- Background Information
See Background Information (Section II) in Lab Documentation.
- Lab Preparation and Equipment
- NaOH pellets, 4.0 g / person
- Potassium hydrogen phthalate (KHP, KHC8H4O4), 3 g / person
- Phenolphthalein indicator
- 0.1 M H2SO4 solution, 50 mL / person
- sand (particle size anywhere between 1 mm and 710 micrometers), 90g / person
- Al(OH)3, 1.5 g / person (Aldrich catalog No. 23,918-6; contains 50-57% Al2O3)
- 8-hydroxyquinoline, 0.1 g / person
- 95% ethanol, about 300 mL / person
- 6 M NH3, 6 mL / person
- crushed limestone (particle size between 1 and 2 mm), 13.5 g / person
- limestone powder (optional; particle size << 63 micrometers), 5 g / person
- montmorillonite clay (Wardâ€™s Clay Mineral Standard), 3 g / person
- molecular sieves (sodium aluminum silicates; 1.6 mm size pellets; Type 4A)
- aluminum sulfate, Al2SO4 • 18H2O, 0.07 g / person
- 1 L glass screw-cap bottles, 2 / person
- 50 mL and 25 mL volumetric pipets
- 50 mL and 250 mL Erlenmeyer flasks
- 10 mL and 50 mL burets
- 1.8 cm diameter glass tubing, ~2 feet / person
- 0.6 cm diameter glass tubing, ~2 inches / person
- No. 2 rubber stoppers with bored holes, 1 / person
- Tygon tubing, 2 inches / person
- Electrical tape, 6 inches / person
- Cotton, 5 nickel-sized wads / person
- Buret clamps
- Iron ring stands
- 50, 100, and 250 mL beakers
- glass stirring rods with rubber policeman
- sintered-glass filter crucibles (30 mL, medium porosity)
- 500 mL filter flasks
- Walter crucible holders
- Wash bottles
- Approximate Student Time Required
- Preparation and standardization of a 0.1M NaOH solution: 2 hrs.
- Preparation and standardization of 5x10-3M H2SO4 solution: 30 mins.
- Sand Preparation: 30 mins.
- Construction of column apparatus: 10 mins.
- Passage of acid through Al(OH)3 column: 5 mins.
- Passage of acid through Ca(CO3) column: 2 mins.
- Passage of acid through clay column: 1 hr.
- Passage of acid through molecular sieve column: < 30 sec.
- Analysis of acid solution
- Titration: 20 mins.
- Precipitation of the aqueous cation: 4 hrs.
- Enrichment Options for Parts One and Two of The Project
The entire procedure for Part One can be repeated by passing H2SO4 solutions of varying concentrations through Al(OH)3 columns. These varying concentrations simulate acid rain of three additional pHâ€™s. The following concentrations are recommended:
- 0.025 M H2SO4 (simulates acid rain of pH 1.3)
- 0.0025 M H2SO4 (simulates acid rain of pH 2.3)
- 5x10-5 M H2SO4 (simulates acid rain of pH 4)
- All solutions should be standardized before use in the project.
Because the particle size of the limestone seems to play a role in its effectiveness at consuming H+, the entire procedure for Part Two can be repeated by using limestone of a different particle size. We recommend the use of limestone which has been ground into a powder (particle size << 63 micrometers). Because the powder packs more densely in the column than the crushed limestone of a larger particle size, use approximately 5 g limestone instead of the 13 g used above, and pack the layers as recommended for the packing of the Al(OH)3 column. This will prevent an extremely slow percolation of acid through the column.
Suggestions on how this five-part project can be executed
- Treated it as a multi-session lab project to be completed by each individual.
- Treat it as a group / class project, and assign one part to each group or set of students. Then have each group share their results in a class discussion.
- Treat it as a set of mini-projects, and select just one or two parts for the individual student to complete.
- Preparation and standardization of NaOH and H2SO4 solutions.
- To save time, these solutions can be made up and standardized for the students ahead of time by the lab instructor or lab assistant.
- Be sure that the burets are clean, and have been rinsed with the NaOH solution before titrating.
- Be sure that the pipets are clean, and have been washed with the H2SO4 solution to be standardized.
- After standardizing the NaOH solution, keep a layer of parafilm over the bottle cap while not in use. NaOH readily reacts with CO2 in the atmosphere to form carbonates.
Filling a column with a soil mineral
- Make sure that the columns are clean and dry before attempting to place any minerals in them. It is difficult to pack wet columns because the minerals adhere to the wet sides of the glass instead of falling to rest on the bottom.
- In packing the column with minerals which are finely ground into powders (e.g. Al(OH)3 and limestone powder), it is important to layer the Al(OH)3 or limestone thinly between layers of sand. When the mineral layers are too thick, the acid percolates through the column at an extremely slow rate.
If Enrichment Option for Part One is included in the project, the following directional tips are important
- Each Al(OH)3 column should be used only once and not re-used for additional concentrations of acid. That is, each time a 100 mL portion of acid is passed through the column, it should be through a freshly packed Al(OH)3 column. The re-use of an Al(OH)3 column will result in extremely slow percolation of acid through the column.
- Due to the higher molarity of the 0.025 M H2SO4 solution, a larger volume of NaOH will be needed to reach an endpoint during titration. Therefore, it will be advantageous to use a 125 mL Erlenmeyer flask and a 50 mL buret instead of the 50 mL flask and 10 mL buret used to titrate the 5x10-3 M H2SO4 solution after its passage through a column.
- Due to the low molarity of the 5x10-5 M H2SO4, such a small volume of NaOH is needed to reach an endpoint that it is too difficult to observe accurately. Therefore, a pH meter must be used to determine the new molarity after its passage through a column.
Hydration of the molecular sieves
- Hydrating the molecular sieves with 100 mL of distilled water is an exothermic process, so a small amount of vapor and slight warmth should be anticipated.
- Safety and Hazards
All work should be done in a fume hood, and appropriate eye protection should be worn at all times. Dispose of Al(C9H6NO)3 and Ca(C9H6NO)2 precipitates in an appropriate waste bottle. The use of rubber gloves is recommended when handling 0.1 M sulfuric acid and 0.1 M NaOH solutions.
CAS Registry Numbers
- NaOH: 1310-73-2
- KHC8H4O4: 877-27-7
- Al(OH)3: 21645-51-2
- C9H6NO (8-hydroxyquinoline): 148-24-3
- Ethanol: 64-17-5
- NH3: 7664-41-7
- CaCO3: 471-34-1
- Al2(SO4)3 x 18H2O: 10043-01-3
- Molecular sieves (Sodium aluminosilicate): 73987-94-7