6. A TITRIMETRIC DETERMINATION OF HYDROGEN CARBONATE CONCENTRATION IN A WATER SAMPLE

6.1 Objective

Use a miniaturised and simplified acidimetric titration to determine the concentration of hydrogen carbonate in a water sample. Use a syringe instead of a burette, and use a Coulter-counter container as a replacement for an Erlenmeyer flask.

6.2 Background

Write the reaction on which the titration is based.

6.3 Equipment and reagents

• A device for miniaturized titration.
• A standard solution of hydrochloric acid, concentration 0.1 mol/L.
• A solution of bromochresol green (Phenol, 4,4'-(3H-1,2-benzoxathiol-3-ylidene) bis[2,6-dibromo-3-methyl-, S,S-dioxide) indicator.

Hazards

6.4 Procedure

1. Fill the syringe with the standard solution of HCl.
2. Fill the Coulter-counter container with the water sample up to the first mark (V=10 ml).
3. Add a drop of the bromocresol green indicator solution to the sample.
4. Titrate the examined solution until its colour changes to pale green. Be careful, the solution must not turn yellowish-green.
5. Record the volume of HCl solution consumed during the titration from the syringe.

Note.
The procedure can be made easier by first estimating the volume of HCl standard solution that is needed to reach the end-point of the titration. Start with a preliminary titration in which the HCl solution is added in 0.05-ml increments.

Calculation of the result:

Calculate the amount and the mass concentration of hydrogen carbonate in the water sample.

M_HCO3^_ = 61.00 g/mol

Write both results in the table “Results – Major components”, under the serial number of the sample.