Quantitative Determination of Sulfate by Gravimetric Analysis Essay

1. Synopsis: This report is written about determining the quantitative amount of sulphate inside barium sulphate, BaSOâ‚„, using the method of gravimetric analysis. This quantitative determination is done by the addition of a dilute solution of barium chloride slowly to a hot unknown sulfate solution slightly acidified by concentrated hydrochloric acid, HCl. The white precipitate of barium sulphate is filtered off, washed with water, oven-dried, and weighed as barium sulphate. The quantitative amount of sulphate is deduced from mathematical calculations. The results of the experiment, however, did not yield positively, probably due to inadvertent human error over the course of the experiment. The percentage yield of sulphate inside barium sulphate attained from our results was not up to expectations. The percentage yield of sulphate was expected to be at least 90% and above, with >90% as a good percentage yield. Instead, we attained 54% percentage yield of sulphate. 2. Objective: The purpose of the experiment is to determine the quantitative amount of sulphate inside barium sulphate using the method of gravimetric analysis. 3. Theory: 3.1 Summary: Throughout the duration of the experiment, there are many procedures, techniques, chemicals, and instruments used to produce the results of the experiment. There are a total of three simple sets of procedures required, in the gravimetric analysis method, in order to create the results of the experiment. The first procedure is the precipitation of BaSOâ‚„, barium sulphate, followed by the second procedure, the washing and filtration of BaSOâ‚„ precipitate. The third and final procedure is the drying and weighing of the dry sample of BaSOâ‚„ precipitate. From there, the results are gathered by methodical mathematical calculations. 3.2 Technique: Gravimetric Analysis: Gravimetric analysis is a series of methods in analytical chemistry for finding the quantitative amount of a certain analyte based on a sample of solid. To perform gravimetric analysis, one of the most common methods is to convert the analyte into a solid via the use of precipitation with the appropriate reagent chemicals. After that, the precipitate is collected via filtration, washed, dried, off all moisture content, and weighed. Then, the quantitative amount of analyte in the sample is calculated from the mass of the precipitate and its chemical composition. There are many advantages using gravimetric analysis. It allows for extremely precise analysis, such as the determination of many elements’ atomic masses up to six decimal places. It also does not require expensive scientific equipment to perform such analysis and, furthermore, it can even be used to calibrate scientific instruments in lieu of international reference standards. 3.3 Chemicals: During the experiment, some chemicals were used to obtain the barium sulphate, BaSO4, from which the quantitative amount of sulphate can be found from within. The chemicals used were dilute 10% barium chloride solution, BaCl2, dilute 0.5% sodium sulphate solution, (Na)2SO4, and concentrated hydrochloric acid solution, HCl. In order to obtain barium sulphate, a chemical process, known as the displacement reaction, was utilised. In the displacement reaction, the cations and anions switch places from their original compounds to form entirely different compounds. In this experiment, 10% barium chloride solution is added to 0.5% sodium sulphate solution (which is slightly acidified by adding concentrated hydrochloric acid), resulting in the formation of soluble barium sulphate. 3.4 Instruments: In the experiment, various scientific instruments were used in the determination of the quantitative amount of sulphate. The following instruments were used, were the 250ml beaker, the bulb filler and vacuum-assisted pipette, the measuring cylinder, the watch glass, the laboratory crucible, the vacuum pump, the hot air oven, the desiccator, and the digital analytical weighing balance. The 250ml beaker is a cylindrical container with a flat bottom, which is used as a simple container to stir, heat, or mix various liquids. The vacuum-assisted pipette is a hollow narrow cylinder that has a large bulge with a single graduation mark as it is calibrated for its specific volume, generally between 10ml, 25ml, and 50ml. The bulb filler is the simplest form of the pipette dispenser, using pinch valves to draw air within to create a vacuum within the vacuum-assisted pipette. The two pieces of laboratory equipment are generally used in conjunction with one another. The bulb filler is carefully inserted on top on the vacuum-assisted pipette. The pinch valves can be manipulated to draw the liquid inside the pipette. The measuring cylinder is a narrow cylinder with a flat base that is used to measure amounts of liquid with the corresponding markings along the cylinder. The watch glass is a circular, slightly convex-concave piece of glass that is generally used to evaporate a liquid, hold solids being weighed, or as a cover for the beaker. The laboratory crucible is a cup-shaped piece of laboratory equipment made to contain chemical compounds as they are heated to extremely-high temperatures. The hot air oven is an electrical oven used to dry chemical compounds or sterilise articles. The desiccator is a sealable enclosure that is used to preserve items sensitive to moisture in the open air, such as cobalt chloride paper. The digital analytical weighing balance is type of electronic balance made to measure small amounts of mass up till several decim al figures. 4. Procedures: In order to determine the quantitative amount of sulphate, the procedure that is split up into three smaller sections. The first section is the precipitation of barium sulphate. The second section is the washing and filtration of the barium sulphate precipitate. And, the third section is the drying and weighing of the barium sulphate precipitate. 4.1 Precipitation of BaSO2: 1. Use the bulb filler and vacuum-assisted pipette to pipette 25ml of the 0.5% sodium sulphate solution into a 250ml beaker. 2. Add 50ml of water and 5 drops of concentrated hydrochloric acid into the beaker. Note: Concentrated hydrochloric acid is highly corrosive. Add the concentrated hydrochloric acid into the beaker while handling it in the fume hood with protective gloves and goggles. 3. Heat the beaker until it is boiling. Use a glass rod to stir the solution vigorously, while adding 10ml of 10% barium chloride solution from a measuring cylinder drop-by-drop. 4. Use a watch glass to cover the beaker and adjust the heat to just below temperatures. Leave it there to digest for 20 minutes. 5. To test for complete precipitation, add a few drops of barium chloride and observe to see if there is clear supernatant liquid. 4.2 Washing and Filtration of BaSO4 Precipitate: 1. Take two pieces of filter paper and place them at the base of the dry and weighed laboratory crucible. Ensure that the filter paper pieces cover the base of crucible completely. Then, use the vacuum pump to decant the clear supernatant liquid by filtration into the crucible. 2. Dislodge any particles in the beaker and rinse it with warm deionised water. Empty the contents into the crucible while the vacuum pump is at work. Make sure that all the solids in the beaker have been transferred to the crucible. 3. Wash the barium sulphate precipitate further with warm deionised water at the vacuum pump twice more. 4. Discard the filtrate. 4.3 Drying and Weighing of BaSO4 Precipitate: 1. Place the crucible, containing the BaSO4 precipitate, into the hot air oven. Set the temperature to 150 °C and leave it for half an hour. 2. Use the desiccator to cool the crucible and precipitate for 10 minutes. 3. Once the crucible has cooled down, weigh it using the digital analytical weighing balance. 4. The weight of the BaSO4 precipitate is calculated from the difference between this weight and the weight of the empty crucible including the filter papers. If there is still sufficient time, you may repeat the above Steps 1-4 until a constant weight of the precipitate is successfully obtained. 5. Results and Calculations: | 1st Drying:| 2nd Drying:| Mass of Crucible + Filter Paper + Sample:| 31.9078g| 32.0188g| Mass of Crucible + Filter Paper:| 31.7975g| 31.9071g| Mass of Sample (BaSO4):| 0.1103g| 0.1117g| The mathematical calculations to attain the results of this experiment are listed below: 0.5% of sodium sulphate (NaSO4) = 5100 Ãâ€"25g = 0.125g Composition by mass of SO42- = Molecular weight of sulphate ionMolecular weight of sodium sulphate Ãâ€" 0.125 = 0.0845g (4 significant figures) Composition by mass of SO42- prepared = Molecular weight of sulphate ionMolecular weight of barium sulphate Ãâ€"0.1103g = 0.04544g (4 significant figures) Percentage yield of sulphate = 0.045440.0845 Ãâ€"100% = 53. 775% ≈ 54% 6. Discussions: The objective of this experiment was to determine the quantitative amount of sulphate using the gravimetric analysis method. The quantitative amount of sulphate was measured in percentage yield, which we attained 54% instead of the expected percentage yield of 90% and above. It became obvious that somewhere along the way, in conducting the experiment, a significant error had been committed. After much analysis, it was found that there had been some sources of error that accounted for the less-than-satisfactory results. One major source of error could be the contamination of the intended precipitate through the use of laboratory instruments and vessels that were not cleaned properly. When the instruments and vessels are unclean, any left-over remains of chemicals and compounds could be unintentionally released to the intended precipitate and polluted it through a process known as co-precipitation. The foreign species could have reacted with the intended precipitate and resulted in the loss of much of the sulphate ions, leaving only 54% instead of the intended 90% and above. To avoid any possible error of contamination, one must keep in mind to properly clean the instruments and vessels to use in the experiment. One way to minimise the co-precipitation of substances would be leaving the solution, containing the soluble precipitate of barium sulphate, in the process of forming the precipitate, to digest longer than the standard 20 minutes. Another source of error could be the decomposition of the precipitate itself during the process of removing moisture content in the hot air oven. The ignition can result in the losses via decomposition of the potentially-volatile precipitate. 7. Conclusion: In conclusion, the results were not up to expectations due to a few sources of error that caused the less-than-satisfactory results. Gravimetric analysis is a proven set of methods to use in the field of analytical chemistry. It allows for extremely precise results, if the procedures were followed very carefully, and no errors were committed over the course of the experiment. However, we did not attain 90% and above for the percentage yield of sulphate as we committed some errors unknowingly. Contamination was a major issue in the experiment that would have been avoided if only we had properly cleaned the instruments before performing the experiment. In short, the objective of the experiment was fulfilled by attaining sulphate using the gravimetric analysis method, although not all of it was attained. 8. References: Online References: Theory: 1. Wikipedia: Gravimetric Analysis Available from: http://en.wikipedia.org/wiki/Gravimetric_analysis Accessed from: 20th June 2013 2. Wikipedia: Instruments Available from: http://en.wikipedia.org/wiki/Beaker_(glassware) http://en.wikipedia.org/wiki/Pipette http://en.wikipedia.org/wiki/Measuring_cylinder http://en.wikipedia.org/wiki/Watch_glass http://en.wikipedia.org/wiki/Crucible http://en.wikipedia.org/wiki/Hot_air_oven http://en.wikipedia.org/wiki/Desiccator http://en.wikipedia.org/wiki/Analytical_balance#Analytical_balance Accessed from: Accessed from: 20th June 2013 3. R.L. Watters, Jr, 1997, Gravimetry as a Primary Method of Measurement Available from: http://www.rminfo.nite.go.jp/common/pdfdata/4-002e.pdf Accessed from: 20th June 2013