Page 83 - Chemistry ICSE Class X

Study of Acids, Bases and Salts 71
The pH Scale

The strength of any solution of an acid or a base can be determined from the
+
–
concentration of H (aq) or OH (aq) in the solution. It is commonly described
in terms of a parameter called pH. This is discussed below.
What is meant by the pH of a solution
A Danish biochemist S. P. Sorensen (1909) proposed that the strength
of acidity of a solution be expressed in terms of a parameter called pH.
According to him, the R* QH C UQNWVKQP KU FGſPGF CU VJG PGICVKXG NQICTKVJO
(to the base 10) of the hydrogen ion concentration in the solution.
Mathematically,
+
pH = – log [H ] = log 1
+
[H ]
The pH values in aqueous solutions usually vary from 0 (highly acidic)
to 14 (highly alkaline). Relationship between pH values and the acidic or
basic nature of any aqueous solution (at 298 K) is shown below in Fig. 3.2. dŚĞ ŶŽƚĂƟŽŶ Ɖ, ŚĂƐ ďĞĞŶ ĚĞƌŝǀĞĚ
ĨƌŽŵ ƚŚĞ &ƌĞŶĐŚ ǁŽƌĚ puissance d’
hydrogen ;ŵĞĂŶŝŶŐ͗ ƉŽǁĞƌ ŽĨ
ŚLJĚƌŽŐĞŶͿ͘
ͻ
dŚĞ Ɖ, ŽĨ Ă ƐŽůƵƟŽŶ ĐĂŶ ďĞ
ŵĞĂƐƵƌĞĚ ďLJ ƵƐŝŶŐ ƌĞĂŐĞŶƚƐ ĐĂůůĞĚ
Ɖ, ŝŶĚŝĐĂƚŽƌƐ Žƌ Ɖ, ƉĂƉĞƌƐ͘ dŚĞ
ŝŶƐƚƌƵŵĞŶƚ ǁŚŝĐŚ ŝƐ ƵƐĞĚ ĨŽƌ
ŵĞĂƐƵƌŝŶŐ ƚŚĞ Ɖ, ŽĨ Ă ƐŽůƵƟŽŶ ŝƐ
Fig. 3.2 The pH values and the acidic or basic nature of any aqueous solution at 298 K (~ 25°C)
ŬŶŽǁŶ ĂƐ Ɖ, ŵĞƚĞƌ͘
How is acidic, neutral and basic character of a solution
related to its pH
The nature of an aqueous solution depends upon the relative concentrations
+
–
of H and OH ions in it. If,
–
+
ƒ [H ] = [OH ], then the solution is said to be neutral.
+
–
ƒ [H ] > [OH ], then the solution is said to be acidic. dŚĞ Ɖ, ǀĂůƵĞƐ ŽĨ ƐŽŵĞ ĐŽŵŵŽŶ
–
+
ƒ [H ] < [OH ], then the solution is said to be alkaline or basic. ƐŽůƵƟŽŶƐ
At 25°C (298 K), ^ŽůƵƟŽŶ pH
ƒ For pure water or a neutral solution pH = 7.0 ůŽŽĚ ϳ͘ϯϲ ʹ ϳ͘ϰϮ
ƒ For an acidic solution pH < 7.0 ^ĂůŝǀĂ ϲ͘ϱ ʹϳ͘ϱ
hƌŝŶĞ ϱ͘ϱ ʹϳ͘ϱ
ƒ For an alkaline solution pH > 7.0
'ĂƐƚƌŝĐ ũƵŝĐĞ ϭ͘Ϭ ʹ Ϯ͘Ϭ
Thus, at 298 K,
ŽīĞĞ ϰ͘ϱ ʹ ϱ͘ϱ
ƒ The pH values over the range 0 to 7 lie on the acidic side of the pH
scale. ĞĞƌ ϰ͘Ϭ ʹ ϱ͘Ϭ
ƒ The pH values over the range 7 to 14 lie on the alkaline (or basic) side dŽŵĂƚŽ ũƵŝĐĞ ϰ͘Ϭ ʹ ϰ͘ϰ
of the pH scale. tŝŶĞ Ϯ͘ϴ ʹ ϯ͘ϴ
>ĞŵŽŶ ũƵŝĐĞ Ϯ͘Ϯ ʹ Ϯ͘ϰ
What is the importance of pH in everyday life sŝŶĞŐĂƌ Ϯ͘ϱ ʹ ϯ͘ϱ
The concentration of hydrogen ion, and therefore, pH has marked effect on , ů ;ϭDͿ 0
CPKOCN CPF RNCPV NKHG 5QOG OCLQT ſGNFU QH KVU CRRNKECVKQPU CTG FGUETKDGF EĂK, ;ϭDͿ ϭϰ
below: E, K, ;ϭDͿ ϭϭ͘ϲ
4
ƒ The pH in our digestive system: pH plays a vital role in the digestion DŐ;K,Ϳ ;ƐĂƚƵƌĂƚĞĚͿ ϭϬ͘ϱ
2
of food and other biochemical reactions inside the human/animal ĂƩĞƌLJ ĂĐŝĚ Ϭ͘ϱ
body. The pH of gastric juice is 1–2 due to the secretion of hydrochloric ;ϰϬй , SO Ϳ
4
2
acid in stomach. During ingestion, the stomach produces more acid.