<P> The known solution should then be allowed out of the burette, into the conical flask . At this stage we want a rough estimate of the amount of this solution it took to neutralize the unknown solution . The solution should be let out of the burette until the indicator changes colour and the value on the burette should be recorded . This is the first (or rough) titration volume and should be excluded from any calculation . </P> <P> At least three more titrations should be performed, this time more accurately, taking into account roughly where the end point will occur . The initial and final readings on the burette (prior to starting the titration and at the end point, respectively) should be recorded . Subtracting the initial volume from the final volume will yield the amount of titrant used to reach the end point . The end point is reached when the indicator just changes colour permanently . </P> <P> Acid--base titration is performed with a bromothymol blue indicator, when it is a strong acid--strong base titration, a phenolphthalein indicator in weak acid--strong base reactions, and a methyl orange indicator for strong acid--weak base reactions . If the base is off the scale, i.e. a pH of> 13.5, and the acid has a pH> 5.5, then an Alizarine yellow indicator may be used . On the other hand, if the acid is off the scale, i.e. a pH of <0.5, and the base has a pH <8.5, then a Thymol Blue indicator may be used . </P> <P> The pH of a weak acid solution being titrated with a strong base solution can be found at different points along the way . These points fall into one of four categories: </P>

Indicator for titration of weak acid and strong base