You are using a 3.00 M solution of HNO3 to titrate a sample of 0.300 M KOH. Belo

Question

You are using a 3.00 M solution of HNO3 to titrate a sample of 0.300 M KOH. Below is a table of common indicators that shows the range over which they change colors.

(a) Which indicator would be best for the endpoint of the titration? (b) Is there a second indicator that would also be appropriate for the endpoint of the titration?

pH 10 11 12 cator Methyl violet Yellow Violet Red Yellow (acid range) Blue (base range) Thymol blue Yellow Methyl orange Red Yellow-orange Yellow Blue Bromcresol green Red Yellow Methyl red Yellow Red Chlorphenol red Brom thymol blue Blue Yellow 3 Red Phenol red Phenolphthalein Colorless Red Thymolphthalein Colorless Blue Alizarin yellow Yellow Violet

Explanation / Answer

HNO3 is a strong acid and KOH is a stron base. For such titration equivalence point is mainly around pH 7 hence the most suitable indicator will be Phenolphthalein. We may wonder why to use it when it has pH range of 8.2 to 12. The answer is that for Strong acid and strong base titration pH curve is very step and pH changes largely with addition of very small amount of titrant near equivalence point.

There is no other second indicator for this titration.

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