Chemistry 119: Experiment 8. Inorganic and organically-bound sulfur may be determined in a sample by conversion of all S to the SO 4

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1 Chemistry 119: Experiment 8 Assay of SO3 by Gravimetric Analysis of Sulfate Inorganic and organically-bound sulfur may be determined in a sample by conversion of all S to the SO 4 2- ion. Usually, this step is accomplished by ignition of the sample in oxygen, in a closed environment. Once the conversion of sulfur to sulfate has been accomplished, the sulfate can be conveniently measured by gravimetry. In this experiment, we will analyze an inorganic sulfate to avoid the need for any combustion step, which is difficult and dangerous for beginners to attempt. The most common precipitant for sulfate is the barium ion. The reaction is Ba 2+ + SO4 2- = BaSO4 (s) (3.1) This apparently simple gravimetric determination has a few difficulties. The most significant difficulty with collecting and weighing BaSO4 is the tendency for BaSO4 to coprecipitate other species, especially those containing potassium and iron (III) cations. Rapid precipitation from hot solution of the sample, something not usually done because it produces small particles, is advised here to minimize - but not eliminate - the coprecipitation of these cations. Another difficulty is that BaSO4 has no good solvents, and it cannot be redissolved, and reprecipitated. If we could dissolve it, we could purify the precipitate by repeating the precipitation in a solution with less of the ions likely to coprecipitate. The lack of good solvents causes another problem. The finely divided crystals of BaSO4 produced must be digested to make particles large enough for collection on filter-glass mats. Filtering crucibles cannot be used, as the BaSO4 cannot be removed from the crucible by any solvent. Instead, after filtering and washing the digested precipitate, the filter paper mat must be charred carefully, then ashed to remove the paper so that it does not contribute to the weight of the precipitate. BaSO 4 is stable to very high heats, and heating with a Bunsen burner has no effect on it. In this experiment, we will report the amount of sulfur present not as %S or %SO4 2- but as %SO3. This is known as an assay. In an assay, we measure one material and infer from that measurement the amount of another material. An assay presumes a stoichiometric relation between what is measured and what is sought. Here, BaSO4 is measured, but the weight percent (written as %(w/w) to indicate that this is a percentage by weight of the solid ) of SO3 is sought. There is a 1:1 stoichiometric relationship between SO3 and BaSO4, so these two species are related by the gravimetric factor 1

2 %(w/w) SO3 = wt BaSO4 FW BaSO4 * FW SO 3 wt sample * 100% (3.2) where FW SO 3 / FW BaSO 4 is the gravimetric factor for reporting %(w/w) SO 3 when one measures BaSO 4. A similar expression could be used to assay any other S- containing species from measurements of the weight of BaSO4. The procedure given below has shown to lead to the most accurate results for weights of BaSO4. Prelaboratory Assignment A g sample of unknown sulfate was dissolved in about 25 ml of distilled water, and precipitated as BaSO4 with excess BaCl2. The precipitate was digested, then ashed. The final weight of BaSO4 was g. Calculate the weight percentage of sulfur in the unknown as % (w/w) SO3. Apparatus 2 porcelain filter crucibles 2 fiber-glass filter mats mL beakers 25-mL graduated cylinder polyethylene wash bottle with distilled water mL beakers 2 stirring rods, each with rubber policeman 2 watch glasses to fit 600-mL beakers dessicator with Drierite desiccant Bunsen burner 2 wire triangles 2 ring stands with iron rings or tripods filtration apparatus (500-mL filter flask, Walters adapter, 3-ft section of vacuum tubing) Chemicals hydrochloric acid (6 M) BaCl2 (0.05 M) AgNO3 (0.1 M) in dropper bottle HNO3 (8 M) in dropper bottle distilled water 2

3 Procedure: 1. Dry about 2 g of the unknown sulfate sample in a 105 o C oven for at least one hour. Use a small glass vial as a weighing bottle (without lid) to contain the solid sample. Label a 150-mL beaker, put the weighing bottle in the beaker and cover the beaker with a watch glass. Always use this method to dry solid samples. 2. Clean and mark two porcelain filter crucibles. Fit each with a fiber-glass filter mat. 3. Mount a crucible on the wire triangle at a 45 o angle, and heat for 20 minutes with the Bunsen burner. Cool to room temperature in a desiccator (this will take at least 30 minutes) and weigh. 4. Repeat step 3 with the other crucible. The heating step for one can be done while the other crucible is cooling. 5. Weigh out two samples of the dried, cooled unknown to ±0.1 mg. These samples should each weigh about 0.5 g, but anything in the range 0.4 to 0.6 g is acceptable. 6. Weigh each of the cooled crucibles to the nearest 0.1 mg. 7. Transfer each sample to a 600-mL beaker, dissolve in 200 ml of distilled water, and then add 4 ml of 6 M HCl. Heat these solutions to near boiling with a Bunsen burner. 8. Heat about 100 ml of 0.05 M BaCl2 (this amount should contain and excess of the BaCl 2 precipitant) to near boiling, and add it quickly with good stirring to the first beaker containing unknown. Stir with a clean glass stirring rod. Leave the stirring rod in the solution. Warning: Soluble barium salts are toxic! Handle BaCl2 carefully and wash your hands thoroughly when you complete this laboratory. Dispose of excess BaCl2 as instructed. 9. Repeat step 8 for the other sample solution. 10. Cover each beaker with a clean watch glass and heat to near boiling for 45 minutes. Also heat about 50 ml of distilled water in a 150-mL beaker. 3

4 11. After allowing the solutions to cool to the point that the beakers can be safely handled, carefully decant the hot supernate through the filter, using suction to draw the liquid through. Then wash the precipitate remaining in the 600-mL beaker with three small (about 10 ml) portions of hot water. Decant the washings through the filter. 12. Quantitatively transfer the precipitate to the filter as shown in Figure 3.1. Use the rubber policemen to loosen any particles that stick to the wall of the beaker. Wash the beaker with water to help the transfer. Figure 3.1 Quantitative Transfer of a Precipitate to Filter Crucible. 13. Wash the precipitate in the filter crucible with two or three 5-mL portions of hot water and collect the filtrate in a clean 150-mL beaker. Acidify the filtrate with 2 drops of 8 M HNO3. 4

5 Warning: Even 8 M HNO3 is corrosive! Handle this acid very carefully, and avoid contact with your skin or clothes. If you do spill some, wash it off at once. Wash your hands carefully when you complete this laboratory. Check the filtrate for chloride ion by adding 1 drop of 0.1 M AgNO3 from the dropper bottle, then mixing. If the filtrate turns cloudy, chloride ion is present (AgNO3 forms a white precipitate of AgCl when chloride is present) If the filtrate contains chloride, continue to wash the precipitate. Empty the filtrate containing the AgCl into the waste bottle provided, and rinse the 150-mL beaker with distilled water. The rinse may be discarded in the sink. 14. Repeat step 13 until the filtrate is clear or only very slightly cloudy upon addition of the AgNO3. Draw air through the filter for 1-2 minutes to dry the solid barium sulfate partially. 15. Repeat steps for the other sample. 16. Heat the first crucible with a moderate flame to dry the precipitate completely (about 15 minutes) without melting the fiber-glass mat. 17. Cool the heated crucible in a desiccator for 30 minutes. Weigh the cooled crucible. 18. Heat the cooled crucible again for 15 minutes, cool for 30 minutes, and reweigh. If the weights do not agree to about ±0.3 mg, one would normally repeat the heating, cooling and reweighing until they agree. However, in this student exercise there is not sufficient time to heat to constant weight. Instead, you should use the second weight as the best estimate of the weight of the crucible, mat and dry precipitate. 19. Repeat steps with the other crucible. Calculations and Report: 1. From the weights of the crucibles and weights with precipitate, calculate the amount of BaSO4 in each of the marked crucibles. 2. Using the gravimetric factor given in equation 3.2, calculate the %(w/w) SO3 in each of the three samples analyzed. 5

6 3. Calculate the mean %(w/w) SO3 from your two results. 4. In order to estimate the precision of your results, it is important to have at least three replicate analyses. Then the calculated standard deviation will be meaningful. Because of time limitations, you were only able to obtain duplicate results. Inexperienced students typically are able to achieve standard deviations of 5 parts per thousand in this analysis. Compare your precision with this standard. If your two results differ by much more than this amount, discuss suspected or likely sources of error that might cause one result to be high or low. 5. Suppose instead of %(w/w) SO 3, you have been asked to report %(w/w) S. Compute %(w/w) S for each of your samples and report the mean. 6. It is possible to use a variant of this procedure to determine %(w/w) Ba in a sample containing a soluble barium compound. Describe how you would alter the procedure for such an assay of barium. This experiment has been adapted from a laboratory manual authored by Professor S. D. Brown. 6

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