INTRODUCTION
In this article, we will learn to measure the pH of different samples. Let’s get started with understanding the concept.
BASIC BUILDING CONCEPTS
1. The substance which has a sour taste and changes the colour of blue litmus to red is known as an acid like vinegar.
2. The substance which has a bitter taste and changes the colour from red litmus to blue is known as a base like vinegar.
3. The mixture of two acids, two bases or one acid and one base is known as salt like NaCl.
4. The scale which is used to measure the strength of the acid, base and salt is known as the pH scale.
5. pH = -log[H+] = Log 1/[ H+].
6. pH has a scale from 0-14
7. The acidic character decreases from 0 to 6,
8. 7 is neutral.
9. Basic character increases from 8 to 14.
10. A pH metre is an instrument used to measure the pH of a solution.
11. pH paper or universal indicator is used to measure the approximate value of pH of a solution.
INDICATORS
Indicators are a kind of substance that can change their colour by changing the pH of a solution. There are many types of indicators available, but we mostly use litmus paper, methyl orange, and phenolphthalein. We must know that litmus paper is a natural indicator. On the other hand, methyl orange and phenolphthalein are synthetic (man-made) indicators.
| INDICATOR | ACIDIC MEDIUM | BASIC MEDIUM | NEUTRAL |
|---|---|---|---|
| Blue litmus | Red | Blue | Blue |
| Red litmus | Red | Blue | Red |
| Phenolphthalein | Colourless | Pink | Colourless |
| Methyl orange | Pink | Yellow | Pink |
UNIVERSAL INDICATOR
This homogenous indicator is formed by mixing common indicators. This indicator changes the colour by changing the pH.
| pH | COLOUR | pH | COLOUR |
|---|---|---|---|
| 0-2 | RED | 6 | GREENISH-YELLOW |
| 3 | ORANGE – RED | 7 | GREEN |
| 4 | ORANGE | 8-9 | BLUISH-GREEN |
| 5 | YELLOW | 10-11 | BLUE |
| – | 12-14 | VIOLET |
pH PAPER
Coated with a universal indicator used to measure the approximate pH value of a solution.
NATURE OF CHEMICALS USED IN THIS EXPERIMENT
1. HYDROCHLORIC ACID
The chemical formula for hydrochloric acid is HCl. Its pH value is less than 7. It is a very strong acid. In the laboratory, mainly, it is prepared by dissolving hydrogen chloride gas in water. In an aqueous solution, it is completely ionised and forms hydrogen and chloride ions.
HCl → H+ (aq) + Cl- (aq)
HYDROCHLORIC ACID HYDROGEN ION CHLORIDE ION
2. SODIUM HYDROXIDE
The chemical formula for sodium hydroxide is NaOH. Its pH is more than 7, and that’s why it is a strong base. In water, it is completely ionised to form sodium and chloride ions.
NaOH(aq) → Na+(aq) + OH-(aq)
SODIUM HYDROXIDE SODIUM ION HYDROXIDE ION
3. ETHANOIC ACID
The other name for ethanoic acid is acetic acid, and its chemical form is CH3COOH. Its pH value is less than 7, and it is a weak acid. In water, it gives hydrogen ions and acetate ions.
CH3COOH → H+(aq) + CH3COO-(aq)
ETHANOIC ACID HYDROGEN ION ACETATE ION
4. SODIUM BICARBONATE
The other name for sodium bicarbonate is sodium hydrogen carbonate and baking soda. Its pH value is greater than 7. Its chemical formula is NaHCO3. It is a very weak base. In water, it gives out sodium ions and bicarbonate ions.
NaHCO3 → Na+(aq) + HCO3-(aq)
SODIUM BICARBONATE SODIUM ION BICARBONATE ION
5. LEMON JUICE
The nature of lemon juice is known to be acidic, and it has a pH which is less than 7.
6. WATER
Water is known to be neutral and has a pH, whose value is equal to 7. Its chemical formula is H2O.
AIM
To determine the pH of the following solutions by using pH paper or a universal indicator.
1. Dilute HCl,
2. Dilute NaOH,
3. Dilute CH3COOH,
4. Lemon juice,
5. Water,
6. NaHCO3
MATERIAL REQUIRED
1. Test tubes,
2. Test tube stand,
3. Glass rod,
4. pH paper or universal indicator,
5. Standard colour chart,
6. Glazed white tile,
7. Samples of dilute HCl, dilute NaOH, dilute CH3COOH, lemon juice, distilled water, and NaHCO3.
THEORY
1. The measurement of the hydrogen ion concentration of a solution is the pH of that solution.
2. For an acidic solution, hydrogen ion concentration is greater than 10^(-7) mol/l and pH is less than 7.
3. For a basic solution, hydrogen ion concentration is less than 10^(-7) mol/l, and pH is greater than 7.
4. For a neutral solution, hydrogen ion concentration is equal to 10^(-7) mol/l, and pH equals 7.
5. Using pH paper and universal indicators, we can measure a solution’s pH value.
PROCEDURE
1. Take six test tubes and in each test tube, put the above solution separately and make them stand on the test tube stand as shown in the figure.
2. Place six strips of pH paper on the glazed tile.
3. By Using a glass rod or dropper, drop one of the six solutions on pH paper.
4. Compare the colour obtained on pH paper with the colour chart for pH paper.
5. Note down the value given in front of the pH colour chart.
6. Perform this same thing with the remaining five solutions by using fresh pH paper and a new dropper every time.
7. Write down your observation in the given table.
8. Now, drop a few drops of universal indicator to each of the solutions.
9. Compare the colour obtained with the colour on the universal indicator bottle.
10. Note down your observation.
| S.NO | SAMPLE SOLUTIONS | COLOUR PRODUCED ON pH PAPER | COLOUR PRODUCED ON UNIVERSAL INDICATOR | APPROXIMATE pH | INFERENCE |
|---|---|---|---|---|---|
| 1. | Dil. Hydrochloric acid | acidic | |||
| 2. | Dil. Sodium hydroxide | basic | |||
| 3. | Dil. Ethanoic acid | acidic | |||
| 4. | Lemon juice | acidic | |||
| 5. | Water | neutral | |||
| 6. | Dil. Sodium bicarbonate | basic |
RESULT
1. We have found that dilute hydrochloric acid, dilute ethanoic acid, and lemon juice are acidic in nature, and their pH is less than seven.
2. We have found that dilute sodium hydroxide and dilute sodium bicarbonate are basic, and their pH is greater than seven.
3. Water is neutral as its pH is equal to seven.
PRECAUTION
1. Note the colour of each pH paper carefully.
2. Do not touch the solution with naked hands.
3. Don’t use used pH paper.
4. Compare the colour carefully.
5. Touching the pH paper with dirty hands is forbidden.
CONCLUSION
In this way, we have learned to determine the pH of samples.
VIVA QUESTIONS WITH ANSWERS
Q.1 What do you mean by the acidic solution?
ANS. The substance which has a sour taste and changes the colour of blue litmus to red is known as acid like vinegar.
Q.2 What do you mean by the basic solution?
ANS. The substance, which has a bitter taste and changes the colour from red litmus to blue, is known as a base like vinegar.
Q.3 What do mean when you say salt?
ANS. The mixture of two acids, two bases or one acid and one base is known as salt like NaCl.
Q.4 What do you understand by pH scale?
ANS. The scale which is used to measure the strength of acid, base and salt is known as the pH scale.
Q.5 Write the full form of pH?
ANS. Potenz de hydrogen?
Q.6 Who discovered pH?
ANS. Peter Sorensen
Q.7 What was the aim of our experiment?
ANS. To measure the pH of different solutions.
Q.8 What is the chemical formula of ethanoic acid?
ANS. CH3COOH
Q.9 What is the other name given to ethanoic acid?
ANS. Acetic acid
Q.10 What is the chemical name of baking soda?
ANS. Sodium bicarbonate
Saquib Siddiqui is a Mechanical Engineer with expertise in science projects and experiments. Saquib’s work focuses on integrating scientific concepts with practical applications, making complex ideas accessible and exciting for learners of all ages. In addition to his practical work, Saquib has authored several articles, research papers, and educational materials.
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