHP
III. Calibrate pH-meter
IV. Titration of vinegar
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Procedure I. Prepare 0.1 M NaOH
(1) Take10 mL of 1 M NaOH (2) Place in 100 mL volumetric flask (3) Add water till mark to dilute
(1) Invert the flask several times (2) Mix thoroughly
(1) Use approx. 5 mL of 0.1 M NaOH
(3) Pour into beaker (2) Rinse buret twice and fill with solution
Read initial volume of buret (Vi) to 0.01 mL 11
Procedure II. Standardize NaOH with KHP (1) Dissolve with 50 mL distilled water (2) Add 2 d. of phenolphthalein (3) Titrate with 0.1 M NaOH
Measure ca. 0.2~0.22 g KHP with analytical balance Place in a 125 mL Erlenmeyer flask Record accurate weight
Titrate
the solution to appear pink and persist for 30 s
Record Carry
Vi and Vf
out a duplicate test
Calculate
average concentration of NaOH 12
Procedure III. Calibrate the pH-meter
Push the “POWER” button, warm up for 10 minutes
Remove the electrode cap
Use washing bottle to clean the electrodes
Blot dry with a tissue
Place electrode and thermoprobe into solution
NT$ 4000 !!
HOLD
Setup of pH meter
Press “HOLD” when cleaning the electrodes and the screen will freeze 13
Procedure III. Calibrate pH-meter (1) Collect standard buffer solution (2) Start calibrating pH meter
Slope button
Calib button
pH 7.0 Immerse thermoprobe and electrodes into pH 7.00 buffer solution Adjust Calib button until meter says ‘7.00’
pH 4.0 Clean thermoprobe and electrodes Immerse in pH 4.00 buffer solution Adjust Slope button until meter says ‘4.00’ 14
Notice: Manipulate pH Meter
The end of the electrode should be fully immersed in the test solution and not touching the walls of the container Do not take the electrode off of the holder Both thermoprobe and the electrode should be placed in solution 100 mL beakers are used for testing in this experiment Position the electrode in the soln so that the stirring bar will not strike the electrode Turn the magnetic stirring bar on during titration Every time the testing solution is changed, 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 periods of time, it should be immersed in 3 M KCl solution
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Procedure IV. Titration of Vinegar Take sample: (1) Transfer 3.0 mL vinegar into a 100 mL beaker
(1) Add 40 mL distilled water (2) 2 d. of phenolphthalein
(2) Record brand and acidity of vinegar
(3) Place stirring bar, the electrode,
Titrate
Setup of apparatus
and thermoprobe in soln
Titrate with standardized 0.1 M NaOH
Add ~1 mL aliquots of NaOH and record Vi, Vf, and pH value after each addition
At pH 6~10: add titrant in 0.2 mL increments
At pH > 10: add titrant in 1 mL increments
When pH is ~12: stop titration
Observe and record the change in color of solution during titration
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After Experiment Clean and check pH electrode Place electrode in plastic-cap that containing 3 M KCl Turn the pH meter off Hand in magnetic stirring bar Wash buret and invert to dip dry Waste liquids (salts) can be discarded in sin after neutralization
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Data Analysis Calculate average standardized concentration of NaOH Calculate 3 equivalence pts and plot 3 graphs in Excel Calculate the molarity of acetic acid in vinegar (0.737 M) N1V1 = N2V2 Change into percent concentration and compare with labels (assume density the same as water) For example: 0.737 M × 60 /1000 × 1 g/cm3 × 100 % = 4.4 % Determine Ka of acetic acid from the half-equivalence point
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Worksheet
(15.10 16.15) V1 15.63 2
First derivative
(15.63 16.63) 16.13 V2 2
Second derivative
V NaOH
pH
V1
pH/V
V2
(pH/V)/V1
15.10
4.99
15.63
0.09
16.13
0.02
16.15
5.08
16.63
0.11
17.11
0.02
17.10
5.18
17.60
0.12
18.14
0.03
18.10
5.30
18.68
0.16
19.21
0.05
19.25
5.48
19.75
0.21
20.23
0.11
20.25
5.69
20.70
0.31
21.14
0.46
21.15
5.97
21.58
0.72
21.84
8.25
22.00
6.58
22.10
5.05
22.20
22.50
22.20
7.59
22.30
9.55
22.40
-34.00
22.40
9.50
22.50
2.75
22.80
-3.00
22.60
10.05 23.10
0.95
23.60
-0.63
23.60
11.00
0.32
24.58
-0.14
24.10
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Graphs of the Titration of Vinegar Titration curve