1
$\begingroup$

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?

$\endgroup$
3
$\begingroup$

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

$\endgroup$
  • $\begingroup$ 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? $\endgroup$ – khajiit Dec 31 '16 at 21:30
  • $\begingroup$ I believe the link was distorted: chemguide.co.uk/physical/acidbaseeqia/sawb2.gif $\endgroup$ – khajiit Dec 31 '16 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 '16 at 21:37
  • $\begingroup$ [volume of solution][conc(H+)] = [volume of added solution][conc(OH-)] $\endgroup$ – Copper Dec 31 '16 at 21:38
  • $\begingroup$ If you're wondering about that specific curve, the volume added is only specific to specific concentration of base, specific concentration of the acid, specific volume of acid. The volume added values all change based on the other 3 "variables" $\endgroup$ – Copper Dec 31 '16 at 21:43
0
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.