phenolphthalein

the pH level below around 8 for phenolphthalein indicate is colourless ...

so when u are titrating an acid with a base ... it will change colour from colourless to pink as pH increase.

on the other hand ... other way round ... if u titrate base with acid ... is changes from pink to colourless

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得罪曬

It is because.. we can know the volume of titrant needed more accurately.
If we titrate from colourless to pink, once the solution changes to pink, we can immediately stop and know the volume of titrant needed. We can know how much titrant we hv added excessively by looking at the intensity of the colour, because even one more drop of titrant is added, the intensity of the colour will increase a lot. If we titrate from pink to colourless, we cannot know if we have added too much titrant. Error is more likely to occur.