quote from http://www.chem.ubc.ca/courseware/pH/section15/content.html [Broken] Intuition may suggest that the endpoint of the titration will occur at the equivalence point if we choose an indicator whose pKa is equal to the pH of the equivalence point. If such an indicator was chosen, the colour change would be half complete at the equivalence point. Unfortunately, it is very difficult to tell when the colour change is precisely half complete, making it difficult to precisely identify the equivalence point. Since it is easiest to tell when the colour first starts to change, we want the equivalence point to occur then. quote from http://www.teachmetuition.co.uk/Chemistry/Acids_and_Bases/acids_and_bases.htm#Selecting%20a%20Suitable%20Indicator [Broken] The equilibrium constant can be expressed as follows: Kind = [H+][Ind-] / [HInd] At the end point of the titration, when the colour of the indicator changes, the concentrations of Ind- and HInd are equal, so Kind = [H+] Taking the log of both of the terms in the above equation gives pKind = pH (Since log of Kind = pKind and log of [H+] = pH) So the indicator that you choose for a reaction must have a pKind value at the pH of the end point of the reaction. Is the information from the first quote wrong? Because endpoint is when the indicator is half way between its two extremes. You want this half way position to coincide with the equivalence point of the titration because you want the intermediate colour to coincide with equivalence point Therefore, you want: pH of the equivalence point = pKa of the indicator. (surely you want pKa of the indicator to match the vertical section of the titration). Something different: I think the first site means: pH of titration = pKa of the titration when reaction is half complete. Can someone please help... I am seriously confused.