Lab Documentation

  1. Experimental Procedure

    Because sodium hydroxide and sulfuric acid are corrosive, they should be handled carefully and with caution. All work should be done in a fume hood to prevent the inhalation of dust from Al(OH)3 and 8-hydroxyquinoline. Contact of molecular sieves with eyes should be avoided. Appropriate eye protection must be worn at all times; the use of lab coats and gloves is recommended.

    Preparation and standardization of a 0.1M NaOH solution

    • Using a top-loading balance, weigh out 4 g of NaOH pellets into a small beaker. Dissolve the pellets in distilled water and then transfer this solution into a 1 L glass screw-cap bottle. Fill the bottle with distilled water up to the neck, and screw the cap on tight. Using both hands, carefully shake the solution back and forth to ensure thorough mixing. Wrap a piece of parafilm over the cap until ready to standardize.

      Dry about 3 g of potassium hydrogen phthalate (KHC8H4O4, or KHP) in a 110 °C oven for one hour, and then place in a dessicator to cool. Using an analytical balance, weigh by difference three 0.7-0.8 g samples of the KHP into 250 mL Erlenmeyer flasks. Dissolve each sample in approximately 50 mL of distilled water, and swirl until all KHP crystals have disappeared. Add 6 drops of phenolphthalein indicator to each KHP solution. Over the sink, use a glass funnel to fill a 50 mL buret with the NaOH solution. Titrate each KHP solution with the NaOH until the endpoint is reached, the point observed when the slightest pink tint remains even after swirling. Record the amount of NaOH (to two decimal places) needed to reach each endpoint, and then calculate the exact molarity of the NaOH solution. Keep a layer of parafilm over the bottle cap while not using the solution.

    Preparation and standardization of a 5x10-3M H2SO4 solution

    • Obtain a 1 L glass screw-cap bottle and fill with about 950 mL of distilled water. Extract a 50 mL aliquot of 0.1 M H2SO4 using a volumetric pipet, and add this aliquot to the distilled water in the bottle. Screw the cap on tightly. Using both hands, carefully shake the solution back and forth to ensure thorough mixing of water and acid. Determine the exact molarity of this acid solution. Pipet a 25 mL aliquot of the acid solution into a 50 mL Erlenmeyer flask. Add 5-6 drops of phenolphthalein indicator. Fill a 10 mL buret with the previously standardized NaOH solution, and titrate the acid sample to the endpoint. Repeat the procedure and take the average of the two results.

    Sand Preparation

    • All sand used in the following procedures must first be acid-washed and air-dried. Using a top-loading balance, weigh out three 30 g samples of sand into 250 mL beakers. Wash each twice with 100 mL of a 0.05 M solution of H2SO4. After two washings with acid, wash each sand sample four times with distilled water. Spread each sample of sand out onto a layer of paper towels and allow to air dry. Place each sample back into its beaker and set aside.

    Part One. The Interaction of Al(OH)3 with Simulated Acid Rain.

    • Construction of column apparatus

      • Obtain a 2-foot glass column with an external diameter of 1.8 cm from the instructor, and make sure that the column is clean and completely dry. Carefully insert a 2-inch piece of glass tubing (0.6 cm external diameter) into the bored hole of a No.2 rubber stopper. Then fit a 2-inch piece of tygon tubing over the end of the glass tubing in the stopper. Insert this rubber-stopper apparatus into one end of the glass column. Wrap about 6 inches of electrical tape over the junction between the glass column and rubber stopper. Obtain a wad of cotton about the size of a nickel, and implant it into the glass column so that it rests on top of the rubber stopper. Clamp the column into a buret clamp attached to an iron ring stand.

        Using a top-loading balance, weigh out approximately 1.3 g of Al(OH)3. Using a medium sized spatula, layer this amount of Al(OH)3 with one of the three previously washed samples of sand to create alternating layers of Al(OH)3 and sand at the bottom of the glass column. Care should be taken not to layer the Al(OH)3 too thickly; the thinner the layers and the more sand between each layer the better. Therefore, we recommend that the Al(OH)3 layers be no more than 0.3 cm thick, and that the sand layers be about 1 cm thick. Failure to pack the column in this manner will result in extremely slow passage of acid through the column.

        First allow a portion (between 10 and 15 mL) of the previously standardized 5x10-3 M H2SO4 to pass through the Al(OH)3 column. Collect the eluate that drips through in a 50 mL beaker. (This solution will appear cloudy due to the initial seeping through of some of the Al(OH)3 powder.) After the 10-15 mL of acid passes through the column, the eluate should become less cloudy. Discard the cloudy solution that was collected in the 50 mL beaker. Now pass 100 mL of the previously standardized 5x10-3 M H2SO4 through the column, and collect the eluate in a 250 mL beaker (at this point, the solution should be clear.) The drip rate should be about 1-2 drops per second. Allow the entire 100 mL to pass through the column before performing any of the analyses described below.

      Titration

      • Take a 25 mL aliquot of the eluate, and place it into a 50 mL Erlenmeyer flask. Add 6 drops of phenolphthalein indicator. Fill a 10 mL buret with the previously standardized 0.1 M NaOH, and titrate the aliquot to the endpoint. Record the amount of NaOH required to reach this endpoint, and then calculate the following:

        1. the new molarity of the acid solution
        2. the moles of hydrogen ion that were consumed by the Al(OH)3 in the column
        3. the percentage that the [H+] decreased by as a result of the acid’s passage through the column
        4. the moles of Al3+ that should now be in solution based upon the stoichiometry of the reaction below.

        Al(OH)3(s) + 3H+(aq) → Al3+(aq) + 3H2O(l)

        Precipitation of aqueous Al3+ as aluminum oxinate. Take another 25 mL aliquot of the eluate, and place it into a 100 mL beaker. Weigh by difference a 0.02-0.03 g sample of 8-hydroxyquinoline into a 50 mL beaker, and dissolve it in about 15 mL of 95% ethanol. Add the dissolved 8-hyroxyquinoline to the 25 mL aliquot, and then add about 2 mL of 6 M NH3. Stir vigorously with a glass stirring rod by scratching the bottom surface of the beaker until a precipitate forms (indicated by cloudiness). Let the precipitate settle for about two hours. On an analytical balance, weigh a clean sintered-glass 30 mL filter crucible. Assemble a filtering apparatus using a 500 mL filter flask, Walter crucible holder, and the previously weighed filter crucible. Apply suction to the filter flask, and decant as much of the supernant as possible through the filter. Then quantitatively transfer all of the precipitate to the filter crucible, using a rubber policeman and wash bottle containing a 1:1 solution of 95% ethanol and distilled water. Dry the precipitate in a 120° oven for 2 hours. Cool in a dessicator, and re-weigh the filter crucible containing the solid. From the amount of aluminum oxinate [Al(C9H6NO)3] that formed, calculate the moles of Al3+ that were mobilized from the Al(OH)3 in the column.

    Part Two. The Interaction of Limestone with Simulated Acid Rain.

    • Construction of column containing limestone (CaCO3)

      • Obtain a 2-foot glass column from the instructor, and make sure that the column is clean and completely dry. Assemble a column apparatus as described under the section entitled Construction of Column Apparatus. Using a top-loading balance, weigh out 13 g of crushed limestone (particle size between 1 and 2 mm) into a beaker. Layer this amount of limestone with one of the previously washed samples of sand to create alternating layers of limestone and sand at the bottom of the glass column. We recommend that the layers of both limestone and sand be about 0.7 cm thick.

      Passage of acid through the limestone column.

      • Pass 100 mL of the previously standardized 5x10-3 M H2SO4 solution through the column, and collect the eluate in a 250 mL beaker. The drip rate should be rather quick, about 4 drops per second. Allow the entire 100 mL to pass through the column before performing any of the analyses described below.

      Analysis of acid solution after passage through the limestone column.

      • Titration

        • Extract a 25 mL aliquot of the eluate, and place it into a 50 mL Erlenmeyer flask. Add 6 drops of phenolphthalein indicator. Fill a 10 mL buret with the previously standardized 0.1 M NaOH, and titrate the aliquot to the endpoint. Record the amount of NaOH required to reach this endpoint, and then calculate the following:

          1. the new molarity of the acid solution
          2. the moles of hydrogen ion that were consumed by the limestone in the column
          3. the percentage that the [H+] decreased by as a result of the acid’s passage through the column
          4. the moles of Ca2+ that should now be in solution based upon the stoichiometry of the reaction below.

          CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O(l)

      Precipitation of aqueous Ca2+ as calcium oxinate.

      • Pipet another 25 mL aliquot of the eluate, and place it into a 100 mL beaker. Precipitate the aqueous Ca2+ by following the same procedure as described under Precipitation of aqueous Al3+ as aluminum oxinate. From the amount of calcium oxinate [Ca(C9H6NO)2] that formed, calculate the moles of Ca2+ that were mobilized from the limestone in the column.

    Part Three. The Interaction of Clay Minerals with Simulated Acid Rain.

    • Construction of column containing montmorillonite Clay [(Na, Ca) (Al, Mg)2 (OH)2 Si4O10]

      • Assemble a column apparatus as described under the section entitled Construction of Column Apparatus. Into a 100 mL beaker, weigh out 2.5 g montmorillonite clay which has been ground up into a powder. To the clay, add about 17 g of washed sand, and mix thoroughly. Transfer this clay / sand mixture into the column.

      Passage of acid through the clay column.

      • Pass 100 mL of the previously standardized 5x10-3 M H2SO4 solution through the clay column, and collect the eluate in a 250 mL beaker. It will take approximately 1 hour for the entire 100 mL of acid to pass through this column. Allow all of the acid to pass through before performing any of the analyses described below.

      Analysis of acid solution after passage through the clay column.

      • Titration

        • Take a 25 mL aliquot of the eluate, and place it into a 50 mL Erlenmeyer flask. Add 6 drops of phenolphthalein indicator. Fill a 10 mL buret with the previously standardized 0.1 M NaOH, and titrate the aliquot to the endpoint. Record the amount of NaOH required to reach this endpoint, and then calculate the following:

          1. the new molarity of the acid solution
          2. the moles of hydrogen ion that were consumed by the montmorillonite clay in the column
          3. the percentage that the [H+] decreased by as a result of the acid’s passage through the column.

    Part Four. The Interaction of Zeolites with Simulated Acid Rain.

    • Construction of column containing molecular sieves (synthetic zeolites).

      • Assemble a column apparatus as described under the section entitled Construction of Column Apparatus. As a laboratory substitute for naturally occurring zeolites, use molecular sieve pellets (1.6 mm size), which have the same composition and structure. On a top-loading balance, weigh out approximately 10 g of the molecular sieve pellets into a beaker, and transfer them into the column. Hydrate the molecular sieves by passing 100 mL of distilled water through the column. This is an exothermic process, so a small amount of vapor and slight warmth should be anticipated.

      Passage of acid through the molecular sieve column.

      • Pass 100 mL of the previously standardized 5x10-3 M H2SO4 solution through the molecular sieve column, and collect the eluate in a 250 mL beaker. The acid should pass through this column as a steady stream. Allow all of the acid to pass through before performing the titration analysis described below.

      Analysis of acid solution after passage through the molecular sieve column.

      • Titration

        • Take a 25 mL aliquot of the eluate, and place it into a 50 mL Erlenmeyer flask. Add 6 drops of phenolphthalein indicator. Fill a 10 mL buret with the previously standardized 0.1 M NaOH, and titrate the aliquot to the endpoint. Record the amount of NaOH required to reach this endpoint, and then calculate the following:

          1. the new molarity of the acid solution
          2. the moles of hydrogen ion that were consumed by the molecular sieves in the column
          3. the percentage that the [H+] decreased by as a result of the acid’s passage through the column.

    Part Five. The Absorption of Aqueous Aluminum by Zeolites.

    • Construction of column containing molecular sieves.

      • Assemble a column apparatus as described under the section entitled Construction of Column Apparatus. Using a top-loading balance, weigh out approximately 33 g of molecular sieve pellets into a beaker, and transfer them into the column. Hydrate the molecular sieves by passing 100 mL of distilled water through the column. This is an exothermic process, so a small amount of vapor and slight warmth should be anticipated.

      Passage of aluminum sulfate solution through the molecular sieve column.

      • Using an analytical balance, weigh by difference 0.07 g aluminum sulfate, Al2(SO4)3 18H2O, into a 250 mL beaker. Dissolve the sample in 100 mL of distilled water. Pass this aluminum sulfate solution through the molecular sieve column, and collect the eluate in a 250 mL beaker.

      Analysis of aluminum sulfate solution after passage through the molecular sieves.

      Precipitation of aqueous Al3+ as aluminum oxinate.

      • Pipet a 25 mL aliquot of the eluate into a 100 mL beaker. Precipitate any Al3+ which was not consumed by the molecular sieves by the procedure described under Precipitation of Al3+ as aluminum oxinate. Due to the effectiveness of the molecular sieves at consuming Al3+ ions, anticipate that it may be more difficult to get a precipitate to form. Therefore, if no precipitate seems to form after several minutes of vigorous stirring, allow the solution to sit overnight, and then stir once again. If a precipitate still fails to form, this indicates that the molecular sieves consumed all of the aqueous Al3+ present. If a precipitate does form, allow it to settle for about two hours before filtering. From the amount of Al(C9H6NO)3 that formed, calculate the moles of Al3+ consumed by the molecular sieves.

  2. Background Information

    Normal, unpolluted rain is already slightly acidic due to the presence of carbon dioxide in the atmosphere. Carbon dioxide combines with water to form carbonic acid, a weak diprotic acid which decreases the pH of rainwater to 5.6 (1, 2). Therefore, the term ‘acid rain’ denotes rainwater which has a pH below that of 5.6. The average pH of rainfall in the Eastern United States is reported to be about 4.2, but pH’s as low as 2.0 and lower and have been observed on occasion (6). The sulfuric acid solution used in this project has a pH of 2, which is on the lower end of observed rain pH’s.

    The two main pollutants responsible for the increased acidity in rainwater are sulfur oxides and nitrogen oxides. Sulfur dioxide is released into the atmosphere primarily from coal-burning factories and power plants, and nitric oxide is emitted mainly from automobile engines (1). The following equations indicate how these two pollutants can become acids by reacting with gases and water in the atmosphere (1, 2):

    S (in coal) + O2(g) → SO2(g)
    2SO2(g) + O2(g) → 2SO3(g)
    SO3(g) + H2O(l) → H2SO4(aq)
    and
    2NO(g) + O2(g) → 2NO2(g)
    4NO2(g) + H2O(l) → 4HNO3(aq)

    There are various mechanisms in soil which attempt to neutralize excess acidity in rainwater. These involve the consumption of hydrogen ions by minerals containing aluminum hydroxide such as bauxite, gibbsite, boehmite, and diaspore; by calcium carbonate in limestone and calcite; and by aluminosilicates such as clay minerals, feldspars, and zeolites (1, 2, 5, 6). As this project demonstrates, these mechanisms vary in their effectiveness at removing acidity, and in the case of calcium carbonate and aluminum hydroxide, also involve the release or mobilization of cations in solution. Of particular concern is the mobilization of the aluminum ion, Al3+, from Al(OH)3 because of the hazards it creates for aquatic life when it reaches rivers, lakes, and streams via run-off water (2). On the other hand, calcium ions mobilized from calcium carbonate-based minerals can be extremely beneficial to aquatic life in bodies of water that have become substantially acidic due to acid rain. Ca2+ ions reduce the permeability of fish gills to H+, thereby preventing an uncontrolled and toxic influx of hydrogen ions (2).

    Another mechanism in soil that can counteract the effect of excess acidity in rainwater is a process called ion exchange (5). Ion exchange is characterized by the exchange of cations located within clay minerals with hydrogen ions in acidic rainwater. Clays consist of stacked sheets of silicon-oxygen tetrahedra and of aluminum in octahedral positions surrounded by oxygen and OH-. In some clays, aluminum frequently replaces some of the tetrahedral silicon to form aluminosilicates. These tetrahedral aluminum have a negative formal charge, and the presence of additional cations is required to maintain electric neutrality. Such cations are commonly Na+, K+, Mg2+, or Ca2+, and they occupy spaces between the stacked sheet layers (5, 6). It is the presence of these cations that enables the process of ion exchange. When H+(aq) ions are present, the aluminosilicate clays consume the hydrogen ions by exchanging Na+, K+, Mg2+, and / or Ca2+ cations for the H+. The driving force behind this exchange is the cations’ affinity for anionic sites within the clay lattice versus their attraction to water molecules and also versus H+ ability to be held more tightly to anions than most other cations (5). When the exchange occurs, the displaced cations enter solution and eventually end up in rivers, lakes or streams. High levels of acidity in rain water may pose limitations to this ion exchange mechanism because cation supply in the clays can become depleted (2).

    Zeolites are aluminosilicates as well, but they differ from clay minerals in that they possess a unique open-holed structure within their framework. These open holes enable zeolites to accommodate various ions and molecules (3). Of particular importance to the issue of acid rain is the ability of zeolites to absorb hydrogen ions as well as aqueous aluminum ions mobilized from minerals containing Al(OH)3 by acidic rainwater. The molecular sieves used in this lab are synthetic zeolites, and they are a good laboratory substitute for naturally occurring zeolites because they have the same composition and structure.

References:

  1. American Chemical Society. Chemistry in Context: Applying Chemistry to Society; William C. Brown Publishing, 1994.
  2. Wellburn, Alan. Air Pollution and Acid Rain: The Biological Impact; Longman Scientific and Technical, 1988.
  3. Swaddle, T.W. Inorganic Chemistry: An Industrial and Environmental Perspective; Academic Press, 1990.
  4. Spiro, Thomas G.; Stigliani, William M. Chemistry of the Environment; Prentice Hall: New Jersey, 1996.
  5. Moore, John W. ; Moore, Elizabeth A. Environmental Chemistry; Academic Press: New York, 1976.
  6. Bunce, Nigel. Environmental Chemistry, 2nd Edition; Wuerz Publishing: Winnipeg, Canada, 1994.

A Simulation of the Interaction of Acid Rain with Soil Materials