Hello Ravi,

1. In the estimation of total carbonates the reactions occuring are;-

CaCO3 + 2HCl --> CaCl2 + H2O + CO2

MgCO3 + 2HCl --> MgCl2 + H2O + CO2

(a) The solution is boiled to remove the CO2, which would otherwise buffer the solution with bicarbonate ions (HCO3-) and cause a false endpoint.
(b) Phenolphthalein has a colour change form colourless to pink at pH8.2 and is ideal for titration  of HCL with NaOH solution.
(c) The excess HCl is then titrated with standard NaOH;-

HCl + NaOH --> NaCl + H2O

For the estimation of Magnesium Carbonate, the solution remaining after the Total Carbonates titration can be used. This solution contains all of the Mg+2 ions in neutral solution.
If the temperature of the solution is kept at 90 to 100oC, it is possible to convert these magnesium ions to magnesium hydroxide which is almost insoluble at 100degC and precipitates out of solution at a pH of 10.3, according to the following reaction;-

MgCl2 + 2NaOH --> Mg(OH)2 + 2NaCl

(a) Thymolphthalein indicator changes colour over the range 9 ( Colourless) to 10 (Blue).
(b) As the pH rises due to the addition of NaOH, the phenolphtpalein (already in the solution from the Total Carbonates titration) turns red and the mixture of blue and red gives a violet colour at pH 10.3, which is the point where all of the magnesium hydroxide has precipitated.
(c) The excess NaOH is then titrated with standard HCl;-

NaOH + HCl --> NaCl + H2O

(2)  A theoretical value of CaCO3 can be calculated from the XRF chemical analysis of limestone or raw meal by assuming that all of the CaO analysed was present as CaCO3. It is then a simple matter of converting the % CaO firstly into moles of CaO, then into moles of CaCO3 and then into % CaCO3 by the following calculations;-

%CaCO3 = %CaO / 56.1 * 100.1
or
%CaCO3 = %CaO * 1.784

The same can be done for MgO, as follows;-

% MgCO3 =  %MgO / 40.3 * 84.3
or
%MgCO3 = %MgO * 2.092

Therefore the theoretical TC = (%CaO * 1.784) + (%MgO * 2.092)

Hope this helps you...

Regards,
Ted.

Hi

What Mr Ted explained is verymuch to the point, I want to add one more point to this.

"every calculation depends upon what you are taking for getting the factor for TC (total carbonate). If CaCO3 is taken than the total carbonate is determined as CaCO3, in that case MgCO3 is converted to CaCO3 by multiplying a factor which is equle to (mol wt of caCO3/mol wt of MgCO3) and added to CaCO3."

chari

Hello Chari,

Thank you for adding that point. You are quite correct, of course.  If the TC is to be expressed as total carbonates as CaCO3 then the formula becomes;-

TC (as CaCO3) =  (%CaO * 1.784) + (%MgO * 100.1 / 40.3 )

or

TC (as CaCO3) =  (%CaO * 1.784) + (%MgO * 2.484)

Regards,

Ted.

It is very kind of "You" for providing me such informative knowledge.

Thankyou Very much Sir