Carbonates (CO3) and bicarbonates (HCO3) in water can be determined by titration a known volume of water against standard H2SO4 using phenolphthalein and methyl orange indicators, respectively.
Principle
Carbonates
When the pH value of a sample of natural water is above 8.4, the CO3 is present, normally as sodium carbonate. The carbonate ion is converted to HCO3. The amount of acid used a measure of the carbonate present.
CO3 + H = HCO3
Bicarbonate
Bicarbonate ions react with mineral acid and release carbon dioxide (CO2) into the solution.
HCO3+ H = H2O + CO2
The pH value at complete neutralization being about 3.8, HCO3 ions are present. Thus HCO3 may be measuring by titration with mineral acid to a pH 3.8, either potentiometrically or using an indicator unaffected by CO2.
Methyl orange is suitable and gives a good color change from green through grey (end-point) to red, which avoids the need for a matching buffer solution.
Apparatus
- Pipettes
- Burette
- Erlenmeyer flasks
- Graduated pipette
- Magnetic stirrer
Reagents
A. Methyl Orange Indicator [4-NaOSO2C6H4N:NC6H4 /-4-N (CH3)2], (F.W. 327.34), 0.1% Dissolve 0.1 g methyl orange indicator in 100-ml volume with DI water.
B. Sulfuric Acid Solution (H2SO4), 0.01 N
- Add 28 ml concentrated H2SO4, to about 600 – 800 ml DI water in a 1-L flask, mix well, let it cool, and bring to 1-L volume. This solution contains 1 N H2SO4 solution (Stock Solution).
- Pipette 10 ml Stock Solution to 1-L flask, and bring to volume with DI water. This solution contains 0.01 N H2SO4.
C. Phenolphthalein Indicator, 1% Dissolve 1 g phenolphthalein indicator in 100 ml ethanol.
Procedure
- Pipette a suitable aliquot of water sample (10 ml natural water sample) in a wide-mouthed porcelain crucible or a 250-ml Erlenmeyer flask.
- Add 1 drop phenolphthalein indicator. If pink color develops, add 0.01N H2SO4 by a burette,drop by drop, until the color disappears.
- Take the reading, y.Continue the titration with 0.01 N H2SO4 after adding 2 drops 0.1% methyl orange indicator until the color turns to orange.
- Take the reading, t.
Calculations
Technical Remarks
- Standard HCl is used because H2SO4 may give rise to turbidity from calcium sulfate with Ca-rich samples.
- The alkalinity of water is a measure of its capacity to neutralize acids and is due primarily to the presence of bicarbonates.
- The acceptable alkalinity for municipal water supplies is generally between 30 and 500 mg/L as CaCO3, but there are many water supplies above and
below these limitations. - Waters with alkalinity greater than 500 mg/L as CaCO3 have objectionable tastes.