pH SCALE: (Puissance of Hydrogen)





Sorenson (1909) introduced a convenient means of expressing the concentration of ions. It is called hydrogen exponent or pH scale value of simply pH. On this scale the ion concentration is written as the power. Thus the pH of a solution is the numerical value of the negative power to which 10 must be raised in order to express the H^+ ion concentration,

h^+ = 10^pH

 

It is –ve logarithm of H_3O^+ or H^+

Or P^H = -\log[H^+] = \log \dfrac{1}{[H^+]}

Mathematically, pH is the logarithm of the reciprocal of the hydrogen ion concentration.

Since for pure water [H^+] =[OH^- ] = 10^{-7}

hence the pure water is neutral.

Similarly for bases pOH is used.

pOH = - \log_{10}[OH^-]

 

= \log_{10} \dfrac{1}{[OH^-]} \\[3mm] pH + pOH = 14

 

For weak acid and base,

 

Here K_a, K_b are dissociation constants of acid and base respectively.

The pH range for some common substances is given  below:

 

Substances with their pH ranges:

1. Apples                                    pH  range 2.9 – 3.3

2. Bollod(Human)                   pH  range = 7.3 – 7.5

3. Battry acid                           pH  range 0 – 1

4. Baking soda                         pH  range 9

5. Bile juice                              pH  range 7.1

6. Coffee                                   pH  range 5

7. Gastric juice                        pH  range 21 – 3

8. Lemons                               pH  range 2.2 – 2. 4

9. Laundry water                  pH  range 10

10. Milk                                   pH  range 6.3 – 6.6

11. Milk of magnesia             pH  range 10.5

12. Rain water                        pH  range 6.0

13. Soda water                       pH  range < 7

14. saliva                                pH  range 6.5 – 7.5

15. soft drinks                       pH  range 2.0 – 4.0

16. sea water                         pH  range 8.5

17. stomach acid                  pH  range 2

18. tomato                             pH  range 4.0

19. tears                                 pH  range 7.4

20. toilet soap                      pH  range 10

21. urine (human)               pH  range 4.8 – 8.4

22. vinegar                          pH  range 2.4 – 3.4

23. water (pure)                pH  range 7

24. boiling water               pH  range 6.56



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