The pH of neutral water at 25°C is 7.0. As the temperature increases, ionisation of water increases, however, the concentration of H+ ions and OH ions are equal. What will be the pH of pure water at 60°C?


The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. Using the simpler version of the Equilibrium :


H2O (l) H+ (aq.) + OH- (aq.)


Then the ionic product of water Kw = [H+] [OH-]


Again, pKw = pH + pOH (by taking negative log ; -logx=px)


o Hence at 25°C [H+] =[OH-] = 107 (as pH=-log[H+] = 7 )


Kw = [H+] [OH-] = 10-14


When temperature increases Kw increases at 60°C Kw >10-14


Therefore, [H+] [OH-] >10-14


Or [H+] > 10-7


Or pH <7.

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