Carbonate and bicarbonate ions in the sample can be determined by titrating it with against standard sulphuric acid (H2SO4) using phenolphthalein and methyl orange as indicators.

2. Theory/Principle:
Carbonate and bicarbonate ions in the sample can be determined by
titrating it with against standard sulphuric acid (H2SO4) using
phenolphthalein and methyl orange as indicators. Addition of
phenolphthalein gives pink red colour in the presence of carbonates
and titration with H2SO4 converts these CO3
- into HCO3
- and
decolourises the red colour as shown below:
2 Na2CO3 + H2SO4 2 NaHCO3 + Na2SO4

3. Carbonate ions in the irrigation water sample
End point: Pink to colourless

4. Thus the carbonates neutralization is only half way. These carbonates along
with the already present ones are then determined by continuing the titration
using methyl orange indicator which gives yellow colour in presence of
bicarbonates. On complete neutralization of bicarbonates the yellow colour
will change to red.
2 NaHCO3 + H2SO4 Na2SO4 + 2 H2O + 2 CO2
Obviously the bicarbonate titre value will be less if carbonates were not
present. (absence of pink colour). In such a situation, either the same aliquot
is used for bicarbonate titration or a fresh sample is analyzed for this. If
carbonates are present and neutralized, the volume of H2SO4 used in the
first phase (carbonate titration) is to be doubled to get the actual volume
needed for complete neutralization of the carbonates.

5. Bi-carbonate ions in the irrigation water sample
End point: Yellow to reddish orange

6. Materials Required:
Burette,
Pipette,
Conical flask,
H2SO4 (0.1 N)
Phenolphthalein and
Methyl orange indicator.

7. Learning Objectives:
To determination the carbonates and bi-carbonates content in
irrigation water sample.

8. Procedure:
Pipette out 10 ml of irrigation water sample into a conical flask.
Add 1 or 2 drops of phenolphthalein indicator; the solution will turn pink.
Titrate the contents against the 0.1 N H2SO4 filled in burette; the end point
is indicated by the disappearance of pink color. Record the burette reading.
If on addition of phenolphthalein the sample does not turn pink, it indicates
the absence of CO3.
In such case add methyl orange indicator to the same aliquot and titrate
against 0.1N H2SO4 till the straw yellow colour changes to rose red.
Note down the burette reading.

9. Observation table:
Sample. No.
Volume of H2SO4
consumed for CO3
2-
neutralization
(ml)
Volume of H2SO4
consumed for HCO3
-
neutralization (ml)
CO3
2- (meq l-1)
HCO3
2-
(meq l-1)
1
2
3
4

10. Calculations
 Amount of 0.1 N H2SO4 required in 1st titration = A ml
 Amount of 0.1 N H2SO4 required in 2nd titration= B ml
Formulae:
 Amount of carbonates (meq l-1) = 20 x A
 Amount of bicarbonates (meq l-1) = 10 x (B-A)

11. Results :
The carbonates and bicarbonates in given irrigation water is ____ and
____ meq l-1, respectively.
Cautions:
While titration the H2SO4 solution should be added drop-wise.