Explanation of the pH Scale
The pH Scale: pH scale is a scale for measuring hydrogen ion (H+) concentration. The concentration of H+ ion is generally small, therefore the concentration of H+ ion is expressed in terms of pH. pH is defined as the negative logarithm of H+ concentration or H3O+ concentration.
pH = -log[H+]
OR
pH = -log[H3O+]
(i) pH stands for the power of hydrogen.
(ii) pH < 7 is acidic, pH = 7 is neutral, pH>7 is basic.
| Solution pH | Solution pH |
| Conc. HCl 0 | Blood 7.4 |
| Dilute H 2 SO 4 1 | Eggs 7.8 |
| Lemon juice 2.5 | Toothpaste 8 |
| Vinegar 4 | Baking soda 8.5 |
| Coffee 5 | Washing soda 9 |
| Soft drink 6 | Milk of magnesia 10.5 |
| Milk 6.5 | Household ammonia 11.6 |
| Pure water 7 | Dilute Na OH 13 |
| Saliva 7.4 | Conc. Na OH 14 |
pH Paper: The paper which is coated with a universal indicator is known as pH paper.
The relation between H+ conc. and pH
pH = -log[H+]
The higher the concentration of H+, the lower will be pH. pH paper or universal indicator solution is used to determine the approximate pH of the solution. Litmus solution can only tell us that the solution is acidic or basic.
pH of Bases
The basic solution also has H+ ions but still, they are bases because [OH–] is more than [H+], therefore, the pH of bases is more than 7.
Measuring of pH
- pH test papers
The pH test papers are impregnated with one or more indicator dyes. To obtain a rough idea of the pH, a drop of the solution to be tested is touched to a strip of the test paper and the resulting colour is compared with a colour code.
- Litmus paper
Litmus paper is often used qualitatively to test whether a solution is acidic or basic. It consists of porous paper impregnated with litmus dye, made either red or blue by exposure to acid or base, and then dried.
- pH meters
In a pH meter, an electrode system sensitive to the hydrogen ion concentration in a solution is first dipped into a solution of known pH to calibrate the instrument. Once calibrated, the apparatus can then be used to measure the pH of any other solution simply by immersing the electrode into it.
Most modern pH meters are able to determine pH values to within 0.01 pH units, and research-grade instruments are capable of even greater precision in the pH range of 0 to 14.
Application of pH in daily Life
- Plants grow in soil having a specific pH.
- The pH of the stomach is 2.0 due to the secretion of HCl. In the case of indigestion, acidity increases which can be neutralized by antacids like milk of magnesium, and sodium bicarbonate are weak bases.
- Digestion of carbohydrates does not take place in the stomach due to acidic pH in which salivary amylase becomes inactive.
- Cold drinks, sweets, and chocolates are most harmful to health as well as the tooth. They produce acids in the mouth which are responsible for tooth decay. You may be surprised to know that the tooth gets dissolved in the cold drink if kept for a long time. Therefore, we must avoid cold drinks and sweets and should brush our teeth after every meal so as to prevent tooth decay.
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