Volumetric Acid Determination

1
Volumetric Acid Determination

• Week 1 - Standardize NaOH w/ pure KHP
  Determine Unknown KHP
• Week 2 Continue Unknown KHP
• pH meter titration of an unknown
  acid

2
From last weekwhy Boil Water for Preparation of
NaOH Solution?
3
KHP is potassium hydrogen phthalate
Ka 3.91 x 10-6
MW 204.2236

HP(aq) OH (aq) ? P2 (aq) H2O(?)
4
Selection of Indicator
pH? Kb HP-OH- -------
2.56 x 10-9 P2- P2-
H2O ? HP- OH- P2--x
x x at equilibrium P2-
n(P2-)/Vol n(P2-) 0.5 g/204.2236 g/mol
.00245 mol V(OH-) .00245 mol/(.08 mol/L) 30
mL .030 L
approximately Vtotal 50 30 mL 0.080
L P2- 0.00245 mol/ (.080 L) .030 M
x2 ----- 2.56 x 10-9 x 8.76 x 10-6
OH- .030 x pOH 5.063
pH 14 - 5.063 8.937
HP- OH- ? P2- H2O complete at
endpoint P2- H2O ? HP- OH- K
? Kb Kw/Ka Lets derive it H P2-
? HP- 1/Ka H2O ? H OH-
Kw ________________________ P2-
H2O ? HP- OH- Kb Kw/Ka
(1.00 x 10-14)/(3.91 x 10-6) 2.56 x 10-9
5
Phenolphthalein

The color change in phenolphthalein is due to a
change in structure of the molecule.  In acid,
the molecule is in its H2In form containing a
central 5-membered ring, which is somewhat
strained. In base the In-2 structure opens up
and becomes flatter.
6
Procedure

• Dissolve KHP in 50 mL DI water, warm if
  necessary
• Add 2-3 drops of indicator
• Titrate until faint pink persists
• For the unknown KHP, adjust mass appropriately
  for 35mL

7
Calculations

• Quick Check of Precision
• Use to calculate
• To determine KHP
• vol NaOH ? moles NaOH ? moles KHP ? mass KHP?
  mass KHP
• (concentration of NaOH) (stoichiometry) (molar
  mass) mass KHP x 100
• mass sample

8
Week 2

• Start unknown KHP
• Then go back to pure KHP if necessary
• pH meter- half the class starts first, then teach
  the second half by 315 PM
• Continue titrations with time left
• Clean up after yourselves

9
Strong Base/Weak Acid Titration Curve
OH-, A-
A-
pH
Inflection Point
HA A-
Equivalence Point
HA only
Volume of NaOH
Use the equivalence pt halfway equivalence
point for calculations...how? End point needs to
be past the equivalence point
10
Determination of Ka

• Use pH meter data to create titration curve
• Dissolve 0.4 g of acid (not KHP, record to .1mg)
  in 250 mL beaker with 75 mL of water
• Record pH every .2-.3 unit change or every 5mL
• Graph paper from me (better than printer)
• Then clean up after yourselves! Wash vials and
  leave on rack near storage dessicators and ovens.
  Otherwise, points will be deducted!!!