[Judy]

You analyze for pyridine (Kb is approximately 10^-9) by dissolving 0.1000 g of the complex in 10 mL of H2O and titrating with a 0.01 M HCl solution. Which of the following indicators should be used to detect the endpoint? (Assume that the initial concentration of pyridine is approximately 0.01 M.)

A) bromophenol blue, pH range of color change = 3.0-4.6
B) methyl red, pH range of color change = 4.8-6.0 C) bromothymol blue, pH range of color change = 6.0-7.6
D) thymol blue, pH range of color change = 8.0-9.6
 E) alizarin yellow, pH range of color change = 10.1-12.0

The answer is A.

I think the answer should be B since pyridine is a weak base, as it is titrated by strong acid its endpoint should be slightly acidic. However, option A is much more acidic than B, why?

[Borek]

You are given enough data to calculate end point pH. Whenever possible don't guess, calculate.

Also: what is the pH at titration midpoint?

[Orcio_87]

Bromophenol blue will change colour when all pyridine will be converted into hydrochloride.

Methyl red will need to be turned into red colour, but will change its colour too soon.

Bromothymol and thymol blues are not suitable, same as the alizarin.

Bromophenol blue is the most accurate in this case, better than methyl red.

[Judy]

Bromophenol blue will change colour when all pyridine will be converted into hydrochloride.

Methyl red will need to be turned into red colour, but will change its colour too soon.

Bromothymol and thymol blues are not suitable, same as the alizarin.

Bromophenol blue is the most accurate in this case, better than methyl red.

Here's my calculation:
[H+] = (C0*Ka)^1/2 = (0.01*10^-5)^1/2
pH = -log[H+] = 3.5, so the answer is A. Is this correct?  Thanks

[Orcio_87]

It is not 100 % correct, as addition of HCl will lower concentration of pyridine (or rather - its hydrochloride), but - it is OK.

[Borek]

approximately

[Judy]

Thanks so much.