The pH scale

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The pH scale

pH of a solution is the negative logarithm of the concentration of hydrogen ions in mole per litre.

We use the pH scale for denoting the concentration of hydrogen ions. pH notation was devised by the Danish biochemist Soren Sorensen in 1909. The term pH means ‘power of hydrogen’. The pH is the logarithm (see box) of the reciprocal of the hydrogen ion concentration. It is written

        pH = - log[H+]

If pH < 7 then solution is acidic
If pH > 7 then solution is basic
If pH = 7 then solution is neutral

pH Scale

» The pH scale ranges from 0 to 14 on this scale.
» pH 7 is considered neutral, below 7 acidic, and above 7 basic.

HC1 (4%) 0
Stomach acid 1
Lemon juice 2
Vinegar 3
Oranges 3.5
Soda, grapes 4
Sour milk 4
Fresh milk 5
Human saliva 6 - 8
Pure water 7
Blood plasma 7.4
Egg white 8
Sea water 8
Baking soda 9
Antacids 10
Ammonia water 11
Lime water 12
Drain cleaner 13
Caustic soda 4% (NaOH) 14

Importance of pH in Everyday Life

» Most of the biochemical reactions taking place in our body are in a narrow pH range of 7.0 to 7.8. Even a small change in pH disturbs these processes.

» When the pH of rain water falls below 5.6, it is called acid rain. When acid rain flows into rivers, the pH of the river water also falls, and it becomes acidic. As a result, the survival of aquatic life becomes difficult.

» Our stomach produces hydrochloric acid, which helps in digestion of food. When we eat spicy food, stomach produces too much of acid which causes ‘acidity’, i.e., irritation and sometimes pain too. To get rid of this, we use ‘antacids’, which are bases like ‘milk of magnesia’ (suspension of magnesium hydroxide in water).

» Tooth enamel is made of calcium phosphate, which is the hardest substance in our body and can withstand the effect of various food articles that we eat. If mouth is not washed properly after every meal, the food particles and sugar remaining in the mouth undergo degradation due to the bacterial present in the mouth.

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