Calcium is determined gravimetrically by precipitating it as CaC2O4â‹… H2O, followed by isolating the precipitate as CaCO3. The sample to be analyzed is dissolved in 10 mL of water and 15 mL of 6 M HCl. After dissolution, the resulting solution is heated
24
to boiling, and a warm solution of excess ammonium oxalate is added. The solution is maintained at 80 °C, and 6 M NH3 is added dropwise, with stirring, until the solution is faintly alkaline. The resulting precipitate and solution are removed from the heat and allowed to stand for at least 1 h. After testing the solution for completeness of precipitation, the sample is filtered, washed with 0.1% w/v ammonium oxalate, and dried at 100–120 °C for 1 h. The precipitate is then transferred to a muffle furnace where it is converted to CaCO3 by drying at 500 ± 25 °C until a constant weight. (a) Why is the precipitate of CaC2O4⋅ H2O converted to CaCO3? (b) In the final step, if the sample is heated at too high of a temperature, some CaCO3 may be converted to CaO. What effect would this have on the reported %w/w Ca? (c) Why is the precipitant, (NH4)2C2O4, added to a hot, acidic solution rather than to a cold, alkaline
Tthe questions
The
(a) The precipitate of CaC2O4⋅H2O is converted to CaCO3 because CaC2O4⋅H2O is a hydrated calcium oxalate compound that contains water molecules in its chemical structure. By heating the precipitate, the water molecules are removed, resulting in the formation of an anhydrous compound, CaCO3. This conversion allows for more accurate and precise determination of the calcium content in the sample since the weight of the anhydrous CaCO3 can be reliably measured.
(b) If the sample is heated at too high of a temperature during the final step, some CaCO3 may be converted to CaO (calcium oxide). This conversion occurs because calcium carbonate (CaCO3) can thermally decompose into calcium oxide (CaO) and carbon dioxide (CO2) at high temperatures. If CaCO3 is converted to CaO, it can lead to inaccurate results in the determination of %w/w Ca because the measurement would no longer reflect the actual calcium content in the sample. CaO has a higher molecular weight than CaCO3, so the reported %w/w Ca would be higher than it should be.
(c) The precipitant, (NH4)2C2O4 (ammonium oxalate), is added to a hot, acidic solution rather than to a cold, alkaline solution because the reaction between calcium ions (Ca2+) and oxalate ions (C2O4 2-) is more favorable under acidic conditions. Adding (NH4)2C2O4 to a hot, acidic solution increases the solubility of calcium oxalate and promotes the formation of a precipitate. In contrast, adding it to a cold, alkaline solution may result in incomplete or slow formation of the precipitate since the reaction requires the presence of acidic conditions to proceed efficiently.