Determination of pH of given solutions using pH paper

Determination of pH Value using pH Paper / Universal Indicator

Name of the Student: ___________________

Date: _______________

Experiment No.: _________

Video:

Aim:

The aim of this experiment is to determine pH value of the given set of solution using pH paper and to indicate its acidic or basic nature.

Scope:

The range of pH value of the samples solutions given below lies between 1 and 14.

  1. Dilute hydrochloric acid (HCl)
  2. Dilute sodium hydroxide solution (NaOH)
  3. Dilute ethanoic acid solution (CH3COOH)
  4. Lemon juice
  5. Water (H2O)
  6. Dilute sodium hydrogen carbonate solution (NaHCO3)

 

Concepts

1. pH is a measure of acidity or basicity of an aqueous solution.

2. It is a mathematical notation that describes the power of an acid or a base. The concentration of the acid is represented as [H+] or [H3O+]

pH = – log [H+]  (or)

pH = – log [H3O+]

3. The pH scale is the range of pH values from 0 to 14. The paper that indicates the complete range of pH value from 0 to 14 is called the universal indicator. pH paper is a strip of filter paper that is soaked in universal indicator and then dried.

Fig.1 – pH Paper

4. When the pH value is between 0 and 7, it indicates the acidic range of the solution. When the value is between 7 and 14, it represents the basic or alkaline range of the solution. When pH = 7, it is called neutral solution. Decrease of pH value indicates the increase of acidity and the increase of pH value shows the increase in alkalinity. Water is neutral and its pH is equal to 7.

Fig.2 – pH Scale

5. The concentration of H+ (aq) ions in pure water is 10-7 mol/litre, i.e.  mol/litre at 25°C. So, its pH is 7. It is in a neutral state.

pH = – log [H+]

pH = – log 10-7

pH = 7

6. In pure water,         concentrations of H+ ions and OH ions are equal. Hence, the product of concentrations these two ions is called ionic product of water (Kw) and its value is 10-14 mol/litre at 25°C.

In pure water,

[H+] = [OH] = 10-7 mol/litre

[H+] [OH] = 10-7 x 10-7 = 10-14 mol2/litre2

Kw = 10-14

7. Universal Indicator is a mixture of organic dyes that shows different colours with the solutions of different pH values.

8. The colour chart given below shows the colour indication of pH paper in solutions of different pH values.

Fig.3 – pH Colour – chart

Apparatus required:

(i)          pH paper

(ii)         Test tubes – 6 numbers

(iii)        A Test tube rack

(iv)       A dropper

(v)        Standard pH colour-chart

(vi)       A glaze tile

(vii)      A beaker containing distilled water

(viii)    Dilute sodium hydrogen carbonate solution (NaHCO3)

 

Chemicals required:

(i)          Dilute hydrochloric acid (HCl)

(ii)         Dilute sodium hydroxide solution (NaOH)

(iii)        Dilute ethanoic acid solution (CH3COOH)

(iv)       Lemon juice

(v)        Distilled water (H2O)

(vi)       Dilute sodium hydrogen carbonate solution (NaHCO3)

 

Procedure:

1. Place the clean and dry test tubes in the test tube rack.

2. Pour 2 ml of distilled water in each tube.

3. Label the test tubes as A, B, C, D, E, and F

4. Add 1 ml dilute HCl in the test tube – A.

5. Add 1 ml NaOH solution in the test tube – B.

6. Add 1 ml ethanoic acid in test tube – C.

7. Add 1 ml lemon juice in test tube – D.

8. Add 1 ml water in test tube – E.

9. Add a pinch of sodium hydrogen carbonate in test tube – F.

10. Shake the contents of each test tube thoroughly.

11. Place 6 strips of pH paper on the dry glazed white tile.

12. Label the pH paper strips as A, B, C, D, E and F

13. Suck the solution from each test tube using a clean dropper and transfer it to the respective pH paper with the labels A to F.

14. Note down the colour change.

15. Compare the colour change in the pH paper with the standard pH-colour chart.

Fig.4 – Test tubes in test tube racket

Observations:

Note down the observations in the table -1 given below.

Table – 1 – Color Changes in pH Paper

Label

Solution

Colour Change in pH Paper

pH Values from pH colour chart

A

Dilute HCl Red

1

B

Dilute NaOH solution Purple

14

C

Dilute ethanoic acid solution Yellow

3

D

Lemon juice Orange

2

E

Distilled water Green

7

F

NaHCO3 solution Blue

9

Precautions:

1. Use good quality pH paper.

2. Ensure thorough cleaning of the dropper with distilled water before proceeding to the next solution.

3. Note down the colour change of the pH paper immediately after dropping the solution.

4. The labels on the test tubes and pH papers should be intact.

5. Shake thoroughly the test tube before sucking with the dropper.

Chances of error:

1. If the test tubes are rinsed with tap water, they will give erratic pH values.

2. If tap water is used for preparing solutions, the pH values may go wrong.

3. Improper and insufficient rinse of test tubes or dropper with distilled water may give erratic values of pH.

 Report:

The pH values of the given set of samples are reported as shown in the table – 2.

Table – 2 – pH of Given Set of Solutions

Label

Solution

pH Values from pH colour chart

Inference

A

Dilute HCl

1

Strong acid

B

Dilute NaOH solution

14

Strong alkali

C

Dilute ethanoic acid solution

3

Weak acid

D

Lemon juice

2

Acid

E

Distilled water

7

Neutral

F

NaHCO3 solution

9

Weak alkali

Signature of the Student                    Signature of the Lab Supervisor

Date: ____________                       Date: _________________

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