pH of Samples

The nature of the chemicals used in laboratories is either basic, acidic or neutral. This characteristic depends on the ions they release. A chemical is said to be acidic if it releases H⁺ ions in its aqueous solutions. A chemical is said to be basic if it releases OH^– ions in its aqueous solutions. In this experiment, we will learn how to find the pH of acids and bases.

Aim:

To determine the pH of the given samples using pH paper or universal indicator. The samples whose pH has to be determined are-

1. Dilute CH₃COOH
2. Dilute NaOH
3. Salt NaCl
4. Dilute NaHCO₃
5. Water
6. Lemon juice

Materials required:

1. Six test tubes
2. Test tube stand
3. Dilute acid CH₃COOH
4. Dilute base NaOH
5. Salt NaCl (preparation: dissolve 1 gram salt in 10 mL distilled water)
6. Water
7. Lemon juice
8. Dilute NaHCO₃
9. Glass rod
10. Measuring cylinder (10 mL)
11. Standard pH colour chart
12. pH paper
13. Glass rod
14. Dropper
15. Universal indicator

Theory:

What is pH?

pH is a measure of hydrogen ion concentration to determine the alkalinity or acidity of a solution.

• If the pH value of a solution is less than 7 it is an acidic solution
• If the pH value of a solution is greater than 7 it is a basic solution
• If the pH value of a solution is equal to 7 it is a neutral solution

What is pH scale?

The pH scale consists of values which range from 0 (very acidic) to 14 (very alkaline). The numbers on the scale help to determine the hydrogen ion concentration.

What is pH paper?

pH paper can help us know if a solution is basic, acidic or neutral. When the pH paper is dipped into a solution whose pH has to be determined, a colour will be developed. This colour is compared with the standard pH colour chart. Instead of pH paper, we can also use universal indicator paper or universal indicator solution.

What is universal indicator?

A universal indicator is a mix of pH indicator solutions that are designed to determine the pH of solutions over a wide range of values. Put a drop of solution on the universal pH indicator paper. The colour developed on the paper is matched with the standard pH colour chart.

pH value chart:

Experimental Setup:

Procedure:

1. Wash six test tubes with distilled water and put them on test tube stand and label them A, B, C, D, E, F.
2. Add 2ml of CH₃COOH in test tube A, Add 2ml of NaOH in test tube B, Add 2ml of NaCl in test tube C, Add 2ml of NaHCO₃ in test tube D, Add 2ml of Water in test tube E, Add 2ml of Lemon juice in test tube F.
3. Take white tile, place 6 pH paper and label them A, B, C, D, E, F.
4. Use a dropper or glass rod to put the respective sample solutions on the labelled pH paper placed on the white tile.
5. Observe the colour change.

Observation:

Result and Conclusion:

Precautions to be taken during the experiment:

• Use freshly prepared test sample for the experiment.
• The fruit juice sample should also be fresh to get the proper pH values.
• Glass rod or dropper used for one sample should be washed thoroughly before using it for the other samples.

Viva Voce:

1. Who was the first scientist to introduce pH?
   Ans: Soren Peder Lauritz Sorensen a Danish chemist.
2. Define the pH of a solution.
   Ans: It is a measure of the basic or acidic power of a solution. It is a scale to measure the hydrogen ion concentration in the solution. Lower the hydrogen ion concentration higher is the pH.
3. What is the pH of pure water at 25 °C?
   Ans: 7.
4. There are two solutions with different pH values. Solution A has a pH value 6 and solution B has a pH value 5. Which solution is more acidic?
   Ans: Solution B whose pH value is 5 is more acidic.
5. If the pH value of a solution is 9 then what is its colour?
   Ans: Light blue.