Food Biochemistry and Food Processing

118 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

The ionization or dissociation constants of inor-
ganic and organic acids and bases are extensive, and
they have been tabulated in various books (for
example Perrin 1965, 1982; Kortüm et al. 1961).

Titration

Titrationis a procedure for quantitative analysis of
a solute in a solution by measuring the quantity of a
reagent used to completely react with it. This meth-
od is particularly useful for the determination of
acid or base concentrations. A solution with a
known concentration of one reagent is added from a
burette to a definite amount of the other. The end
pointis reached when the latter substance is com-
pletely consumed by the reagent from the burette,
and this is detected by the color change of an indica-
tor or by pH measurements. This method has many
applications in food analysis.
The titration of strong acids or bases utilizes the
rapid reaction between H
and OH. The unknown
quantity of an acid or base may be calculated from
the amount used to reach the end point of the titra-
tion.
The variation of pH during the titration of a weak
acid using a strong base or a weak base using a
strong acid is usually monitored to determine the
end point. The plot of pH against the amount of
reagent added is a titration curve. There are a num-

ber of interesting features on a titration curve.

Titration of a weak acid HA using a strong base

NaOH is based on two rapid equilibria:

H

OHH 2 O, K5.56  1015

at 298 K.

As the H
ions react with OH, more H
ions are

produced due to the equilibrium

HAH

A,Ka(ionization constant of the acid).

Before any NaOH solution is added, HA is the dom-

inant species; when half of HA is consumed, [HA]

[A], which is called the half equivalence point.

At this point, the pH varies the least when a little H


or OHis added, and the solution at this point is the

most effective buffer solution,as we shall see later.

The pH at this point is the same as the pKaof the

weak acid. When an equivalent amount of OHhas

been added, the Aspecies dominates, and the solu-

tion is equivalent to a salt solution of NaA. Of

course, the salt is completely ionized.

Polyprotic acids such as ascorbic acid H 2 -

(H 6 C 6 O 6 ) (Vitamin C; Ka17.9  10 -5, Ka21.6

 10 -12) and phosphoric acid H 3 PO 4 (Ka16.94 

10 -3, Ka26.2  10 -8, Ka32.1  10 -12) have more

than one mole of H
per mole of acid. A titration

curve of these acids will have two and three end

points for ascorbic and phosphoric acids, respective-

ly, partly due to the large differences in their dissoci-

ation constants (Ka1,Ka2, etc.). In practice, the third

end point is difficult to observe in the titration of

H 3 PO 4. Vitamin C and phosphoric acids are often

used as food additives.

Many food components (e.g., amino acids, pro-

teins, alkaloids, organic and inorganic stuff, vita-

mins, fatty acids, oxidized carbohydrates, and com-

pounds giving smell and flavor) are weak acids and

bases. The pH affects their forms, stability, and reac-

tions. When pH decreases by 1, the concentration of

H
, [H
], increases 10-fold, accompanied by a 10-

fold decrease in [OH]. The H
and OHare very

active reagents for the esterification and hydrolysis

reactions of proteins, carbohydrates, and lipids, as

we shall see later. Thus, the acidity, or pH, not only

affects the taste of food, it is an important parameter

in food processing.

Solutions of Amino Acids

Amino acids have an amino group (NH 3 
), a car-

boxyl group (COO), a H, and a side chain (R)

[]HO 2

Kw

Figure 5.11.Titration curve of a 0.10 mol/L (or M)
weak acid HA (Ka= 1  10 ^5 ) using a 0.10 mol/L strong
base NaOH solution.