pH Value

pH Value

In 1909 AD, the Danish chemical Sorenson introduced a new method for revealing the concentration of hydrogen ions in a solution, called the pH scale. At this pH, p denotes a Germanic word potenz which means power. Whereas H reveals hydrogen ion concentration [H +]. The pH scale has values ​​ranging from 0 to 14. a pH is a pure number that has no unit.

According to Sorenson, a negative logarithm of the hydrogen ion concentration present in a solution is called the pH value of that solution.
pH = -log [H+]
The pH value of a solution is equal to the negative logarithm of the concentration in gram ions (mole) per liter of hydrogen ions present in it. In general, the pH value of solutions is between 0–14.

pH Value

According to this:

  1. The pH value of distilled water is 7.
  2. If a solution has a pH value of 7, it will be neutral.
  3. If a solution has a lower than pH value of 7, it will be acidic.
  4. If the pH value of a solution is more than 7, it will now be basic.
  5. The pH value decreases with increasing temperature.
  6. The solutions whose pH value is 0 are acidic.

The pH values ​​of some substances are as follows-

Sl No. Substances pH Value Sl No. Substances pH Value
1. Conc. HCl 0.0 7. Tear 7.4
2. Juice of lemon 2.2-2.4 8. Egg 7.8
3. Vinegar 2.4-3.4 9. Grapes 3.1-4.5
4. Milk 6.3-6.6 10. Banana 4.5
5. Tomato 4.7 11. Bleach 12.5
6. Blood 7.4 12. Washing soda 9.0

pH Scale

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

pH value

The pOH value of a solution is equal to the negative logarithm of the concentration of the hydroxyl ions present in the solution while the base of the logarithm is 10 so

pOH = -log [OH

Relation Between pH and pOH

[H+][OH] = KW = 1 × 10-14

Or,      pH + pOH = pKw = 14

  1. pH > pOH = acidic solution
  2. pH < pOH = basic solution
  3. pH = pOH = neutral solution

Acid-Base Indicator

Acid-Base indicators (also known as pH indicators) are substances that change color with pH. In other words, an Acid-base indicator is a substance that gives different colors in acidic and alkaline solutions. An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH) ions changes in an aqueous solution.

pH Range For Indicator

The change in color of an acid-base indicator occurs in a certain range rather than at a certain pH, which is called the pH range. pH range and colors in acidic and basic solutions of the different indicator are mentioned below-

Indicator solutions   pH range Color in acidic solutions Color in basic solutions
Methyl Orange 3.1-4.4 Red Yellow
Litmus Paper 5.0-8.0 Red Blue
Phenolptheline 8.3-10.0 Colorless Pink
Phenol Red  6.8-8.4 Yellow  Red
Methyl Red 4.2-6.3 Red Yellow


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