# pH Of Acids And Bases

## What is pH?

We know that all the acids and bases do not react with the same chemical compound at the same rate. Some react very vigorously, some moderately while others show no reaction. To determine the strength of acids and bases quantitatively, we use a universal indicator which shows different colours at different concentration of hydrogen ion in solution. Generally, the value of pH of acids and bases are used to quantitatively determine their strength. pH is defined as the negative logarithm of H+ ion concentration. Hence the meaning of the name pH is justified as the power of hydrogen.

#### pH of acids and bases:

The pH of a solution varies from 0 to 14. Solutions having a value of pH ranging 0 to 7 on pH scale are termed as acidic and for the value of pH ranging 7 to 14 on pH scale are known as basic solutions. Solutions having the value of pH equal to 7 on pH scale are known as neutral solutions.

Solutions having the value of pH equal to 0 are known to be strongly acidic solutions. Further, the acidity decreases as the value of pH increases from 0 to 7 whereas, solutions with the value of pH equal to 14 are termed as strongly basic solutions.

The basicity decreases as the value of pH decreases from 14 to 7. The strength of acids and bases depends on the number of H+ and OH ions produced. Acids furnishing more number of H+ ions are known to be strong acids and vice versa.

The degree of ionisation of acids and bases differ for different acids and bases. It helps in the determination of strength of acids and bases. The strength of an acid depends on the concentration of hydronium ion (H3O+) too. With the help of comparison between the concentration of hydronium ion and the hydroxyl ion, we can distinguish between acids and bases.

For acidic solution: [H3O+] > [OH]

For neutral solution: [H3O+] = [OH]

For basic solution: [H3O+] < [OH]

### pH Scale

pH Scale

The concentration of hydronium ion is conveniently expressed on a logarithmic scale. This scale is known as pH scale. pH of acids and bases is defined as the negative logarithm (with base 10) of activity of hydrogen ion (H+).

#### How to calculate pH?

Activity has no units and is defined in terms of H+ ion concentration,

$a_{H^+}$ = [H+]/mol L-1

Hence, the pH of a compound is given as,

pH = -log $a_{H^+}$

#### Example:

Thus, the pH of an acidic solution of HNO3 (10–3 M) = 3, a basic solution of KOH having [OH] =10–4M and [H3O+] =10–10 M will have a pH = 10. pH of acids is generally less than 7 whereas for bases it is greater than 7. At 298 K, ionic product of water, Kw can be given as:

Kw = [H3O+] [OH] = 10–14

Taking negative logarithm of RHS and LHS, we deduce

–log Kw = – log ([H3O+] [OH])=   – log 10–14

=>pKw= – log [H3O+] – log [OH] = 14

=>pKw = pH + pOH = 14

The value of pKw controls the relative concentrations of hydrogen and hydroxyl ions as their product is always constant. pH of acids and bases can be roughly calculated with the help of pH scale while for accurate values we use pH meters.

pH example chart

### Importance of pH:

• Only a narrow range of pH change can be sustained by living organism, any further change in pH can make the living difficult. For example: in case of acid rain, the pH of water is less than 7. As it flows into a river, it lowers pH of river water which makes the survival of aquatic life difficult.
• We know that our stomach contains hydrochloric acid which helps in the digestion of food. When stomach produces too much of hydrochloric acid during indigestion, we feel a lot of pain and irritation. Hence, we generally use antacids or a mild base which increases the pH of the acidic stomach and thus decreases the pain.
• Bacteria present in our mouth sometimes lower the pH of our mouth by producing acids through degradation of food particle. Hence, we are instructed to clean our mouths with toothpaste (which are generally basic) to prevent their decay by maintaining the pH.
• We experience a lot of pain in case of bee-sting as the bee injects the methanoic acid through its sting. Hence, we are generally advised to apply baking soda or other mild bases on the surface as it helps in maintaining the pH of the surface.

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#### Practise This Question

Blue color X changes to white color Y on heating and When we moisten Y, Blue color X reappers. This is because?