[positiveion]

The indicator bromophenol blue HIN has a form that is yellow and an IN- form that is blue.


State and explain the color of bromophenol blue at the equivalence point of a titraiton.

HIN <--> H+ + In-

(<--> = equilibrium arrow)

the answer says blue-yellow because both HIN and In- are present

i dont understand this because i thought the answer would be blue because at equivalence point the H+ ions are fully used up leaving only IN- ions which would make the solution blue?

[desadevil]

I think when all the H+ is used up, equilibrium shifts to the right, some HIN is converted into H+ and IN- to oppose the change, but not all the HIN will be used up, when eqm is reached again, there will still be some HIN left

[igloo5080]

At equivalence the concentration of HIN molecules equals the concentration of In- ions, so there are therefore an equal number of yellow HIn molecules as there are blue In- ions, so the appearance of the mixture is blue-yellow, ie green.

H⁺ ions do remain at equivalence - they are not all used up. Because [HIn]=[In-] at equivalence the indicator dissociation constant, K_In equals [H⁺], which in the case of bromophenol blue is about 1x10^-4. This does not represent a neutral solution, nor does it for most other indicators. However, the shape of the pH titration curve shows that this is irrelevant, since pH often changes very dramatically at or near equivalence.

You need to look up the principles involving the use of indicators.

[Borek]

State and explain the color of bromophenol blue at the equivalence point of a titraiton.

This question doesn't make sense. Not knowing what you are titrating it is impossible to say what is present in the solution, as ratio of concentrations of In^-/HIn is a function of pH, and equivalence point pH is a function of pKa of titrated substance.

Could be they are asking about color at the end of titration of bromophenol blue (after all it is a weak acid, it can be titrated) - if so, at the end of the titration all HIn has been neutralized, so you can safely assume there is no HIn in the solution.

At equivalence the concentration of HIN molecules equals the concentration of In- ions

No way, this statement is too general to be true.

You need to look up the principles involving the use of indicators.

And here we agree, some of us need to