Chemistry Experiment 5 - Experiment 5 pH Study of...

Experiment 5: pH Study of Phosphoric Acid in Cola
Drinks
By Jennifer Knotts
Lab Partners: Katlin Geldbach
Chemistry 1312, Section F-1, February 22, 2007

Introduction:
The lab was preformed to provide a study of phosphoric acid concentration in cola drinks
and provide experience in proper K
a
’s calculations. The “pleasant tartness” found in cola drink
is due to the phosphoric acid content it contains. Phosphoric acid also slows the growth of molds
and bacteria, which would otherwise multiply rapidly in the sugary solution. Almost all of the
acidity of soda pop comes from the concentration of phosphoric acid and not from the carbonic
acid. This can be verified by measuring the pH of fresh and flat soda pop; there's very little
difference.
Ph titration of Weak Acids and polyprotic acids differs slightly mainly because a weak
acid only has one ionizable protein whereas a polyprotic acid has more than one ionizable proton
per molecule. The equilibrium expressions for each of the two types of acids are shown below.
• Weak Acid equilibrium: HA
(aq)
+ H
2
O  H
3
O + + A -
(aq)
1
st
equivalence pt:
[ ][ ]
[ ]
HA
A O H
K
a
- +
=
3
• Polyprotic Acid equilibrium: (H
3
PO
4
) two equivalence points
H
3
PO
4
+ H
2
O ↔ H
3
O
+
+ H
2
PO
4
_
K
a1
= 7.5 x 10
-3
H
2
PO
4
- + H
2
O ↔ H
3
O + + HPO
4
2- K
a2
= 6.2 x 10 -8
HPO
4
2- + H
2
O ↔ H
3
O + + PO
4
3- K
a3
= 2.2 x 10 -13
1 st equivalence pt:
[ ]
2 1
3
2
a a
b
w a
K K
K
K K
H ⋅ =
⋅
= +
2
nd
equivalence pt:
[ ]
2 3
2
3
a a
b
w a
K K
K
K K
H ⋅ =
⋅
= +