Gravimetric Determination of Calcium

Determining the mass of a pure compound is a method of a gravimetric analysis. One of the gravimetric analyses is the precipitation; it is a method of separating the analyte from the unknown sample as a precipitate where it will be filtered and converted into a known composition that can be weighed to determine its mass (Skoog et al, 2013). Determining the mass of calcium by using gravimetric analysis was the objective of the experiment. A 25 mL of unknown sample was used to analyze its calcium component. This sample was diluted with 25mL of distilled water in a beaker.

It was converted into a soluble precipitate by adding 25 mL of ammonium oxalate solution and 15 g of solid urea. Since the solution is acidic, the Ca2+ and C2O42- were dissolved. By boiling the solution, the pH of the urea increases thus large, pure crystals of precipitate was able to obtain. Subtracting the mass of the petridish alone from the mass of the petridish with CaC2O4 2H2O precipitate, one can get the mass of calcium oxalate dihydrate. And from that using stoichiometry, one can determine the mass of calcium. Based from what we have computed the resulted mass of calcium is 0.3267 grams.

I. INTRODUCTION
Gravimetric methods are quantitative methods that are based on determining the mass of analyte and one of the methods is the precipitation. Precipitation gravimetry involves converting analyte into a soluble precipitate where it will be filtered, washed, and converted to product of known composition by heat treatment, and then this will be weighed. This scientific report will focus more on being familiar with the gravimetric methods of analysis and determining the weight of calcium in the unknown sample. The mass of calcium in an unknown sample can be determined by using gravimetric method. Precipitating the calcium with oxalate anion, C2O42-, will form a precipitate. Ca2+ (aq) + C2O42-(aq) → CaC2O4 2H2O (s). Wherein, it is soluble in acidic solution because the oxalate anion is a weak base. A successful gravimetric determination depends on the ability to obtain large and pure crystals. One of the ways to obtain it is to carry the precipitation slowly and for this experiment, it is done by dissolving Ca2+ and C2O42- and raising the pH by thermal decomposition of urea.

The solution will be filtered to obtain the precipitate, and then it will be washed to remove impurities and dried to remove water as a possible source of error in the mass. Calcium oxide is the precipitate. CaC2O4 2H2O(s) → CaO(s) + CO(g) + CO2(g). By computing using correct gravimetric factors, one can get the mass of the calcium from the calcium oxalate dihydrate.

II. MATERIALS AND METHODS
In performing the experiment, one will need 0.1 M HCl solution, ammonium oxalate, distilled water, four filter paper, methyl red indicator and urea as reagents. We got our reagents from our laboratory assistant of our school. As for the apparatus, one will need 25-mL volumetric pipette, two 250-mL beaker, analytical balance, desiccator, funnel support, glass funnel, glass rod, hot plate, oven, two petridish, one suction bulb and watch glasses. Before we had starting the experiment, we cleaned all the apparatuses to avoid possible source of error in the process. The methods we use in conducting the experiment are as follows: first, we rinsed the 25-mL pipette by using a small amount of the unknown sample. We prepared two 250-mL beaker and labeled it as trial one and the other trial two. We transferred the duplicated 25mL of the unknown by using the pipette in to each 250-mL beaker.

Then we diluted it with 25mL of distilled water. We added up 5 drops of methyl red indicator and 25 mL of ammonium oxalate solution to each beaker while stirring it with a glass rod. We made sure to rinse the glass rod before removing it to the beaker. Then we added up 15 g of solid urea to each sample. We boiled the solution for 30 minutes up until the indicator turns yellow. After boiling, we filtered it each through a funnel. The remaining solid from the beaker will be transfer to the funnel by adding 3mL of ice cold water to the beaker. We made sure that all of the precipitate has been transferred. After that, we used 10 mL of ice cold water to rinse each beaker and pour the washings over the precipitate.

We transferred the precipitate in to the petridish, but before that we weighted first the petridish with the filter paper so that we can get the grams of the CaC2O4 2H2O precipitate later. We dried the precipitate in an oven set at 105oC for 1 hour. After that, we placed it inside the desiccator for 30 minutes to cool down the heat. Then instantly we weigh the petridish with CaC2O4 2H2O precipitate. In order to get the weight CaC2O4 2H2O precipitate, we subtracted the mass of the petridish alone from the mass of the petridish with CaC2O4 2H2O precipitate. And for determining the weight of Ca in the CaC2O4 2H2O precipitate, we use stoichiometry. We also computed for the average weight of calcium.

III. RESULTS AND DISCUSSION
Sample No.
Trial 1
Trial 2
Volume of sample analyzed, mL
25 mL
25 mL
Weight of petridish with filtered paper, g
71.5650 g
98.2324 g
Weight of petridish with CaC2O4 2H2O precipitate, g
72.5422 g
99.9302 g
Weight of CaC2O4 2H2O precipitate, g
0.9772 g
1.6978 g
Weight of Ca in CaC2O4 2H2O precipitate, g
0.2387 g
0.4147 g
Average weight of Ca in CaC2O4 2H2O precipitate, g
0.3267 g

For determining the mass of the calcium in an unknown sample, we used the precipitation method of gravimetric analysis. A 25 mL of unknown sample was used to analyze its calcium component. We converted it into a soluble precipitate by diluting it with 25mL of distilled water, and adding it with 25 mL of ammonium oxalate solution and 15g of solid urea to the unknown sample while stirring with a glass rod. We did this for calcium oxalate dihydrate to form as a precipitate. Since the solution is acidic, the Ca2+ and C2O42+ was dissolved, and by boiling the solution, the pH of the urea increases thus large, pure crystals of precipitate was able to obtain. We filtered the solution through a funnel. We made sure that all of the precipitate has been transferred. Then we weighed the petridish with filter paper in analytical balance before we put the precipitate on it. For trial 1, the weight of the petridish with filter paper only is 71.5650 grams while in trial 2, it is 98.2324 grams.

Then finally we put the precipitate on the petridish then we weighed. For trial 1, the weight of the petridish with CaC2O42H2O precipitate is 72.5422 grams while in trial 2, it is 99.9302 grams. Then we got the mass of CaC2O42H2O precipitate by just subtracting the mass of the former petridish to the latter with CaC2O4 2H2O precipitate. For trial 1, the weight of CaC2O4 2H2O precipitate is 0.9772 grams while in trial 2, it is 1.6978 grams. Then we were able to get the mass of the calcium in calcium oxalate dihydrate by multiplying the weight of CaC2O4 2H2O precipitate to 1 mole over its molar mass times the molar mass of the calcium. For trial 1, we got 0.2387 grams of calcium while in trial 2, we got 0.4147 grams. Then we also computed for the average weight of calcium which is 0.3267 grams.

IV. CONCLUSION
We therefore conclude that by using a precipitation method of gravimetric analysis, one can get the weight of calcium in an unknown solution. We had converted the unknown solution into a soluble precipitate. It was filtered, washed and converted to product of known composition which is calcium oxalate dihydrate by heat treatment, and then it was weighed. We had determined from the data we obtained in the experience that the average mass of calcium in a 25mL of unknown sample is 0.3267 grams.

V. REFERENCES
http://chemweb.chem.uconn.edu/teaching/chem-232/Laboratory_Manual/GA3_Ca_by_homo_precp_revLS1.pdf http://www.chm.uri.edu/jdwyer/chm212_fall_10/CHM212_Exp5_2010_Final.pdf http://jupiter.plymouth.edu/~jsduncan/courses/2011_Spring/Techniques/Labs/10-GravimetricDeterm.pdf http://www.ausetute.com.au/gravimetric.html