DETERMINATION OF MgO WITH EDTA
Standardization with CaCO3. Obtain about 1g of the standard CaCO3. Weigh accurately between 0.6 and 0.7 g of the CaCO3 into a 250 mL conical flask. Carefully add about 5 mL of con. HCl to the CaCO3. After dissolution is complete add about 50 mL of water, rinsing the sides of the flask with DI water. Boil for 5 minutes to remove carbon dioxide. Quantitatively transfer the cooled solution to a 500 mL volumetric flask and accurately dilute this solution with water, mixing well. This is your stock solution. Transfer your stock solution to a clean 1 L polyethylene bottle that has been rinsed with a small portion of your stock solution for storage.
Pipet five 50 mL aliquots of the standard CaCO3 solution into separate 250 mL conical flasks. Immediately before titration add about 10 mL of the ammonia buffer solution, about 20 mL of deionized water, and enough EBT indicator to see the wine red color due to the Ca-In complex (do not add too much indicator). Titrate to a pure blue end point (the color will go from a red slowly to purple, and then quickly to blue). If unsure of the endpoint record your current volume and add a small volume of titrant from your buret. If there is a color change repeat this procedure until the color does not change with an additional amount of titrant. The endpoint will be given by the last volume recorded which caused a color change. Repeat the procedure on the remaining solutions. Calculate the molarity of calcium ions in the stock solution using the mass of CaCO3 dissolved and the final volume of the stock solution. The M.W. of CaCO3 is 100.087. Use the molarity of Ca2+ and the volume pipetted to calculate the molarity of EDTA in your solution (the Ca-EDTA complex has a 1:1 ratio of ligand to metal.) Perform a statistical analysis of the EDTA molarity values.
Determination of MgO purity. Dry your unknown sample (magnesium sulfate) in a weighing bottle for 3 hours at 160oC before weighing. Cool your sample in a desiccator, and when cool quickly weigh into separate conical flasks 5 samples. Choose sample sizes that will require between 30 and 50 mL of titrant. [A typical 'first guess' might be about 0.30g of unknown.] Be sure to weigh with the lid to the weighing bottle ON to prevent water from absorbing into the unknown during weighing. Dissolve the samples in about 75 mL of deionized water, and immediately prior to the titration add 10 mL of the pH 10 buffer and the indicator to each flask. Add the same amount of indicator to all samples. Titrate the samples with your EDTA until the endpoint is reached.
Determine the percent MgO for individual samples and complete a statistical analysis. Notice that the magnesium is reported as percent MgO in the sample, and that the Mg-EDTA complex formed has a 1:1 ratio of metal to ligand. The M.W. of MgO is 40.31.