The Gravimetric Determination of Calcium Introduction The point of this experiment was to develop an understanding of gravimetric analysis to learn how to determine the percent CaO in an unknown sample and to gain the ability to use precipitation to crystallize a sample In order for a procedure to be considered gravimetric it must exhibit a quantitative conversion of a species into a isolatable compound the precipitate must be of known purity and the precipitate must be in a form that can be easily weighed and handled One real life example of gravimetric determination would be the calculation of carbon dioxide dissolved in water The amount of carbon dioxide in water needs to be verified to meet a specification so that it is safe to be used in boilers 1 Procedure Three glass crucibles were labeled AW 1 AW 2 and AW 3 using pencil Then the crucibles and the calcium carbonate was dried for one hour at 110oC The crucibles and the calcium carbonate were dried by putting two of the crucibles into a 400 mL glass beaker and the calcium carbonate along with the third crucible into a 150 mL beaker These two beakers were covered with watch glasses and heated at 110oC for an hour Then using tongs the crucibles and calcium carbonate were placed in a desiccator to cool to room temperature The crucibles were then cleaned with Kimwipes to remove any smudges that would alter the mass of the crucibles After the crucibles were cleaned they were weighed to the tenth of a milligram using an analytical balance A 250 mL beaker another 250 mL beaker and a 400 mL beaker were respectively labeled 1 2 and 3 0 35 grams of the unknown calcium carbonate was placed in each of these beakers The mass of the unknown calcium carbonate was recorded to the tenth 1 The Journal of the American Chemical Society Vol 13 New York John Polhemus Printing 99 Web 30 Jan 2012 of a milligram using the weighing by difference technique 100 mL of deionized waster was added to each of the three samples Then 6mL of 6M HCl was slowly added to each beaker while swirling the solutions The pH of the sample was then tested using pH paper Because the pH was greater than one still several drops of HCl were added to the solutions to lower their pH Four drops of methyl red indicator were then added to beaker 1 2 and 2 20 mL of saturated ammonium oxalate solution was placed in three 50 mL beakers 1 0 mL of 6M HCl was added to these beakers using a Mohr pipet The three 50 mL beakers were mixed by swirling The pH of these three beakers was verified to be less than one using pH paper One of the 50mL beakers was added to each beaker 1 2 and 3 Next 25 g of solid urea was added to each of the numbered beakers The numbered beakers were then mixed by swirling The solutions were then covered with Para film to save for the second lab period At the beginning of the second lab period Beaker 1 was covered with a clean watch glass The solution was then heated to near boil using a hot plate until the methyl red indicator changed from pink to yellow Note that the solution was not allowed to actually boil Also note that the urea will release gas bubbles when heated but these will look different than boiling This process took about 35 minutes When the color did not change within this time period 5 g of urea was added and the solution was swirled When the color still did not change after an additional 10 minutes 5 more grams of urea were added This process was repeated every 10 minutes from this point until the color changed The condensation on the watch glass was then rinsed into the beaker using a wash bottle A filtration system was assembled according to the diagram in the lab manual using two flasks two clamps heavy wall vacuum hose an aspirator and a sintered glass crucible Beaker tongs were used to pour the hot solution into the sintered glass crucible labeled 1 A rubber policeman and a small amount of deionized water were used to transfer the precipitate completely into the crucible Because boiling occurred in the suction flask the filtration of the solution occurred over a six minute time period The procedure above that occurred during the second lab period was repeater for beakers 1 and 2 The solutions were covered in Parafilm for the next lab period At the beginning of the third lab period the crystals from the crucible 1 were washed with 15mL of ice cold deionized water After all the water from the first wash passed through the same crucible was rinsed with 15mL of ice cold water again Then the crystals were rinsed twice with 10 mL of acetone Air was drawn through the precipitate for 5 minutes to allow them to partially dry The crystals were allowed to dry in open air for 30 minutes While the crystals from crucible 1 were drying the process to isolate the crystals was repeated for the crystals in crucible 2 and then 3 After the precipitates from all three crucibles were dried they were placed in an oven for one hour at 105oC The crucibles were placed in the desiccator to cool The crucibles were then weighed to the nearest tenth of a milligram using the analytical balances The crucibles were once again placed in the desiccator for 15 minutes to cool and reweighed This process was repeated until a constant mass was recorded within 0 5mg Note the time in the air that the crucibles were out was minimized since the precipitate was hygroscopic The mass of the CaC2O4 H20 was then recorded for each trial Lastly the calcium content as percent of calcium oxide CaO was calculated and the results of the three trials were averaged The preceding procedure was taken from the Hayden McNeil 2011 2012 Chemistry 203 205 Lab Manual Table of Reagents Chemical Water Formula H20 Molar Mass g mol 18 02 Hydrochloric Acid HCl 26 46 Density g mL 1 0 1 dependant on concentration Calcium Oxide CaO 56 08 3 5 Ammonium Oxalate NH4 2C2O4 28 053 1 5 Strong Acid Irritant when inhaled or in contact with eyes Gives off irritating fumes when burned Urea Methyl Red Indicator Hazards N A CO NH2 2 60 06 1 32 irritating to skin C15H15N3O2 296 3 0 791 N A Questions to Consider 1 Ca2 aq C2O42 aq H2O l CaC2O4 H20 s 2 The precipitate formed through direct combination is finely divided and has to be very carefully filtrated to isolate When the reaction takes place in a cold solution the di or trihydrate is formed instead of the desired monohydrate The HC2O2 is formed by carrying the reaction under acidic conditions which in turn slowly forms the oxalate ion In order to maximize yield and purity