Food Biochemistry and Food Processing (2 edition)

BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come

5 Water Chemistry and Biochemistry 95

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, forminghydrated protons(H 3 O+

or (H 2 O) 4 H+), is calledself-ionization. However, the extent of

ionization is small, and pure water is a very poor conductor.

Self-Ionization of Water

The self-ionization of water is a dynamic equilibrium,

H 2 O(l)↔H+(aq)+OH−(aq),

Kw=[H+][OH−]= 10 −^14 at 298 K and 1 atm

where [H+] and [OH−] represent the molar concentrations of

H+(or H 3 O+)andOH−ions, respectively, andKwis called the

ion product of water. Values ofKwunder various conditions

have been evaluated theoretically (Marshall and Franck 1981,

Tawa and Pratt 1995). Solutions in which [H+]=[OH−]are

said to beneutral. Both pH and pOH, defined by the following

equations, have a value of 7 at 298 K for a neutral solution

pH=−log 10 [H+]=pOH=−log 10 [OH−]= 7 .(at 298 K)

The H+represents a hydrated proton (H 3 O+), which dynami-

cally 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 perchloric acid (HClO 4 ), Chloric acid (HClO 3 ),

hydrochoric acid (HCl), Aitric acid (HNO 3 ), and Sulphuric acid

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 −,andHSO 4 −, respec-

tively. Strong bases NaOH, KOH, and Ca(OH) 2 also completely

ionize to give OH−ions and Na+,K+,andCa^2 +ions, respec-

tively. 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 −^14 mol/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 phospho-

ric acid (H 3 PO 4 ) also ionize in their aqueous solutions, but not

completely. The ionization of acetic acid is represented by the

equilibrium

HCH 3 COO(aq)↔H+(aq)+CH 3 COO−(aq),

Ka=

[H+][CH 3 COO−]

[HCH 3 COO]

= 1. 75 × 10 −^5 at 298 K

whereKa, as defined above, is theacid dissociation constant.

The solubility of CO 2 in water increases with partial 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 weakdiprotic acidionizes in

two stages with acid dissociation constantsKa1andKa2:

H 2 O+CO 2 (aq)↔H+(aq)+HCO− 3 (aq),

Ka 1 = 4. 30 × 10 −^7 at 298 K

HCO− 3 (aq)↔H+(aq)+CO^23 −(aq),Ka2= 5. 61 × 10 −^11.

ConstantsKa1andKa2increase as temperature rises, but the solu-

bility 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 sur-

vive 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 ion-

izable, and it is atriprotic acid. Acids having more than one

dissociable H+are calledpolyprotic acids.

NH 3 and many nitrogen-containing compounds are weak

bases. The ionization equilibrium of NH 3 in water and thebase

dissociation constant Kbare

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

Kb=

[H+][OH−]

[NH 4 OH]

= 1. 70 × 10 −^5 at 298 K.

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

The ionization or dissociation constants of inorganic and or-

ganic acids and bases are extensive, and they have been tabulated

in various books (for example Perrin 1965, 1982, Kortum 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 method 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. Theend pointis reached when the

latter substance is completely consumed by the reagent from the

burette, and this is detected by the color change of an indicator

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 titration.

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 atitration curve. There are a

number 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++OH−=H 2 O,K=

[H 2 O]

Kw

= 5. 56 × 1015 at 298 K.