thermometric titration ( tas)

For strong acids and strong alkalis the molar enthalpy of neutralisation is effectively constant at about -57kJ per mol. This is because strong acids and strong alkalis, and the salts they form, are all completely ionised in dilute solution, so that the reaction is simply the formation of unionised water from H+ and OH- ions.
H+(aq)　+　OH-(aq)　--->　H2O(l)　　delta Hm(298K) = -57 kJ per mol
With weak acids and/or alkalis, neutralisation produces an enthalpy change due to the formation of water from H+ and OH- ions but, in the course of the reaction, previously un-ionised acid and/or alkali has to be converted into ions and this involves ethalpy changes which may be either positive or negative. (For the case of ethanoic acid, it is positive.) As a result, the ethalpy of neutralisation involving either weak acids or alkalis may be greater or smaller than -57 kJ per mol. (For the case of ethanoic acid, it is smaller than -57 kJ per mol.)
I hope this can help your understanding. =)

enthalpy change of neutralisation is a constant value by definition