Limitations of pH scale

Limitations of pH Scale

(i) pH values of the solutions do not give us immediate idea of the relative strengths of the solutions. A solution of pH =1 has a hydrogen ion concentration 100 times that of a solution pH = 3 (not three times). A  is twice concentrated of a  solution, but the  values of these solutions are 4.40 and 4.70 (not double).

 

(ii) pH value zero is obtained in  solution of strong acid. If the concentration is , etc. the respective pH values will be negative.

(iii) A solution of an acid having very low concentration, say , can not have pH 8, as shown by pH formula but the actual pH value will be less than 7.

pK value: p stands for negative logarithm. Just as  and  ion concentrations range over many negative powers of 10, it is convenient to express them as pH or pOH, the dissociation constant (K) values also range over many negative powers of 10 and it is convenient to write them as pK. Thus, pK is the negative logarithm of dissociation constant.

 and

Weak acids have higher  values. Similarly weak bases have higher  values

             For any conjugate acid-base pair in aqueous solution,

 (at 298o K)

 

Calculation of the pH of  &

If we use the relation  we get  equal to 8, but this is not correct because an acidic solution connot have  greater than 7. In this condition concentration of water cannot be neglected.

Therefore,

Since  is strong acid and completely ionised,

,

Similarly if  concentration is

Then, 

 ;  

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