Gravimetric Determination of Sulfur Trioxide in a Soluble Sulfate Salt Introduction Sulfate ion is conveniently precipitated as barium sulfate with barium chloride solution as shown in the reaction: Ba+2(aq) + SO4-­‐2(aq) → BaSO4(s) After filtering, the paper is charred off and the precipitate ignited to constant weight. The SO3 content is calculated from the weight of BaSO4. Students will notice that this experiment is similar to the gravimetric chloride experiment. Solutions needed for this experiment: Solutions prepared by the student Solutions provided by the instructor 0.25 M barium chloride Concentrated hydrochloric acid 1:4 sulfuric acid solution Concentrated sulfuric acid Concentrated nitric acid 0.2 F silver nitrate Specialized equipment needed: You will need 3 ignition crucibles (Figure 1). These are shorter and fatter than the filtering crucibles, and your instructor can help you identify them. Figure 1. Ignition crucible 1
Experimental Procedure 1. Clean three ignition crucibles using the following procedure. a. Wash the crucibles with soap and water. Rinse with tap water, followed by three rinsings with DI water, ~5 mL per rinse. b. Dry the crucibles with paper towels to remove excess water. c. Place the cleaned crucibles and covers over Fisher or Tirrill burners and heat to the maximum temperature of the burner for 10-­‐15 minutes (see Figure 2 below). Allow the crucibles to cool for 15 minutes, and then cool them in a desiccator for at least one hour. d. Accurately identify and weigh the crucibles and covers using the analytical balance. MAKE SURE TO HANDLE THE CRUCIBLES AND COVERS WITH TONGS!! Figure 2. Heating crucibles with Fisher/Tirrill burner. 2
2. Record your unknown number, and transfer the unknown to a weighing bottle. Oven dry the unknown for about 2 hours at 105 – 110 oC. After drying, store the unknown in the weighing bottle in your desiccator and allow it to cool. 3. Wash 3 beakers (600 mL) using soap and water, and rinse them with 3 portions of deionized water (25 mL portions). Clearly label these beakers so that you can tell them apart! 4. Weigh 3 individual samples of your unknown on the analytical balance, and put each sample into its own clean 600 mL beaker. Each sample should be between 0.5 and 0.7 grams. Samples slightly above 0.7 grams, or below 0.5 grams, will work. Samples significantly outside the indicated range should be avoided. 5. Dissolve each sample in about 250 mL of deionized water. Add about 0.5 mL of concentrated hydrochloric acid to each beaker. 6. Assume that the unknown is pure sodium sulfate, and calculate the volume of 0.25 M barium chloride necessary to react stoichiometrically with each soluble sulfate sample. 7. Working with one sample at a time, heat the sulfate solutions prepared in step 5 nearly to boiling, and add the calculated amount of barium chloride solution, slowly, from a burette. Continue adding barium chloride solution until 10% excess has been added. Stir each mixture vigorously while adding barium chloride. 8. Allow the precipitate to settle, and test for completeness of precipitation by adding a few drops of barium chloride solution without stirring. If additional precipitation occurs, add 5 mL additional barium chloride solution. Repeat this procedure until no further precipitation occurs. 9. Leave the stirring rods in the beakers, cover with watch glasses and digest the samples over low heat until the supernatant liquid is clear. This normally takes 30-­‐60 minutes, but will sometimes take longer. If the volume of solution falls below ~200 mL, add DI water to adjust the volume to ~200 mL. Alternatively, digestion can be accomplished by covering the beakers with watch glasses and storing overnight in the locker drawer. 10. Prepare three 11 cm No. 42 Whatman “ashless” filter papers (or equivalent) for filtration. The filter papers should be well fitted to the filtering funnels: if not the filtration will be slow. 11. Reheat the precipitate solutions (if necessary) and filter the precipitates while hot. Do NOT overfill the funnel, since barium sulfate will tend to 3
“creep” up the filter paper and the funnel. Fill the filtration funnel 1/2 to 2/3 full, but not more than 2/3 full. 12. After filtration, wash the precipitate with hot distilled water. Test the washings by collecting a few milliliters in a test tube, acidifying with a few drops of nitric acid, and adding 2 or 3 drops of silver nitrate. If precipitation occurs additional washing is necessary. If no precipitate forms on addition of silver nitrate, stop washing and continue with the procedure. 13. Loosen the filter paper, and allow it to drain for a few minutes. Fold the filter paper into a compact package, and place it into a previously weighed ignition crucible. 14. Heat each crucible gently until all moisture has been driven off and the paper begins to smoke and char. Continue heating until all paper has been charred. IF THE PAPER BURSTS INTO FLAMES, EXTINGUISH THE FLAMES IMMEDIATELY WITH THE CRUCIBLE COVER! 15. After the paper has completely charred, heat the crucible until the contents glow. The contents should be white, with no black particles. 16. Allow the crucible to cool, and place it upright on the pipe stem triangle. Moisten the precipitate with 3-­‐4 drops of dilute (1:4) sulfuric acid. Heat the crucibles gently to drive off the sulfuric acid. (This treatment converts any barium sulfate that has been reduced to barium sulfide (BaS) back into barium sulfate). Cover the crucibles and heat to dull redness for 15 minutes. 17. Cool the crucibles in the air for a few minutes, and then transfer them to the desiccator. Make sure to cover the crucibles with the correct crucible lid. Allow the crucibles to cool for at least 1 hour in the desiccator. 18. Weigh the crucibles and precipitate on the analytical balance. Re-­‐heat the crucibles to redness for 10 – 15 minutes, allow them to cool and reweigh the crucibles and precipitates. Continue the cycle of heating and cooling until two successive weighings agree within 0.4 mg. 19. Calculate and report the percentage SO3 in each sample, and the average value and sample deviation for your unknown. Lab report A sample lab report is shown at the end of this procedure. 4
Notes: 1. Use a separate stirring rod for each sample and leave it in the beaker throughout the analysis. WASTE DISPOSAL: The precipitates can be thrown in the trash. Excess solutions of all types (except silver nitrate) can be flushed down the sink. Unused silver nitrate is disposed of in the waste bottle in the hood. 5
SAMPLE REPORT: ALL VALUES ARE FICTIONAL AND ARE USED FOR ILLUSTRATION ONLY!! Gravimetric Sulfate Unknown #1 x 10-­‐13 V. Frankenstein 1st 2nd 3rd Mass unknown, g 0.5876 0.6693 0.5932 Mass crucible, g 22.1986 20.2955 19.2289 Mass, crucible + 22.5279 20.6149 19.5033 precipitate, g Mass, precipitate, g 0.3293 0.3194 0.2744 Mass SO3, g 0.11296 0.10956 0.094128 % SO3 in unknown 19.22 16.36 15.87 Average % SO3: Standard deviation: Sample calculations: Mass SO3 % SO3 17.15 1.81 % SO3 = 17 + 2% = precipitate mass X (F.W. SO3/ F.W. BaSO4) = 0.3293 g X (80.06/233.39) = 0.11296 g = (mass SO3/mass unknown) X 100 = (0.11296/0.5876) X 100 = 19.22% 6