Determination of Bicarbonate

Bicarbonate is a significant anion in all wall-wash samples, especially those taken from limestone.

For the purpose of ion balance accounting, I assume that the number of gmoles of hydrochloric acid required to reduce the pH of the sample to 4.5 is indicative of the bicarbonate concentration.

The procedure is as follows –

  1. 10ml aliquot of sample
  2. add 50ml DIW
  3. add 2 drops of bromocresol green indicator
  4. add 1 drop methyl red indicator
  5. titrate against 0.01M hydrochloric acid until olive green end-point

1 gmole of bicarbonate in aliquot = gmoles of chloride titrated

Hydrochloric Acid 0.01M:

prepared from commercial standardised 1.0M AR hydrochloric acid

Bromocresol Green Indicator:

  • weigh out 200mg of dyestuff
  • dissolve in 50ml of DIW

Methyl Red Indicator:

  • weigh out 50mg of sodium salt of dyestuff
  • dissolve in 25ml of absolute ethanol
  • make up to 50ml with DIW

Notes:

  • mixed indicator necessary to get sharp transition at the bicarbonate end-point
  • the blank is significant and should be included. It is typically around +6 umoles