Bromocresol Green as an Indicator

[monae]

Give an example of a titration assay in which bromocresol green could be
used. In your example give the concentrations of each component. Based
on the conditions of the titration, calculate the pH at equivalence.

Since Bromocresol Green has a pKa of 4.6, I figured that this must be a titration of a weak base and a strong acid.
I decided to use 5.0mL of 0.1M NH3 and 10.0mL of 0.020M HCl.

The pKb is 4.76.
I solved for the pH and got 9.42.

Would I be able to use this particular acid base combination?

 

[nate808]

Yes, this is a perfect example of a titration assay in which bromocresol green could be used. In this case, bromocresol green would act as an indicator to determine the endpoint of the titration. The concentrations of each component would be 5.0mL of 0.1M NH3 (the weak base) and 10.0mL of 0.020M HCl (the strong acid).

At the beginning of the titration, the solution would have a basic pH due to the presence of the weak base. As the strong acid is added, it will react with the weak base and neutralize it, causing a decrease in pH. At the equivalence point, where all of the weak base has been neutralized, the solution will have a pH equal to the pKa of bromocresol green, which is 4.6.

Using the Henderson-Hasselbalch equation, we can calculate the pH at equivalence:

pH = pKa + log ([A-]/[HA])

Where [A-] is the concentration of the conjugate base (in this case, the weak base NH3) and [HA] is the concentration of the acid (HCl).

Substituting in the values, we get:

pH = 4.6 + log (0.1/0.020) = 4.6 + 0.699 = 5.299

Therefore, at the equivalence point, the pH of the solution would be approximately 5.3.

Overall, bromocresol green is a useful indicator in titration assays involving weak bases and strong acids, as it changes color at a pH close to the equivalence point, making it easier to determine the endpoint of the titration.

 

[Blueshift5]

Yes, you can use this acid-base combination for a titration assay using bromocresol green as an indicator. The concentration of NH3 is 0.1M and the concentration of HCl is 0.020M. In this titration, bromocresol green would change color at the equivalence point, which is when the moles of NH3 are equal to the moles of HCl. At this point, the pH would be equal to the pKa of bromocresol green, which is 4.6. Therefore, the pH at equivalence would be 4.6. This titration could be used to determine the concentration of an unknown acid or base solution.