Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 117

drated ions. The concentration of all ions and
molecular substances in a solution contributes to the
osmotic potential.
Water can also be an acid or a base, because H 2 O
molecules can receive or provide a proton (H
).
Such an exchange by water molecules in pure water,
forming hydrated protons(H 3 O
or (H 2 O) 4 H
), is
called self-ionization. However, the extent of ion-
ization is small, and pure water is a very poor con-
ductor.

Self-Ionization of Water

The self-ionization of water is a dynamic equilibri-
um,

H 2 O(l) ↔H
(aq)
OH(aq),

Kw[H
][OH]  10 -14at 298 K and 1 atm

where [H
] and [OH] represent the molar concen-
trations of H
(or H 3 O
) and OHions, respective-
ly, and Kwis called the ion product of water. Values
of Kwunder various conditions have been evaluated
theoretically (Marshall and Franck 1981, Tawa and
Pratt 1995). Solutions in which [H
] [OH] are
said to be neutral. Both pHand pOH,defined by
the following equations, have a value of 7 at 298 K
for a neutral solution.

pHlog 10 [H
] pOHlog 10 [OH]  7
(at 298 K)

The H
represents a hydrated proton (H 3 O
), which
dynamically exchanges a proton with other water
molecules. The self-ionization and equilibrium are
present in water and all aqueous solutions.

Solutions of Acids and Bases

Strong acids HClO 4 , HClO 3 , HCl, HNO 3 , and
H 2 SO 4 completely ionize in their solutions to give
H
(H 3 O
) ions and anions ClO 4 , ClO 3 , Cl,
NO 3 , and HSO 4 , respectively. Strong bases
NaOH, KOH, and Ca(OH) 2 also completely ionize
to give OHions and Na
, K
, and Ca^2
ions, re-
spectively. In an acidic solution, [H
] is greater than
[OH]. For example, in a 1.00 mol/L HCl solution
at 298 K, [H
] 1.00 mol/L, pH 0.00, [OH] 
10 -14mol/L.
Weak acids such as formic acid (HCOOH), acetic
acid (HCH 3 COO), ascorbic acid (H 2 C 6 H 6 O 6 ), oxalic

acid (H 2 C 2 O 4 ), carbonic acid (H 2 CO 3 ), benzoic acid

(HC 6 H 5 COO), malic acid (H 2 C 4 H 4 O 5 ), lactic acid

(HCH 3 CH(OH)COO), and phosphoric acid (H 3 PO 4 )

also ionize in their aqueous solutions, but not com-

pletely. The ionization of acetic acid is represented

by the equilibrium

HCH 3 COO(aq) ↔H
(aq) 
CH 3 COO(aq),

where Ka, as defined above, is the acid dissociation

constant.

The solubility of CO 2 in water increases with par-

tial pressure of CO 2 , according to Henry’s law. The

chemical equilibrium for the dissolution is

H 2 O(l) 
CO 2 (g) ↔H 2 CO 3 (aq)

Of course, H 2 CO 3 dynamically exchanges H
and

H 2 O with other water molecules, and this weak

diprotic acidionizes in two stages with acid disso-

ciation constants Ka1and Ka2:

H 2 O 
CO 2 (aq) ↔ H
(aq) 
HCO 3 (aq), Ka1

4.30  10 -7at 298 K

HCO 3 (aq) ↔H
(aq) 
CO 3 2-(aq), Ka2

5.61  10 -11.

Constants Ka1and Ka2increase as temperature rises,

but the solubility of CO 2 decreases. At 298 K, the

pH of a solution containing 0.1 mol/L H 2 CO 3 is 3.7.

At this pH, acidophilic organisms survive and grow,

but most pathogenic organisms are neutrophiles, and

they cease growing. Soft drinks contain other acids—

citric, malic, phosphoric, ascorbic, and others. They

lower the pH further.

All three hydrogen ions in phosphoric acid

(H 3 PO 4 ) are ionizable, and it is a triprotic acid.

Acids having more than one dissociable H
are called

polyprotic acids.

Ammonia and many nitrogen-containing com-

pounds are weak bases. The ionization equilibrium

of NH 3 in water and the base dissociation constant

Kbare

NH 3 
H 2 O ↔NH 4 O ↔NH 4 
(aq) 
OH(aq),

Other weak bases react with H 2 O and ionize in a

similar way.

Kb

+

4

H[OH

NH OH

==×at 298 K.

−

[] ] −

[]

170 10.^5

Ka

3

HCHCOO

HCH COO

==×at 298 K

+−

[][ ] −

[]

(^3) 175 10. 5