Determination of Carbonate and Bicarbonate in Soil

When phenolphthalein is used as an indicator, strong alkalis like KOH or NaOH are completely neutralized whereas weak alkalis like Na2C03 or K2CO3 are neutralized to the stage of NaHCOg or KHCOg according to the equation. 

The NaHCOg thus formed requires more H2SO4 to get completely neutralized according to the equation. 

It is evident from the above equations that the quantity of H2SO4 required in both the stages of neutralization of NagCOg is the same. The second stage of neutralization of Na2COg (i.e. the neutralization of NaHCOg) can be indicated by methyl orange which can also indicate complete neutralization of alkali carbonate or bicarbonate. Thus phenolphthalein and methyl 

Let volume of aliquot taken from soil extract or water sample be V ml. 

Volume of 0.1 (N) HgSO required for the first titration (with phenolphthalein) = ml. Total volume of H2SO4 required = ml (phenolphthalein plus methyl red) 

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