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Ionic product of water and pH scale
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It may be defined as the product of the molar concentrations of H3O+ and OH- ions i.e. Kw = [H3O+][OH-] Its value at 25°C is 10-14 and increases with increase of temperature because dissociation of H2O increases.
(1) pH = – log [H3O+]. The term pH was introduced by Sorensen to express the acidic or basic strength of a solution.
(2) pH scale is from 0 to 14 as shown below :
Acidic character increases Neutral Basic character increases
(3)pKa= - log Ka where Ka is the ionization constant of the weak acid HA.
(4) pKb =- log Kb where Kb is the ionization constant of the weak base BOH.
(5) Pkw=-log Kw. As Kw = [ H3 O+][OH-] =- log10-14
\ - log[H3 O+ ][OH-] = -log 10-14 or -log[H3O+]+(-log[OH-]) =14 or pH+pOH =14
Note : q pH of a solution decreases on heating as the dissociation of water is an endothermic process.
q pH of solutions can be calculated by pH metres (accurately) or by emf method or by universal indicator (roughly).
q pH of boiling water is 6.5625, although it is neutral.
q The word p in pH stands for french word puissance or german word potenz means power.
q pH value is zero for 1N solution of strong acid. In case of 2N,3N,4N etc. solutions pH values are negative.
q pH range for some important substances are : Gastric juice – 1.0 to 3.0, Vinegar – 2.4 to 3.4 Human saliva– 6.5 to 7.5, Tears – 7.4, Soft drinks – 2.0 to 4.0, Human urine – 4.8.
q 10-8 M HCl has a pH =+ 6.958. In such cases as the H+ ion concentration of acid is less than that originally present in water, therefore H+conc. of water is also taken into account.
Total H+ion concentration = 10-8+ 10-7 =10-8 ( 1+ 10) , [H+] = 11 x 10-8 pH =- log 11 x 10-8 = 6.958
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