14D2 Bicarbonate_/SE and carbonate_/SE – indicator method

This is an alternative to Method 14D1 when indicators to detect relevant end-points are preferred.

Additional Reagents – To Method 14D1

Metacresol Purple Indicator (pH 8.3 Indicator)

Dissolve 100 mg metacresol purple (C₂₁H₁₈O₅S) in 100 mL deionised water.

Bromocresol Green Indicator (pH 4.5 Indicator)

Dissolve 100 mg bromocresol green, sodium salt (C₂₁H₁₃O₅Br₄SNa) in 100 mL deionised water.

Procedure

Pipette a 5 mL portion of SE into an Erlenmeyer flask, as described for the potentiometric titration procedure (14D1). Add 0.2 mL (5 drops) of each indicator solution (metacresol purple indicator solution for pH 8.3 and bromocresol green indicator solution for pH 4.5) and titrate over a white surface to the persistent colour change/s characteristic of the equivalence point/s. Check colour at the end-points by adding the same amounts of indicator solutions to buffer solutions at the designated pH values. Otherwise, titrate and record mL of titrant added as described for the potentiometric titration procedure.

Calculations

As for Method 14D1. Report as detailed in Method 14D1.