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Bromophenol blue (represented as BH ) is frequently used as a pH indicator, an electrophoretic colour marker and a dye. It is a weak acid which acts as an acid-base indicator according to the following equilibrium: BH(aq) yellow ​⇌B−(aq)+H+(aq) blcie ​Ka​=6.3×10−5M At high pH,BH exists mainly as its conjugate base, B−, which is blue. At low pH, it exists mainly as the acid, BH, which is yellow in colour. An intermediate colour is observed when the concentration of the acid and the concentration of the conjugate base are similar. (a) Write an expression for Ka​ - (1 mark) (b) (i) When [BH]=[BB], the mixture appears green. Deduce the pH of this mixture. (2 marks) (ii) Calculate the ratio /[BH] when the pH of the bromophenol blue solution is 7 . (2 marks) (iii) What colour will the indicator solution appear at pH7? Justify your answer. (2 marks)
The table below shows the pKind, the pH range and the colour changes of the two indicators. The titration of sodium carbonate solution with dilute hydrochloric acid is presented in the pH curve below. Besides methyl orange, could you use bromophenol blue or methyl red to detect the second equivalence point of this reaction? Explain your answer. (3 marks)