Titrations (Review)
In a titration a solution of accurately known concentration is added gradually added
to another solution of unknown concentration until the chemical reaction between the
two solutions is complete.
Equivalence point – the point at which the reaction is complete
Indicator – substance that changes color at (or near) the
equivalence point
Slowly add base
to unknown acid
UNTIL
The indicator
changes color
(pink)
1
Alternative Method of Equivalence Point Detection
monitor pH
5
Strong Acid-Strong Base Titrations
NaOH (aq) + HCl (aq)
H2O (l) + NaCl (aq)
OH- (aq) + H+ (aq)
H2O (l)
7
Weak Acid-Strong Base Titrations
CH3COOH (aq) + NaOH (aq)
CH3COOH (aq) + OH- (aq)
CH3COONa (aq) + H2O (l)
CH3COO- (aq) + H2O (l)
At equivalence point (pH > 7):
CH3COO- (aq) + H2O (l)
OH- (aq) + CH3COOH (aq)
8
Exp 16B – An Acid-Base Titration Curve
Weak acid=strong base specific titration curves
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When titrating a weak acid with a strong base (or the other way around) the
acid dissociation constant, Ka, for the weak acid can be determined from the
titration curve
It coincides with the middle of the “buffer region” of the titration curve
Exp 16B – An Acid-Base Titration Curve
Purpose of the experiment
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Perform an acid-base titration of potassium hydrogen phthalate (“KHP”)
with sodium hydroxide, measure and record the changes in pH
Plot the change of pH as a function of volume of base added
Determine the Ka for the weak acid potassium hydrogen phthalate, KHP
Determine the equivalence point
Exp 16B – An Acid-Base Titration Curve
Titration of potassium hydrogen phthalate with sodium hydroxide
CO 2K
CO 2K
+
+
NaOH
CO 2H
H 2O
CO 2Na
potassium acid phthalate
Standardization of an NaOH solution
• NaOH needs to be standardized before it can be used in a titration
– solid NaOH is hygroscopic and attracts H2O from the air
• you cannot weigh NaOH accurately
• you’ll also weigh absorbed water
– an NaOH solution absorbs CO2 from the air, which will react to form
carbonic acid, making the solution more acidic than the dissolved
amount of NaOH suggests
– standardization with potassium hydrogen phthalate (KHP)
HC8H4O4-(aq) + OH-(l)  C8H4O42-(aq) + H2O(l)
– KHP is a weak acid, molar mass = 204.224 g/mol
HC8H4O4-(aq) + H2O(l)  C8H4O42-(aq) + H3O+(aq)
Exp 16B – An Acid-Base Titration Curve
Standardization of an NaOH solution
• Equivalence point
[H3O+] = [OH-]
– If [H3O+] is known, [OH-] can be calculated
 Volacid x Molarityacid= Volbase x Molaritybase
 Vacid (L) x Macid (mol/L) = Vbase (L) x Mbase (mol/L)
moles of acid = moles of base
Prelab problem #2
 25 mL 0.10 M NaOH neutralizes how many grams of KHP? KHP is a solid!
Answer
 25 mL 0.10 mol/L NaOH = 25 mL x 1L/1000 mL x 0.10 mol/L NaOH = ? mol NaOH
 At equivalence point: moles of KHP = moles of NaOH = ? mol
 ? mol KHP x 204.224 g/mol = ?? g KHP
Exp 16B – An Acid-Base Titration Curve
Determine pKa
• Acid dissociation constant Ka = [H3O+] [A-]/[HA]
[H3O+] = Ka [HA]/[A-]
pH = -log [H3O+] = -log Ka + (-log [HA]/[A-]) = pKa - log [HA]/[A-] =
pKa + log [base]/[acid]
(Henderson-Hasselbalch equation)
When is pH = pKa?
•
During a titration of the weak acid HA:
HA(aq) + OH-(aq)  A-(aq) + H2O(l)
 HA(aq)(acid) decreases because it reacts with OH-(aq), forming H2O
 A- (conjugate base) increases
 When [HA] = [A-], ½ of HA has reacted with OH pH = pKa + log [A-]/[HA] = pKa + log 1 = pKa
 This point is halfway between the start of the titration and the equivalence point
 Remember: at the equivalence point all acid has reacted with OH-: no HA is left
Exp 16B – An Acid-Base Titration Curve
Experimental
1.
2.
3.
4.
Calibrate the pH meter. Use buffer pH 4 or pH 7
Clean the buret and rinse with water followed by a small amount of the NaOH
Weigh between 0.50 – 0.53 g KHP. Record the exact mass to 4 decimal places!
Dissolve KHP in 50.0 mL (exact!!) of dH2O in a 150 mL beaker*. Dissolve
completely (why?)
5. Add 2 drops of phenolpthalein indicator (NOT done)
6. Immerse pH electrode in the KHP solution
7. Set up the buret and fill it with 50 mL 0.1 M NaOH solution. Be careful not to spill
NaOH in the KHP solution! (Why does that matter? You are going to mix the
solutions anyway)! Record the initial volume reading of the burette in your lab
manual. The tip of the buret will be positioned slightly above the surface of the
solution
8. Read and record the initial pH of the KHP solution before adding any NaOH
9. Start the titration by adding 1-mL aliquots of NaOH solution to the KHP solution
while stirring.
10. Record the burette reading and the pH after each addition
11. When the pH changes fast (>0.3 units/mL of base) reduce the amount of base that
you add to ~0.2 mL
12. You will quickly pass the equivalence point. When the pH changes start to lag again,
return to the 1-mL additions. Keep titrating until the pH ~ 11.5-12
 To get a good titration curve you need to go at least 5 mL beyond where the pH >11
13. Repeat the titration with a 2nd sample of KHP
Acid-Base Titration Curve
Exp 16B – An Acid-Base Titration Curve
Calculations
Sample
Vol (equivalence point) (mL)
pH (equivalence point)
Vol at half-way point (mL)
pH
pKa
Ka
1
2
Next Monday: Experiment 18 Spontaneity
DUE Monday Nov. 19:
Exp 16B: An Acid-Base Titration Curve
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Datasheet, Calculations, Titration curves
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Post lab questions 1-2
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Postlab question 3 (5 bonus points)
Exp 18: Spontaneity
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Prelab preparations
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Goal
Procedure
Physical and Chemical Properties of Ammonium nitrate
Prelab Questions