Question

For which trial(s) is phenophthalein a good indicator? For which trial(s) was it a poor indicator? For those trials in which it was a poor indicator, suggeest a better indicator and indicate its pH range on the titration curve.

So for phenophthalein the equivalence point has to be from 8.3 to 10 right?

Trial 1: 7.29 pH so bromothymol blue indicator

trial 2: 8.14 so phenol red indicator

Trial 3: 9.21 falls between 8.3 and so that's good

trial 4: 7.01 so bromothymol blue

Answer 1

So much of this depends upon how steep the curve is at the equivalence point AND for how large a range. But if we have short ranges and the curve is not too steep, then I would pick phenol red for trial 1. My text lists that as changing from 6.8-8.4 and 7.29 seems closer to mid-range for phenol red than for bromothymol blue (6.2-7.6....here 7.29 is so close to the high end). Trial 2 at 8.14 is so close to the high end of phenol red (6.8-8.4); it appears cresol purple (7.6-9.4) might be more in the middle. Trial 3 looks good for phenolphthalein. Trial 4 looks good. Actually, all of those you picked will work if the curve is steep enough and the range is high enough.