Science and Technology for UPSC Examinations [Chemistry] Part 5- pH, Acids and Bases

pH

In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion. Pure water has a pH very close to 7 at 25°C. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. The maintenance of pH is very essential in biological systems where life processes can take place only within a limited range of pH. The functioning of enzymes is sharply pH dependent. The pH of blood and intra cellular fluids in our body also has to be maintained to a particular range for proper functioning of the different systems. In industries, many chemical reactions take place only at a specified pH. Hence, it is necessary to understand the term pH and the related facts.

Acids:

The term acid is derived from the Latin word acidus meaning sour. Acids can be defined in many ways but generally their aqueous solutions have the following properties.

• They are sour in taste.
• They turn blue litmus red.
• They react with certain metals and liberate hydrogen gas.
• They react with oxides and hydroxides of metals forming salt and water.
• Their aqueous solutions conduct electricity

Bases

Bases are defined in various ways but generally substances having the following characteristics are called bases.

• They have a bitter taste.
• Their aqueous solutions have a soapy touch.
• They turn red litmus blue.
• They react with acids to form salt and water.
• Their aqueous solutions conduct electricity.

According to Arrhenius, an acid is a substance which gives hydrogen ions in its aqueous solution. A base is a substance which gives hydroxyl ions in its aqueous solution. For example, substances such as Nitric acid, Hydrochloric acid, acetic acid are acids whereas substances such as sodium hydroxide, potassium hydroxide and ammonium hydroxide are bases according to this concept

Weak and Strong acids and bases

Acids such as Hydrochloric Acid (HCl), Sulphuric Acid (H₂SO₄) and Nitric Acid (HNO₃) which are almost completely ionised in aqueous solution are termed as strong acids. Acids such as acetic acid (CH₃COOH) is partially ionised and is called a weak acid. Similarly, bases like NaOH and KOH are almost completely ionised in aqueous solution and are therefore called strong bases. Ammonium Hydroxide is partially ionised and is called a weak base.

According to the Bronsted and Lowry theory, an acid is a substance that has the tendency to lose a proton and a base is a substance that has the tendency to accept a proton.

Acids and Bases: The notable points:

• Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases.
• Acidic nature of a substance is due to the formation of H+(aq) ions in solution. Formation of OH–(aq) ions in solution is responsible for the basic nature of a substance.
• When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed. When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.
• When an acid reacts with a metal carbonate or metal hydrogen carbonate, it gives the corresponding salt, carbon dioxide gas and water.
• Both acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.

pH Scale

The acidity or basicity of a solution is usually expressed in terms of function of the hydrogen ion concentration. This function is called pH of a solution. pH of a solution may be defined as the negative logarithm (to the base 10) of hydrogen ion concentration expressed in moles per litre.

pH = log₁₀
[ H]

For pure water and neutral solutions, at 298 K, concentration of hydrogen ions is 1 x 10-7 mol L-1. These hydrogen ions are formed by ionisation of some of the water molecules.

H2O(l) ↔ H(aq) ⁺ +
OH(aq) ^–

Thus, for pure water at 298 K,

[ H+ ] = [ OH- ] = 1 x10^-7 mol L-1

Sp pH of distilled water is :

-log (1 x 10^-7) = 7

The Approximate values of pH for some familiar solutions are as follows:

[table id=29 /]

Red Cabbage Juice as Acid Base Indicator

Red cabbage juice is known as an acid/base indicator because it has pigments in it that react differently to acids and bases. These pigment change color when exposed to an acid or a base. Cabbage juice is naturally neutral and it has a purplish color. If an acid is added, it turns pink. If a base is added, it turns green.

For example, the juice will turn pink when lemon juice is added to it.

It changes to green when toothpaste is mixed with it, because toothpaste is basic in nature. The tooth paste should be basic due to the reason, that some acids are formed by the bacteria in our mouth and these are neutralized by the bases present in the tooth paste.