pH is used as a unit to determine the nature of an aqueous solution, a measure of the acidity, alkalinity or neutrality of the solution. The pH of a solution is the negative logarithm of the concentration of hydrogen ion present in the solution. It is expressed as,
Where p is written in small hand letter and H is capitalized. [H+] expresses the molar concentration of H+ ion.
The presence of H+ in 1 liter distilled water is 10-7 moles. The pH of distilled water is
So, we get the pH of distilled water = 7. Any ion within the third bracket in molarity unit, the concentration of that ion is expressed in this way.
If acid is added to distilled water and for this reason, the number of H+ ion increases 10 times to make it 10-6 mole per liter. Hence, the pH of the solution will drop down.
The more the molarity of H+ ion, the lesser the value of pH will be.
When alkali is added to distilled water, the OH– of alkali will undergo reaction with H+ and lessen the number of H+. For example, due to adding alkali in water, if the number of H+ drops down to 10-10 mole per liter then the pH will be:
That means the pH value will rise from 7; the pH value of the alkaline solution is more than 7.
The significance of pH value 7 is that the solution is neither alkali nor acid. Hence, it is a neutral solution. When the value of pH is less than 7, the solution is acid and when the value of pH is more than 7, the solution is alkaline.
Measurement of pH is done using the pH scale.
Although in mathematical terms, the value of pH can be any number from negative to positive. In reality, the value is measured on a scale of 0 to 14. The pH of a neutral solution is 7. You have already known that the value of the acidic solution is less than 7 and the value of the alkaline solution is more than 7.
So, according to the scale, the strongest acid will have a value of 0 and the strongest alkali will have a value of 14.
You have already learned how to measure pH of a solution from the molarity of its hydrogen ion. Now you will learn how to measure it in an experiment. The process is involved in the universal indicator, pH paper, pH meter etc.
The Universal indicator is the mixture of various acid and base indicators. The universal indicator turns to different colours for different value of pH. There is a chart of colours for different values of pH in the Universal indicator. This is known as Universal Indicator Colour Chart. To know the value of pH of an unknown solution, you have to add few drops universal indicator in the solution and then compare the colour of solution with the standard colour chart in it and determine the value of pH.
pH Paper is used for knowing the value of an unknown solution. If you add a small piece of pH paper in the solution then it will change its colour. There is a standard colour chart for various pH values. Comparing the colour of the pH paper with that standard colour chart one can determine the value of pH.
pH Meter is used for knowing the value of pH of an unknown solution. Inserting the electrode of the pH meter in the unknown solution you can know the value of pH directly from the digital display of the pH meter.
Litmus paper is the easily available cheaper alternative way to know the approximate value of pH. The litmus paper is red in colour when the value of pH is less than 7 and the colour is blue when the value is more than 7.
Importance of pH
pH has limitless significance in terms of biochemical reactions in the animal body and use of cosmetics.
pH has a great importance in agriculture. Plants absorb various ions, water etc from soil for its own nutrition. The best value of pH of the soil is 6.0-8.0. If the value is less than 3.0 or more than 10.0 the beneficial microbes in the soil will die.
When the value drops down, a measured amount of CaO is used in the soil. On the other hand, if the value of pH of soil rises up, a measured amount of ammonium sulfate [(NH4)2SO4] or ammonium phosphate [[NH4)3PO4] fertilizers are used to settle the pH of the soil.
pH in Different Biochemical Reactions of Body
Various types of biochemical reactions occur in different parts of the animal body. They require different values of pH in various parts of the body.
Photo- Human biology lab
Human beings use the different type of chemicals to clean their skin, beautification of skin, clean the hair and other actions. When the pH of skin remains 4.8- 5.5, the skin is acidic and that prevents the attack of microbes on the skin. Therefore, the pH of cosmetics is maintained at 4.8-5.5.
Source- NCTB Books