Dear Sir,

pl refer this paper.

Data Method for Estimation of Calcium Carbonate in Soils from Iraq

Dear Durga,

Determination of carbonate by titration method:

DETERMINATION OF CARBONATES AND BICARBONATES

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  Gives 2 NaHCO3 + Na2SO4

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 Gives 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.

Reagents :

1) Saturated H2SO4 (0.01N) : Carefully add 2.8 ml of conc. H2SO4 to one litre volumetric flask and dilute to one litre with distilled water, the strength will be approximately 0.1N H2SO4. Dilute 100 ml of this solution to 1 litre to obtain 0.01N H2SO4. Standardize it against primary standard, Na2CO3.

2) Standard Na2CO3 (0.01N) : Dissolve 5.3 gm of A.R. grade Na2CO3 in one litre volumetric flask with distilled water, the strength will be 0.1N Na2CO3. Dilute 100 ml of this solution to get 0.01N. This may be used for standardization of 0.01N

H2SO4.

3) Phenolphthalein (0.25%) : Dissolve 25 gm of pure Phenolphthalein powder in 100 ml of 60% ethyl alcohol.

4) Methyl Orange (0.50%) : Dissolve 0.5 gm of dry methyl orange powder in 100 ml of 95.0% ethyl alcohol.

Procedure :

1) Transfer 25 ml of water sample to a 150 ml conical flask. Add 2-3 drops of Phenolphthalein.

2) If pink red colour appears, titrate it against standard H2SO4 till colour disappears. The burette reading (volume used) is designated as Y ml.

3) To this colourless solution or in original sample (25 ml) add 2-3 drops of methyl orange. This will develop the yellow colour.

4) Again titrate with standard H2SO4 till colour changes from yellow to rosy red. Record the volume of H2SO4 as Z ml. This volume corresponds to initial carbonate changed to bicarbonates plus initial bicarbonates present in irrigation water.

5) Run a blank (25 ml distilled water) and subtract from the titre value to avoid error due to any impurity of chemicals.

For more details on this procedure, please see pages 93-95 in the attached file.

Another method which may interest you is the determination of carbonate by rapid gasometric method. For more on this method, please see the publication contained in the following link:

 http://pubs.acs.org/doi/pdf/10.1021/jf60130a002

Hoping this will be helpful,

Rafik

Dear Durga Singh Nagar

You can also check the following publications:

- Routine method for calcium carbonate equivalent of soils.Moore et al. Communications in Soil Science and Plant Analysis 18(3):265-277 · March 1987

An acetic acid method for determination of soil CaCO3 equivalent has been adapted for the rapid, routine analysis of large numbers of samples. The method proved to be very sensitive, as assessed by making standard additions of CaCO3 to soils.

-Methods of Soil Analysis. Allison and Moodie 1965.

Cordialy

Marius