Determination of carbonates and bicarbonates in water sample

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:
Carbonateandbicarbonateionsinthesamplecanbedeterminedby
titratingitwithagainststandardsulphuricacid(H
2SO
4)using
phenolphthaleinandmethylorangeasindicators.Additionof
phenolphthaleingivespinkredcolourinthepresenceofcarbonates
andtitrationwithH
2SO
4convertstheseCO
3
-
intoHCO
3
-
and
decolourisestheredcolourasshownbelow:
2Na
2CO
3+H
2SO
4 2NaHCO
3+Na
2SO
4

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

4. Thusthecarbonatesneutralizationisonlyhalfway.Thesecarbonatesalong
withthealreadypresentonesarethendeterminedbycontinuingthetitration
usingmethylorangeindicatorwhichgivesyellowcolourinpresenceof
bicarbonates.Oncompleteneutralizationofbicarbonatestheyellowcolour
willchangetored.
2NaHCO
3+H
2SO
4 Na
2SO
4+2H
2O+2CO
2
Obviouslythebicarbonatetitrevaluewillbelessifcarbonateswerenot
present.(absenceofpinkcolour).Insuchasituation,eitherthesamealiquot
isusedforbicarbonatetitrationorafreshsampleisanalyzedforthis.If
carbonatesarepresentandneutralized,thevolumeofH
2SO
4usedinthe
firstphase(carbonatetitration)istobedoubledtogettheactualvolume
neededforcompleteneutralizationofthecarbonates.

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

6. Materials Required:
Burette,
Pipette,
Conical flask,
H
2SO
4(0.1 N)
Phenolphthalein and
Methyl orange indicator.

7. Learning Objectives:
Todeterminationthecarbonatesandbi-carbonatescontentin
irrigationwatersample.

8. Procedure:
Pipetteout10mlofirrigationwatersampleintoaconicalflask.
Add1or2dropsofphenolphthaleinindicator;thesolutionwillturnpink.
Titratethecontentsagainstthe0.1NH
2SO
4filledinburette;theendpoint
isindicatedbythedisappearanceofpinkcolor.Recordtheburettereading.
Ifonadditionofphenolphthaleinthesampledoesnotturnpink,itindicates
theabsenceofCO
3.
Insuchcaseaddmethylorangeindicatortothesamealiquotandtitrate
against0.1NH
2SO
4tillthestrawyellowcolourchangestorosered.
Notedowntheburettereading.

9. Observation table:
Sample. No.
Volume of H
2SO
4
consumed for CO
3
2
-
neutralization
(ml)
Volume of H
2SO
4
consumed for HCO
3
-
neutralization (ml)
CO
3
2-
(meql
-1
)
HCO
3
2-
(meql
-1
)
1
2
3
4

10. Calculations
Amount of 0.1 N H
2SO
4required in 1st titration = A ml
Amount of 0.1 N H
2SO
4required in 2nd titration= B ml
Formulae:
Amount of carbonates (meql
-1
)= 20 x A
Amount of bicarbonates (meql
-1
)= 10 x (B-A)

11. Results :
Thecarbonatesandbicarbonatesingivenirrigationwateris____and
____meql
-1
,respectively.
Cautions:
WhiletitrationtheH
2SO
4solutionshouldbeaddeddrop-wise.