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 –
- 10ml aliquot of sample
- add 50ml DIW
- add 2 drops of bromocresol green indicator
- add 1 drop methyl red indicator
- 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