TM-26, Determination of Calcium Carbonate and Magnesium Carbonate in Lime and Pigment Materials

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This method determines the percent calcium carbonate and percent magnesium carbonate in Agricultural Lime and calcium carbonate paint pigments. Reagents and Apparatus

(a) Sargent - Malmstadt Automatic Spectro-Electro titrator, Model S-29700

(b) Hexaver Solution

Dissolve 65 g Hexaver (Disodium Dihydrogen 1, 2, Cyclohexanediaminetetracetate) in 2.0 liters of H2O

(c) Magnesium Chloride Solution

Dissolve 8.00 g MgCl3.6H2O Reagent Grade) in H2O and dilute to 1 liter

(d) Calcon Indicator

Dissolve 0.30 g Calcon in 50 ml of Methanol

(e) EBT Indicator

Dissolve 0.30 g of Erichrome Black T in 50 ml of Methanol

(f) Potassium Hydroxide Solution

Dissolve 100 g KOH (Reagent Grade) in 200 ml H2O Standardization of Hexaver Solution

(a) Weigh 0.5801 g Calcium Carbonate (Primary Standard Grade) and transfer to a 500 ml volumetric flask. Slowly add 15 ml HCl (Sp.Gr. 1.19), and boil for a few minutes to expel CO2. Add 2 g NH4Cl (Reagent Grade) and 200 ml H2O. Add with a pipette 10.00 ml of the MgCl2 solution, and make alkaline to methyl red with NH40H (Sp.Gr. 0.90). Cool to room temperature and dilute to volume.

(b) Turn on the power switch of the automatic titrator and allow to warm up for about 15 minutes. Set the controls as follows:

Function Switch - Spectro
Polarity Switch - No. 2
Wavelength Selector - 650
The Hupp Cadmium Sulfide photocell should be used.

Pipette 25.00 ml aliquots into two 100 ml tall form beakers. To one beaker add 3 ml KOH Solution, 10 ml H2O, and 10 drops of Calcon indicator. Place the beaker on the titration platform and start the titrator. The burette should be adjusted so that the rate of delivery is about 45 seconds between the 35 ml mark and the 45 ml mark. When the titrator shuts off, record the burette reading as Tca. To the second beaker, add 10 ml NH4OH and 8 drops of EBT Indicator. Titrate as described above, and record the burette reading as Tmg.

Calculate the Calcium and Magnesium equivalents of the Hexaver as follows:

CaO Equiv. (Fca) = 65 / Tca
MgO Equiv. (Fmg) = Fca / 1.391
K = Tmg - Tca Procedure

Weigh 0.5000 g sample of the material and transfer to a 250 ml beaker. Moisten with H2O and add 10 ml HCL. Remove the insoluble matter, Si02, and R2O3 by the methods set forth in ASTM C25. Collect all the filtrates and washings from the R2O3 filtration in a 500 ml volumetric flask. Pipette 10 ml MgCl2 solution into the flask, cool and dilute to volume. Tritrate 25 ml aliquots for calcium and magnesium as described above in Section 3. Record the burette readings as Tca and Tmg. Calculations

Calculate the percent Calcium Carbonate and percent Magnesium Carbonate as follows:

% CaCO3 = Fca x Tca x 1.7848
% MgCO3 = Fmg x (Tmg - Tca - K) x 2.0915

Report the results, to the nearest 0.1 percent, as follows:

% Calcium Carbonate CaCO3)
% Magnesium Carbonate (MgCO3)