Lab Manual: pH of Samples | Lab Manuals for Class 10 PDF Download

Objective

• Dilute hydrochloric acid (HCl)
• Dilute sodium hydroxide (NaOH) solution ?
• Dilute ethanoic acid (CH₃COOH) solution
• Lemon juice
• Water (H₂O)
• Dilute sodium bicarbonate (NaHCO₃) solution

Materials Required
Six test tubes, test tube stand, glass rod, pH paper, dropper, standard pH colour chart, universal indicator, sample of solution—dilute HCl, dilute NaOH, dilute CH₃COOH, dilute NaHCO₃, water, lemon juice.

Theory

• If pH > 7, solution is basic.
• If pH < 7, solution is acidic.
• If pH = 7, solution is neutral.
  Pure water is neutral because of the absence of free ions. A paper impregnated with the universal indicator is used for measuring pH. Now-a-days, pH meter, an electronic device, is used to measure the pH. pH is defined as the negative logarithm of hydrogen ion concentration in moles per litre. The pH value has no unit. It can be written as:
  pH = -log[H⁺] or pH = – log [H₃O⁺]

pH of a solution can be determined by using pH paper. When the pH paper is dipped in the solution whose pH is to be found out, a colour is developed on the pH paper. The colour is matched with the standard colour chart (as shown in Fig. 2).In the place of pH paper, we can use universal indicator solution or universal indicator paper to measure the pH. When we take a drop of the test solution and place it on universal indicator paper, the paper turns a particular colour. The colour of the indicator paper is then matched against a colour chart showing the pH value for different colours. Universal indicator is also available in the form of solution.
The colours produced by universal indicator at different pH values are given in table below.
Note: Violet/blue colour indicates alkaline solution, red/pink colour indicates acidic solution and green colour indicates neutral solution.

Procedure

• Take each sample solution in a separate test tube and label them as A, B, C, D, E and F. Put all the test tubes one by one in a test tube stand (as shown in Fig. 3).
  
• Take a strip of pH paper and place it on cleaned and dry white surface.
• Dip glass rod into the sample solution of test tube A (dilute HCl).
• Put one or two drops of hydrochloric acid on the pH paper using clean glass rod as shown in the Fig. 4 given below.
  
• Observe the colour produced on pH paper and compare it with the different colour shades given in the standard pH colour chart.
• Note down the colour produced on pH paper and pH value in the observation table.
• Clean the glass rod properly, before taking another sample solution.
• Repeat the procedure from step (2) to (6) with acetic acid solution, lemon juice, water and dil. NaHCO₃ separately.
• Observe the colour change in each sample and record pH in observation table.
• Decide nature of solutions of each sampie, i.e. acidic, basic or neutral with the help of respective pH value.

Observation & Inference
Result
The pH of the given samples by using pH paper are as follows:

• pH of dilute HCl is ______ and nature of solution is ______ 
• pH of dilute NaOH is ______  and nature of solution is ______ 
• pH of dilute CH₃COOH is ______  and nature of solution is ______ 
• pH of lemon juice is ______  and nature of lemon juice is ______ 
• pH of water is ______  and it is ______
• pH of NaHCO₃ is ______ and nature of its solution is ______

Precautions

• Use the dilute solution of all samples and label the test tube carefully.
• Do not touch, smell or taste any of the solution directly.
• Colour developed on the pH paper should be compared carefully with that of the chart.
• Keep the pH strips away from chemical fumes.
• Do not touch the pH paper with dirty or wet hand.
• Clean the glass rod properly before using.
• The test sample solutions should be freshly prepared and the lemon juice samples should also be fresh.
• Glass rod used for one sample should be used for the other sample only after washing it with water.