16 Gravimetric Determination of a Precipitate

16 Gravimetric Determination of a Precipitate Purpose isolate sulfate from an unknown sample by precipitation with barium chloride. Collect, dry and weigh the precipitate. Calculate the amount of analyte in the unknown from collected data. Background Gravimetric analysis is one of the oldest and most accurate quantitative methods for determining the amount of an analyte in a sample. A sample containing a sulfate of an alkali metal is first weighed then dissolved in dilute hydrochloric acid. The sulfate is then isolated by precipitation with barium chloride. The precipitate is digested in the hot solution to form coarser, easily filtered particles and to purify the precipitate. The precipitate is then collected by filtration, washed, dried and weighed. The amount of analyte in the original sample can then be calculated from the mass of the precipitate and its chemical composition. Materials

Equipment • Xplorer GLX • Glass stirring rods (3) • Temperature probe (Fast- response • Analytic balance Probe is recommended) • Beaker, 400 mL (4) • Plastic beral popets • Beaker, 100 mL (3) • Rubber policeman (3) • Funnel (3) • Graduated cylinder, 100 mL • Watch glass, 100 mm (3) • Dessicator • Support Stand and ring • Bunsen burner • Clay or wire triangle Consumables • Unknown sample containing an • 0.5 M Barium chloride (BaCl) solution alkali sulfate • 6 M Hydrochloric acid (HCl) • Ashless filter paper • 0.1 M Silver Nitrate solution • (AgNO3)

Safety Precautions • Wear gloves and protective clothing when handling solutions • Wear safety glasses and follow all standard laboratory safety procedures

Equipment Setup 1) Place a hot plate on a support stand and clamp a buret filled with 0.5 M barium chloride solution above it.

2) Plug the temperature probe into one of the temperature ports () on the left side of the GLX. Procedure Sample preparation 1) Obtain 3-400 mL beakers, 3 stirring rods and 3-100 mm watch glasses. Clean and rinse them with distilled water and dry. Num- ber the beakers and watch glasses for future reference.

2) Obtain a weighing bottle containing a sample of unknown substance from your instructor. Record the number of the unknown in your lab manual.

3) Carefully weigh by difference ~.35 g of unknown into each of three 400 mL beakers. Record the actual weight of unknown in each beaker to 0.0001 g.

4) Add 50 mL of distilled water to each beaker, then add 5 mL of 6 M HCl followed by 200 mL of additional distilled water. Cover the beakers with 100 mm watch glasses and, if necessary, store them until you are ready to proceed with the determination. Precipitation 1) Place the beakers of solution on a hot plate and begin heating to about 90 °C. Do not allow the solutions to boil as some of the solution may be lost through spatter- ing. Keeping the solution hot will promote the formation of large, filterable crystals and minimize the inclusion of impurities in the precipitate.

Note: You can use the GLX and stainless steel temperature probe to monitor the temperature of the solution in one of the beakers. When you remove the temperature probe from the solution, be sure to rinse the solution adhering to the probe back into the beaker with a small quantity of distilled water.

1) Begin adding 0.5 M barium chloride solution dropwise from the buret into one of

the beakers. When 25 to 35 mL of BaCl2 solution has been added, stop the addition and let the precipitate settle. At this point you can begin the addition of

BaCl2 solution to the next beaker.

2) When the precipitate has settled test for completeness of precipitation by adding 1-

2 drops of BaCl2. If you see any sign of precipitation, add 5 more mL of BaCl2.

3) After adding BaCl2 to all three beakers, cover the beakers with watch glasses and continue heating at 90 °C for one hour. After this “digestion” the precipitate should be coarse and the supernatant should be clear. Filtration 1) Place a piece of whatman ashless #42 filter paper in each of three clean, dry funnels. Support the funnels over beakers or flasks of sufficient size to hold all of the supernatant.

2) Heat 200 mL of distilled water to about 80 °C for rinsing the precipitate.

3) Pour the supernatant through the filter and then transfer the precipitate into the filter with the aid of a rubber policeman and small washes of hot distilled water. Be sure to scrape all the precipitate from the sides of the beaker.

4) Wash the material in the funnel with three 5 mL portions of hot distilled water. Collect the washes separately in small beakers.

5) Add two drops of 0.1 M AgNO3 solution to the last wash. If any cloudy white precipitate is observed, the precipitate still contains chloride ions and must be washed a fourth time.

6) Clean and dry by heating to constant mass 3 porcelain crucibles with lids by heating until a constant mass is obtained. Record the final mass of each empty crucible and lid. 7) Remove the filter paper containing the precipitate from the funnels. Carefully fold the filter paper so that the precipitate is trapped inside and final shape is small enough to be stuffed into a clean crucible. To prevent the loss of precipitate, avoid tearing or breaking the folded filter paper open. Record the identity of the sample that is stored in each crucible. If necessary, store the covered crucibles for later processing. Ashing and heating to constant mass Follow the steps below for each crucible of precipitate 1) Place the crucible on a clay or wire triangle on a ring stand over a bunsen burner.

2) Heat the crucible with a small flame so that the filter paper does not burst into flame. Keep the crucible lid ready to quickly extinguish any flame from the paper before material is expelled from the crucible.

3) Move the burner around and gradually increase the size of the flame so that all parts of the crucible are heated.

4) When all of the carbon residue has been removed, the temperature should be maximized by bringing the tip of the blue cone of the flame to a point just below the crucible. Continue heating the crucible for ten minutes.

5) Allow the crucible to cool for a few minutes then place it into a desiccator until it cools to room temperature. Weigh and record the mass of the crucible, precipitate and lid. Repeat the heating, cooling and weighing procedure until two successive masses are within 0.002 g. Record the final mass. Analyze Record calculations in your lab notebook as you complete your analysis. Data Table

Trial #1 Trial #2 Trial #3 Weight of unknown sample (g) Initial mass of crucible and lid (g) Mass after 1st heating

Mass after 2nd heating Initial mass of precipitate, crucible and lid (g) After 1st heating After 2nd heating Mass of precipitate (g) Percent sulfate Average percent

Analysis Questions

2- 1) Calculate the percentage of SO4 in the unknown for each sample, average percentage for your samples.

2) If ordinary filter paper was used instead of ashless paper, how would your results be affected? Explain.

3) Why are the washes of the precipitate tested with AgNO3?

4) What are the most likely sources of error in your procedure? Would they cause your result to be high or low?