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
From last week…why Boil Water for Preparation of
NaOH Solution?
CO2(g) + H2O()  H2CO3(aq)
H2CO3(aq) 
+
H (aq)
-
+ HCO3 (aq)
Ka = 4.3 x 10
Protons from carbonic acid can neutralize hydroxide ions
thus changing the concentration of OH- in the solution.
Also forms Na2CO3, slightly insoluable salt.
-7
KHP is potassium hydrogen phthalate
KHP
K+(aq) + HP–(aq)
Ka = 3.91 x 10-6
MW = 204.2236
HP–(aq) + OH– (aq) → P2– (aq) + H2O()
Selection of Indicator
pH? Kb = [HP-][OH-]
─────── = 2.56 x 10-9
[P2-]
P2- + H2O
[P2-]-x
→ HP- + OHx
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
HP- + OH- → P2- + H2O complete
at endpoint
─────
= 2.56 x 10-9 ; x = 8.76 x 10-6 = [OH-]
.030 – x
P2- + H2O → HP- + OH- K = ? = Kb = Kw/Ka
Let’s derive it …
pOH = 5.063 ; pH = 14 - 5.063 = 8.937
+
2H + P → HP
1/Ka
H2O → H+ + OHKw
________________________
P2- + H2O → HP- + OH- Kb = Kw/Ka = (1.00 x 10-14)/(3.91 x 10-6) = 2.56 x 10-9
Phenolphthalein
-O
OH
O
OH
O
+ 2OH -
O-
+ 2 H2O
O
O
H2In (colorless)
In-2 (pink)
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.
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
Calculations
Quick Check of Precision
Use
vol(base)
mass(acid )
to calculate
high  low
1000  2 ppt
average
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
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 3:15 PM
Continue titrations with time left
Clean up after yourselves
Strong Base/Weak Acid Titration Curve
OH-, A-
pH
AInflection
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
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!!!