Finding the pH of the samples by using pH paper/universal indicator:

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Aim

To find the pH of the following samples by using pH paper/universal indicator:
(i) Dilute Hydrochloric Acid
(ii) Dilute NaOH solution
(iii) Dilute Ethanoic Acid solution
(iv) Lemon juice
(v) Water
(vi) Dilute Hydrogen Carbonate solution

Materials Required

• Six test tubes
• Test tube stand
• Glass rod
• Measuring cylinder (10 mL)
• Standard pH colour chart
• pH paper
• Glass rod
• Dropper
• Universal indicator
• Samples of dilute HCl, dilute NaOH, dilute CH₃COOH, lemon juice, Water, dilute NaHCO₃

Theory

pH is the 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

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.

Procedure

1. Wash six test tubes with distilled water and put them in test tube stand and label them as A, B, C, D, E, F.
2. Take the given solutions in separate test tubes.
3. Take glazed white tile, place 6 pH papers 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 Table

S. No.	Sample Solution	Colour on pH paper	Approximate pH	Inference
A	Dil. HCl	2	Pink	Acidic
B	Dil. NaOH	14	Purple	Basic
C	Dil. CH3COOH	3	Orange	Acidic
D	Lemon Juice	2	Pink	Acidic
E	Water	7	Green	Neutral
F	Dil. NaHCO3	9	Turquoise (Light Blue)	Basic

Result

• The pH of dil. HCl, CH₃COOH and lemon juice is less than 7 and therefore, they are acidic in nature.
• The pH of dil. NaOH and NaHCO₃ is more than 7 and therefore, they are basic in nature.
• The pH of water is 7 and therefore, it is neutral.

Precautions

• The acids and bases solution should be used with care.
• Use distilled water for preparing solutions.
• Use only standard colour charts for finding the pH value.
• Do not touch the pH paper with dirty and wet hands.
• Glass rod or dropper used for one sample should be washed thoroughly before using it for the other samples.