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It seems that no resource I come across addresses the fact that the pH range for phenolphthalein's color change is around 8.2-10. In other words, it's still colorless at 8.2. Why then, is it claimed that the indicator changes colors when the solution changes from an acid to a base? Wouldn't you want an indicator that changes colors at about pH 7?

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If you analyse a titration curve you will immediately notice the pH shooting up near equivalence point. For this reason, there is very little difference to the amount of base added for pH 7 and pH 8 (due to the logarithmic nature of pH). So, it's not entirely accurate but certainly good enough (for strong acid - strong base and weak acid - strong base), if you want something better use an electronic pH tester

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  • $\begingroup$ Looking at a titration curve clears up a bit of my confusion, but now I'm wondering what the point is of having an indicator at all. If the "volume added" in this graph--chemguide.co.uk/physical/acidbaseeqia/sawb2.gif--matches up with all those different pH values, what's the point of trying to get it close to 7? $\endgroup$
    – user39367
    Dec 31, 2016 at 21:30
  • $\begingroup$ I believe the link was distorted: chemguide.co.uk/physical/acidbaseeqia/sawb2.gif $\endgroup$
    – user39367
    Dec 31, 2016 at 21:31
  • $\begingroup$ pH 7 is the equivalence point where the moles of H+ in the original solution equals the moles of OH- added, so we can construct an equation and solve for unknown $\endgroup$
    – Copper
    Dec 31, 2016 at 21:37
  • $\begingroup$ [volume of solution][conc(H+)] = [volume of added solution][conc(OH-)] $\endgroup$
    – Copper
    Dec 31, 2016 at 21:38
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    $\begingroup$ Looking at the title, the other half of the answer should be that there are few indicators that go from colorless to a color. Phenolphthalein's color change is very easily detectable visually. $\endgroup$
    – MaxW
    Jan 1, 2017 at 3:39
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In order to determine what indicator to use, you have to identify the reactants and strong or weak. For a strong base and strong acid reaction the pH shift will likely occur around 7. If its a strong acid vs a weak base, then the pH will probably lower than 7.

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