What color range and at what pH levels will you observe with bromothymol blue in

Question

What color range and at what pH levels will you observe with bromothymol blue indicator?

Approximately what wavelength will the acid form of the indicator maximally absorb? Explain.

Approximately what wavelength will the base form of the indicator maximally absorb? Explain.

Define the term isobestic point. Explain why the isobestic point would be a poor choice of wavelength for determining pKa? Provide at least one citation for this question.

pKa has been shown to be temperature dependent. Use Equation 26.4 to determine the pKa of an indicator at 273K if the pKa at 298K is 7.10 and the Enthalpy of ionization is 770 J/mol. Show all work.

Explanation / Answer

1) Bromothymol blue works between pH range of 6.0 to 7.6.

Colour ranges from yellow to blue .

2) Protonated form of bromothymol blue has its peak absorption at 427 nm thus transmitting yellow light in acidic solution .

3) Deprotonated form of bromothymol blue has its peak absorption at 620nm thus transmitting blue light in basic solution .

4) Isobestic point is specific wavelength wavenumber or frequency at which total absorbance of sample doesn't change during a chemical reaction or physical change in sample . Isobestic point for bromothymol blue is 325 nm and 498nm.

Here wavelength doesn't depend on concentration . But in pKa determination Henderson Haselbech equation is used and it depends on concentration change , temperature , ionic strength of solvent and many more parameters . Hence the choice as such will be poor .

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