Phenolphthalein is an indicator often used for weak acid-strong base titrations. The acid form of phenolphthalein is colorless and the base form is pink. Do you think the pKa of phenolphthalein could be determined using same method from this experiment to find the pKa for bromothymol blue? Explain why or why not.

Explanation:

In chemistry an indicator is any of many substances, such as litmus, used to indicate the concentration  and some other properties of a substance, or the degree of a reaction.

Titration is a concept in analytical chemistry where one determines the concentration of some substance in a solution by slowly adding measured amounts of some other substance (normally using a burette) until a reaction is shown to be complete, for instance by the colour change of an indicator.

 Phenolphthalein is a quinone derivative used as a pH indicator in chemistry.

Bromothymol Blue is also a dye 4,4'-(3H-2,1 benzoxathiol-3-ylidene) bis [2 bromo-3-methyl-6-(1-methylethyl)phenol] S,S-dioxide that is used as an acid-base indicator.

When a chemical changes the phenolphthalein into its ion the solution turns the solution pink this happens around pH of 9.

Phenolphthalein is acidic and colorless.

A spectrophotometric analysis can be used to perform an acid-base titration to find the pKa of an acid-base indicator such as bromothymol blue.

Spectrophotometric analysis is a quantitative analysis of electromagnetic spectra by use of a spectrophotometer; especially in order to determine the structure or quantity of a substance including finding the pKa.

The following are applicable to solve the above;

• use a pH meter/titrant curve

• to treat the indicator like a weak base,

but what titrant would I use

• use a strong acid like HCl (HCl would work with a pH meter).

• know the mass of base (indicator)

• know the equivalents of a strong acid e.g. 0.5 eq.of HCl and determine the pH.

According to the Henderson-Hasselbalch equation when the weak base would be half protonated, the pH would be equal to the pKa or simply solve for pKa and the equivalence after the full titration.

Remember that Bromothymol blue is yellow in acidic solutions, blue in basic solutions and green in neutral solution.

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