GRAVIMETRIC DETERMINATION OF SULFATE
You will be given a solid powder, which has been dried to constant mass. The sample is water soluble. The purpose of this experiment is to determine the concentration of sulfate in your solid sample using gravimetric analysis.
The goal of most quantitative chemical analysis measurements is to estimate the relative abundance of an analyte in a chemical sample. For solid and liquid samples, a very common expression of analyte content is based on the mass fraction. This fraction is commonly expressed as a percentage (w/w %), as parts per million (ppm) or as parts per billion (ppb), depending on the concentration level of the analyte. Classical chemical analysis methods are excellent for the determination of analyte concentrations in the range of 1−100 w/w %. In order to estimate the analyte mass fraction of any solid sample, we typically need two measurements, one to estimate the sample mass and one to estimate the analyte mass. In gravimetric analysis, measurements of mass are used to determine the analyte concentration in a sample. In the most common form of gravimetric analysis, the analyte is separated from the rest of the sample in the form of a solid precipitate. In this experiment, the analysis of sulfate is performed using barium chloride as a reagent to precipitate sulfate from the dissolved sample. The mass of sulfate in the sample can be calculated by simple stoichiometry from the mass of the weighing form of the sulfate precipitate. In order to ensure maximum filterability and minimum contamination, the sulfate is precipitated under conditions that maximize the particle size. These conditions include: (a) avoiding excessively high concentrations of precipitating reagent; (b) slow addition of reagent to a hot solution, with vigorous stirring; (c) avoiding a great excess of reagent. The digestion of the precipitant after the reaction minimizes errors due to coprecipitation. For accurate measurements, we must be sure that the composition of the weighing form is known exactly. You must be very careful in your mass measurements throughout this experiment − for example, do not handle your weighing bottle with your bare hands, to avoid leaving fingerprints that would increase the mass of the bottle. Of course, the recovery of the sulfate precipitate must be quantitative, and the final weighing form must be pure.
GRAVIMETRIC ANALYSIS: Procedure
Chemicals and materials: 6 M HCl 0.05 M BaCl2 (note: you will be provided with 0.5 M BaCl2) Whatman #42 ashless filter paper Place the sample in a beaker and cover it with a watch glass to protect your sample from dust in the air. Dry the sample for at least one hour at 110 °C. Allow the sample to cool in a desiccator for at least 30 minutes before weighing. Weigh three samples by difference (your instructor will show you how to do this) into 400 mL beakers. The samples should be approximately 0.4 to 0.5 g in size, and their mass must be known accurately (to the nearest 0.1 mg). Be sure to label the beakers; you do not want to mix up your samples. While the sample is drying, clean three porcelain crucibles and covers and identify each numerically according to the instructor’s direction. Heat the crucibles without covers to constant weight at the highest temperature of the burner. To heat the crucibles to constant weight, place the crucibles in triangles, bring the tip of the flame close to the bottom of the crucible, and heat for 15 minutes (see Harris Fig 29-1). Allow the crucibles to cool in air until only moderate radiated heat is felt when the back of the hand is brought near the crucible (about one minute or so), then transfer the crucibles to the desiccator. After the crucibles reach room temperature in the desiccator (about 30 minutes), weigh the crucibles accurately to the nearest 0.1 mg. Repeat the heating, cooling and weighing until successive weights are within ±0.2 mg. Dissolve each...
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