Question

Consider a titration curve for a weak base and a strong acid. Suppose both phenolphthalein and bromocresol green are present in the solution. What color changes would occur at pH 10, pH 7, and pH 3?

Answer 1

This may be illustrated by the titration of 100 mL of 0.1 (M) aqueous ammonia ( K_b= 1.85 * 10^-5 ) with 0.1 M hydrochloric acid at ordinary laboratory temperature .

NH₃ +H₂O + H⁺ --> NH₄⁺ [ OH^-]

K_b= [NH₄⁺] [OH^-]

or, [OH^-] = K_b [ NH₃] / [NH₄⁺ ]

or, pOH = log [salt ] / [ base] + pK_b

_or, pH = pK_w - pK_b - log [salt] / [base]

After the equvalance point is reached, the solution containing excess H⁺ ions, hydrolysis of the salt is supressed, and the subsequent pH may be assumed, with sufficient accuracy, to be due to the excess of acid present.

The results are shown in picture, along with the curve for titrating 100 mL of a 0.1 M solution of a weak base. (K_b= 1*10^-7 )

Phenolphthalein can not be used in the titration of 0.1 M aqueous ammonia .The equivance point is at pH 5.3 , and it is necessary to use indicator with a pH range in the slight acid side ( pH 3 to 6.5 ) . Bromocresol green can be used as indicator as has pH range ( 4.2 to 6.3 ) . Show yellow in acid solution and blue in base solution.

At pH 10 Phenolphthalein will show red colour.

At pH 7 and pH 3 it will no change color with both indicator ( Phenolphthalein & Bromocresol green)