A Brief Note on Determining the pH of Samples
Chemicals have their distinct nature - they can be neutral or acidic or basic. These characteristics primarily depend on how many H+ or OH- ions a particular chemical is releasing in its aqueous solution.
For example, chemicals that release H+ ions are acidic, whereas those which release OH- ions are basic.
These experiments are an integral part of Class 10 practical chemistry syllabus. Hence, to know about pH of samples and how to determine that, keep on reading this article!
Theory:
Definition of pH
pH is defined as the negative logarithm (base 10) of hydrogen ion concentration. It is the most common way to determine the strength of base/acid. Often it is termed as “potential of hydrogen ion”. It is the molarity of H+ ion.
Following is the pH value of different chemicals on a scale from 0 to 14.
Acid pH<7
a pH of Base>7
pH of neutral =7.
pH Scale
A pH scale is used to determine the nature of a substance. Its value ranges from 0 to 14. This scale is logarithmic. It means that if an integer value increases or decreases, the concentration changes tenfold, accordingly.
For example, pH 2 is ten times more acidic than pH 3. Furthermore, pH 12 is 100 times more basic than pH 10.
pH Paper
A pH paper is a piece of paper used to find out if a solution is basic, acidic or neutral. This is determined by dipping part of the paper into a solution and observing the colour change.
A pH paper changes colour in different solutions due to the chemical flavin. This molecule, which is an anthocyanin, is soluble in water and changes colour in the presence of various types of solutions.
The packaging a pH paper comes in often includes a colour-coded scale indicating the pH. This works in the following ways –
Solution Type | Colour |
Acidic | Red |
Basic | Greenish-blue |
Neutral | Light green |
Universal Indicator
It is a mixture of several indicators and demonstrates a range of colours based on the pH of any particular solution. It is generally available in two forms, liquid solution of ethanol and pH paper, soaked in indicator solution.
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Salts: Salts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food.
Characteristics of salt:
- Most of the salts are crystalline soild.
- Salts may be transparent or opaque.
- Most of the salts are soluble in water.
- Solution of the salts conducts electricity in their molten state also.
- The salt may be salty, sour, sweet, bitter and umami (savoury).
- Neutral salts are odourless.
- Salts can be colourless or coloured.
Family of Salt: Salts having common acidic or basic radicals are said to belong to the same family.
Example:
(i) Sodium chloride (NaCl) and Calcium chloride (CaCl2) belongs to chloride family.
(ii) Calcium chloride (CaCl2) and Calcium sulphate (CaSO4) belongs to calcium family.
(iii) Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belongs to the zinc family.
Neutral, Acidic and Basic Salts:
(i) Neutral Salt: Salts produced because of reaction between a strong acid and strong base are neutral in nature. The pH value of such salts is equal to 7, i.e. neutral.
Example : Sodium chloride, Sodium sulphate. Postassium chloride, etc.
Sodium chloride (NaCl): It is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).
Sodium Sulphate (Na2SO4): It is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid ( a strong acid).
Potassium Chloride (KCl): It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid).
(ii) Acidic Salts: Salts which are formed after the reaction between a strong acid and weak base are called Acidic salts. The pH value of acidic salt is lower than 7. For example Ammonium sulphate, Ammonium chloride, etc.
Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).
Ammonium sulphate is formed after reaction between ammonium hydroxide (a weak base) and sulphuric acid (a strong acid).
(iii) Basic Salts: Salts which are formed after the reaction between a weak acid and strong base are called Basic Salts. For example; Sodium carbonate, Sodium acetate, etc.
Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak acid).
Sodium acetate is formed after the reaction between a strong base, sodium hydroxide (a strong base) and acetic acid, (a weak acid).
Cause of formation of acidic, basic and neutral salts:
- When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this, an acidic salt is formed.
- When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this, a basic salt is formed.
- When equally strong acid and a base react, they fully neutralize each other. Due to this, a neutral salt is formed.
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Some Important Chemical Compounds
1. Common Salt (Sodium Chloride): Sodium chloride (NaCl) is also known as Common or Table Salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in the manufacturing of many chemicals.
Important chemical from sodium chloride
Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base. It is also known as caustic soda. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as by products. This whole process is known as Chlor – Alkali process.
Use of products after the electrolysis of brine:
- Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.
- Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in the manufacturing of bleaching powder and hydrochloric acid.
- Sodium hydroxide is used for degreasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.
