120 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
cations. The presence of ions in the solution changes
the ordering of water molecules even if they are not
in the first hydration sphere.
The hydration of ions releases energy, but break-
ing up ions from a solid requires energy. The amount
of energy needed depends on the substance, and for
this reason, some substances are more soluble than
others. Natural waters in oceans, streams, rivers, and
lakes are in contact with minerals and salts. The
concentrations of various ions depend on the solu-
bility of salts (Moeller and O’Connor 1972) and the
contact time.
All salts dissolved in water are completely ion-
ized, even those formed in the reaction between
weak acids and weak bases. For example, the com-
mon food preservative sodium benzoate (NaC 6 H 5
COO) is a salt formed between a strong base NaOH
and weak benzoic acid (Ka6.5 10 -5). The ben-
zoate ions, C 6 H 5 COO, in the solution react with
water to produce OHions giving a slightly basic
solution:
C 6 H 5 COOH 2 O ↔C 6 H 5 COOH OH,
K 1.6 10 -10, at 298 K.
Ammonium bicarbonate, NH 4 HCO 3 , was a leaven-
ing agent before modern baking powder was popu-
lar. It is still called for in some recipes. This can be
considered a salt formed between the weak base
NH 4 OH and the weak acid H 2 CO 3. When NH 4 HCO 3
dissolves in water, the ammonium and bicarbonate
ions react with water:
NH 4 H 2 O ↔NH 3 (aq) H 3 O,
K 5.7 10 ^10 , at 298 K
HCO 3 H 2 O ↔H 2 CO 3 OH,
K 2.3 10 ^8 , at 298 K
H 2 CO 3 H 2 O CO 2 (g).
Upon heating, ammonia (NH 3 ) and carbon dioxide
(CO 2 ) become gases for the leavening action. Thus,
during the baking or frying process, ammonia is
very pungent and unpleasant. When sodium bicar-
bonate is used, only CO 2 causes the dough to rise.
Phosphoric acid, instead of NH 4 , provides the acid
in baking powder.
Buffer Solutions
A solution containing a weak acid and its salt or a
weak base and its salt is a buffer solution, since its
pH changes little when a small amount acid or base
is added. For example, nicotinic acid (HC 6 H 4 NO 2 ,
niacin, a food component) is a weak acid with Ka
1.7 10 ^5 (pKalog 10 Ka4.76):HC 6 H 4 NO 2 HC 6 H 4 NO 2 ,(Henderson-Hesselbalch equation).In a solution containing niacin and its salt,
[C 6 H 4 NO 2 ] is the concentration of the salt, and
[HC 6 H 4 NO 2 ] is the concentration of niacin. The
pair, HC 6 H 4 NO 2 and C 6 H 4 NO 2 , are called conju-
gate acid and base, according to the Bronsted-
Lowry definition for acids and bases. So, for a gen-
eral acid and its conjugate base, the pH can be
evaluated using the Henderson-Hesselbalch equa-
tion:(Henderson-Hesselbalch Equation)Adding Hconverts the base into its conjugate acid,
and adding OHconverts the acid into its conjugate
base. Adding acid and base changes the ratio
[base]/[acid], causing a small change in the pH if the
initial ratio is close to 1.0. Following this equation,
the most effective buffer solution for a desirable pH
is to use an acid with a pKavalue similar to the
desired pH value and to adjust the concentration of
the salt and acid to obtain the ratio that gives the
desired pH. For example, the pKafor H 2 PO 4 is
7.21, and mixing KH 2 PO 4 and K 2 HPO 4 in the
appropriate ratio will give a buffer solution with pH- However, more is involved in the art and science
of making and standardizing buffer solutions. For
example, the ionic strength must be taken into
account.
The pH of blood from healthy persons is 7.4. The
phosphoric acid and bicarbonate ions in blood and
many other soluble biomaterials in the intercellular
fluid play a buffering role in keeping the pH con-
stant. The body fluids are very complicated buffer
solutions, because each conjugate pair in the solu-
tion has an equilibrium of its own. These equilibria
pH = pKbase]
a [acid]
= ⎧⎨
⎩⎫
⎬
⎭log[
10pKHpCHNO
a [HC H NO64
64=+⎧
⎨
⎩⎫
⎬
⎭−
log[](^10) ]
2
2
Ka=
[][+−]
]
HCHNO
[HC H NO
64
64
2
2