S.P.L Sorenson, a Danish biochemist in 1909 devised a scale known as pH to represents the H+ ion concentration of an aqueous solution. The pH value of a substance is a number which simply represents the acidity and basicity of the substance. The pH value of any solution is numerically equal to the logarithm of the inverse of the hydrogen ion (H+) concentration. Hence, the pH of solution is referred to as the negative logarithm of hydrogen ion.
pH = -log [H+]
= log 1/ [H+]
Basic concept of pH value
- pH of Neutral Solution (Pure Water): pH of water is 7. Whenever the pH of a solution is 7, it will be a neutral solution. Such a solution will have no effect on any litmus solution or any other indicator.
- pH of an Acidic Solution: All the acidic solutions have a pH less than 7. So, whenever a solution has a pH less than 7, it will be acidic in nature and it will turn blue litmus into red as well as methyl orange pink and phenolphthalein colourless.
- pH of a basic solution: All the alkaline solution have a pH of more than 7. S, whenever a solution has a pH less than 7, it will be basic in nature and it will turn red litmus, blue, methyl orange yellow and phenolphthalein pink.
pH values of the common substance from our daily life
| |||
Solution
|
pH
|
Solution
|
pH
|
Conic HCL
|
0
|
Saliva (before meal)
|
7.4
|
Oil HCL
|
1.0
|
Saliva (after meal)
|
5.8
|
Gastric Juice
|
1.4
|
Blood
|
7.4
|
Lemon Juice
|
2.5
|
Eggs
|
7.8
|
Vinegar
|
4.0
|
Toothpaste
|
8.0
|
Tomato Juice
|
4.1
|
Baking Soda Solution
|
8.5
|
Coffee
|
5.0
|
Washing Soda Solution
|
9.0
|
Soft Drink
|
6.0
|
Milk of Magnesia
|
10.5
|
Milk
|
6.5
|
Household Ammonia
|
11.6
|
Pure Water
|
7.0
|
Dilute Sodium Hydroxide
|
13.0
|
Concentrated Sodium Hydroxide
|
14
| ||
Things to remember about pH Value and concentration
- An acid solution having low pH is stronger than another solution having higher pH values. A solution having pH of 2 is a stronger than a solution having pH of 5.
- An alkali solution having higher pH value is a stronger than a solution of pH 10.
- Strong acids solution can have pH values less than zero and strong basic solutions can pH greater than 14.
Universal Indicator of colour at pH scale
It is a mixture of different indicators (or dyes) which gives different colours at different pH values of the entire pH scale. The colour produced by universal indicators at various pH values in the given table:
Colour
|
pH
|
Colour
|
pH
|
Dark Red
|
0
|
Greenish Yellow
|
8
|
Red
|
1
|
Blue
|
9
|
Red
|
2
|
Navy Blue
|
10
|
Orange Red
|
3
|
Purple
|
11
|
Orange
|
4
|
Dark Purple
|
12
|
Orange Yellow
|
5
|
Violet
|
13
|
Greenish Yellow
|
6
|
Violet
|
14
|
Green
|
7
| ||
Importance of pH
- In Agriculture: By determining the pH of the soil. We can find whether it is acidic or alkaline. This helps in deciding the type of fertilizer to be used and the types of crops to sown.
- In Biological process: By knowing pH we can adjust the medium of biological processes like fermentation, enzyme hydrolysis, sterilization etc.
- In corrosion research: By measuring the pH of sea-water, the effect of alkaline sea-water on the material used for building ships and submarines is studied.
Buffer Solution
A Buffer Solution or a buffer is defined as a solution whose pH does not change when small amount of an acid or base is added in it. It is used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. In nature, the bicarbonate buffering system is used to regulate the pH of blood. Buffer solutions achieve their resistance to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A−. HA is in equilibrium with H+ + A−.