CHEM 334 Quantitative Analysis Laboratory Determination o f C hloride b y G ravimetric A nalysis Introduction Gravimetric analysis signifies a set of methods for the quantitative determination of a particular analyte based on the mass of a solid. A trivial example is the measurement of solids suspended in a water sample: A known volume of water is filtered and the collected solids are weighed. In many cases, a soluble analyte must be converted to a solid by precipitation with a suitable reagent. The precipitate can then be collected by filtration, washed, dried to remove traces of moisture from the solution and weighed. The amount of analyte in the original sample can then be calculated from the mass of the precipitate and the precipitate's chemical composition. In other cases, it will be easier to remove the analyte by vaporization. The analyte might be collected, perhaps in a cryogenic trap and measured directly. Alternately, the sample being analyzed can be weighed before and after it is treated, for example, dried; the difference between the two masses gives the mass of analyte lost. This is especially useful in determining the water content of varied materials. Gravimetric analysis can provide an exceedingly precise analysis. Gravimetric analysis provides very little room for instrumental error and does not require a series of standards for calculation of an unknown. In addition, methods often do not require expensive equipment. Gravimetric analysis, due to its high degree of accuracy, can also be used to calibrate other instruments in lieu of reference standards. Procedures Dry between 0.4 and 0.5 g of unknown analyte to constant weight. Accurately weigh an approximately 0.1 g sample of the analyte, transfer it into a 250 mL beaker and dissolve it in 4-­‐5 mL of 1 M HNO3 and 20 mL of water. Compute the volume of 0.1 M AgNO3 (silver nitrate solution) corresponding to 1.1 equivalents of the analyte assuming it is pure sodium chloride. Add this quantity of silver nitrate solution to the beaker and swirl the beaker occasionally (do not insert any implement in the beaker for the purpose of stirring the solution) for approximately ten minutes. Let the precipitate settle for a few moments then add a few drops of silver nitrate solution to confirm that the precipitation is complete. If additional precipitate forms, add a small volume (approximately one-­‐tenth of the original addition) of silver nitrate solution and again check for completeness of precipitation. Cover the beaker with aluminum foil, using two pieces, one for the sides of the beaker but not the bottom and a second for a removable top, to exclude light and prevent photooxidation of the solid silver chloride. Do not cover the beaker's bottom surface as this often reduces the rate of heat transfer from the hot plate (remarkably). Heat the beaker still covered with the aluminum foil for ten minutes on a hot plate (150 °C) just short of boiling to cause the silver chloride particles to aggregate. Cool the beaker still covered with the aluminum foil to room temperature. Set up a water aspirator, clean suction flask, rubber crucible adaptor and clean crucible (use water, paper towels and, if necessary, 1 M aqueous ammonia) as shown in the figure. The crucibles used in this experiment are perforated at the bottom with large holes. Cut out three small circles of filter paper to fit Colorado State University 2017.04.06 Page 1 of 2 Determination of a Titration End-­‐Point using a pH Meter CHEM 334 Quantitative Analysis Laboratory the bottom of the crucible, place all three circles in the crucible and dry the combination to constant weight. Place the crucible into the crucible holder and press the filter paper assembly firmly into the bottom of the crucible. Turn on the cold (not hot) water with full force and wet the filter paper with a little water. Confirm that the liquid is quickly drawn through the filter paper and into the suction flask. Quantitatively transfer the contents of the beaker to the crucible. After the liquid has been drawn through the filter, wash the precipitate with water until it is free of silver ions. Ten to twenty milliliters of water will suffice. Note: always "break" the vacuum before turning off the cold water to prevent the transfer of tap water into the suction flask and contamination of the filtrate. Next, remove the water from the precipitate by washing it with approximately ten milliliters of acetone to speed drying. Add the liquid wash slowly, over a period of around ten seconds to thoroughly replace the water on the precipitate and filter paper with the acetone. After the Figure 1. The vacuum filtration apparatus acetone wash, draw air through the precipitate for around including water aspirator, suction flask, one minute. Dry the crucible, filter paper and precipitate in crucible holder, crucible and stand. a 105 °C oven for ten minutes and cool on the bench. Accurately weigh the crucible, filter paper and precipitate combination. Repeat the entire determination process one more time with fresh analyte. Results Report the number of moles and mass of chloride and the percentage (by weight) of chloride in each of the samples. Include a standard uncertainty analysis. Discussion In addition to a customary discussion of the results presented in this Report, explain (1) photooxidation in the context of these reagents (what are the reactants and products of photooxidation) and (2) the effects on the accuracy of the determination if the photo-­‐oxidation of silver chloride occurs. References th
Harris, D.C., "Quantitative Chemical Analysis" (2007) 7 edition, Freeman & Co., NY, Chapter 11, especially sections 11-­‐5, 11-­‐6, 11-­‐7 and 11-­‐8. Gravimetric analysis, Encyclopedia Britannica. http://www.britannica.com/EBchecked/topic/242396/gravimetric-­‐analysis (accessed Oct 2013). Colorado State University 2017.04.06 Page 2 of 2