Salting Effects as an Illustration of the Relative Strength of

In the Classroom edited by

Todd P. Silverstein Willamette University Salem, OR 97301-3922

Salting Effects as an Illustration of the Relative Strength of Intermolecular Forces Eric C. Person* and Donnie R. Golden Department of Chemistry, California State University, Fresno, Fresno, California 93740-8034, United States *[email protected] Brenda R. Royce University High School, Fresno, California 93740, United States

Understanding intermolecular forces and their relative strengths is an essential learning objective for any high school or general chemistry course. There are a wide variety of demonstrations illustrating the impact of intermolecular forces on the properties of materials including viscosity (1), surface tension (2), and vapor pressure (3), but few look at the relative strength of these forces. Solubility offers another way to illustrate these concepts and introduce the important role that solubility will play in upper-division chemistry courses. In the simplest terms, a solute will be soluble in a solvent if the strength of the intermolecular forces formed between the solute and solvent molecules are stronger, resulting in a lower total potential energy, than the intermolecular forces found in the pure substances. In this way, the relative solubility of species can be used as a means to compare the strength of intermolecular forces that are formed in solutions. An example of this relative solubility is the salting out of nonelectrolytes from aqueous solutions upon the addition of electrolytes. In this demonstration isopropyl alcohol, a nonelectrolyte, becomes immiscible with water after the addition of ammonium sulfate, a strong electrolyte. At a simple level, once a sufficient quantity of the electrolyte is added to the solution, water molecules must choose between forming ion-dipole interactions with the dissolved electrolytes and weaker dipole-dipole or hydrogen-bonding interactions with the nonelectrolytes. As the water molecules solvate the ions, the nonelectrolytes are pushed out of solution and will transfer to or form separate phases. Several demonstrations of these salting effects have been described in the literature. Shakhashiri describes salting of methanol from an aqueous solution using potassium carbonate leading to a discussion of phase diagrams and the Gibbs phase rule (4). Smith modified this procedure using ethanol, sodium carbonate, and bromthymol blue to help visualize the formation of separate phases in a classroom setting (5). This demonstration modifies Smith's procedure in three substantive ways. First, the acid-base color change is removed, as the concept may not have been covered in lecture prior to the discussion of intermolecular forces. Second, the demonstration uses materials students are already familiar with: rubbing alcohol, food coloring, and fertilizer. Third, an additional portion of water is added reforming a single phase to emphasize that the separation of layers is the result of a competition of the relative strength of the intermolecular forces that can form between two solutes and a limited number of solvent molecules. 1332

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Demonstrations of salting interactions can provide a useful connection to organic chemistry, as salting effects are used extensively in liquid-liquid and acid-base extractions. For example, the use of saturated salt solutions (brine) to wash organic extracts stems in part from two effects related to salting: first, the brine solution partially dries the organic layer by drawing dissolved water out to form more favorable interactions with dissolved ions, and second, the high salt concentration significantly reduces the solubility of any organic solutes dissolved in the aqueous phase. Procedure Add 15 mL of water and one drop of food coloring to a 50 mL test tube (25 200 mm), cap with a rubber stopper, and mix by inverting. Though most colors of food coloring can work for this demonstration, green or blue food coloring are recommended as they provide a nice contrast and partition more completely than yellow and red colors. Add 15 mL of rubbing alcohol (70% isopropyl alcohol), cap, and mix by inverting (Figure 1A). Add 7 g of ammonium sulfate that has been ground to a powder (Figure 1B), cap, and mix by shaking vigorously for 5-10 s (Figure 1C). Two distinct layers should form in approximately 10-20 s on standing (Figure 2). A colorless layer is observed forming from the bottom and increasing in size until it is approximately 70% of the total solution volume. The food coloring is dissolved in the top alcohol layer while the bottom

Figure 1. Images showing the steps of the main procedure described: (A) 30 mL of the 35% isopropyl alcohol solution with one drop of blue food coloring, (B) the alcohol solution after adding 7 g of powdered ammonium sulfate, (C) the solution after shaking vigorously to aid the salt in dissolving, (D) the solution after the layers have settled, and (E) the solution after adding an additional 15 mL of water.

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Vol. 87 No. 12 December 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100315r Published on Web 10/08/2010

In the Classroom

Figure 2. Images showing the rapid separation of the aqueous and isopropyl alcohol layers. The time required for the separation varies between approximately 10 and 20 s after vigorous shaking of the alcohol solution and added ammonium sulfate.

aqueous layer is nearly colorless (Figure 1D). If desired, use a transfer pipet to remove approximately 1 mL of each layer and place each layer in separate watch glasses for testing by conductivity or flammability. Add another 15 mL of water to the remaining alcohol and water in the test tube, cap, and mix by inverting the tube. A single, uniformly colored layer will be observed immediately and persist indefinitely (Figure 1E). A document camera system can be effectively used to visualize the separation of layers if visibility of the demonstration is a concern in larger classrooms. The identity of the two layers formed after salting can easily be determined using either a conductivity measurement or a simple flammability test. To test for conductivity, use a simple low voltage (∼9 V) conductivity indicator (e.g., Lab-Aids Kit 300) or an ohmmeter after cleaning the electrodes with steel wool or emery paper to ensure good contact with the solution. A document camera or other projection method may be required to make the conductivity indicator visible in a large classroom. High voltage conductivity indicators such as the conventional light bulb apparatus may present a fire hazard if used with the alcohol fraction. The colored alcohol layer will show low but nonzero conductance. The colorless aqueous layer will show relatively high conductivity. The layers can then be tested for flammability with a match or a butane grill lighter. We generally ignite the isopropyl alcohol layer first and then put the match out in the aqueous layer. The alcohol layer should burn for at least 1 min with sufficient yellow color to be easily visible in a classroom. The flame can be easily extinguished by covering with another watch glass if desired. Variation A variation on this procedure using sodium chloride in place of ammonium sulfate creates some interesting effects with the partitioning of the food coloring, which would facilitate discussion of relative solubility of organic molecules. Prepare three large test tubes with 15 mL of water and 15 mL of 70% isopropyl alcohol. Add one drop of yellow food coloring in the first test tube, one drop of green food coloring in the second test tube, and one drop of blue food coloring to the third test tube. Add 4.5 g of sodium chloride to each tube and shake vigorously. The sodium chloride will take longer to dissolve, but as it dissolves, two layers will appear in each of the tubes (Figure 3). The green solution will split into a blue-green top alcohol layer and a bright green aqueous layer. Both layers in

r 2010 American Chemical Society and Division of Chemical Education, Inc.

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Figure 3. Images showing the colors formed by yellow, green, and blue food coloring in 35% isopropyl alcohol before (A) and after (B) adding sodium chloride as described in the variation. The blue-green color formed in the alcohol layer for the green coloring results from the different extent of partitioning of the blue and yellow dyes that make up the green coloring.

the blue tube will be relatively evenly colored, while the yellow tube will not show even color distribution. An additional 15 mL of water can be added to each tube to restore the sample to a single, uniformly colored layer. Commonly Available Reagents The reagents for this demonstration are inexpensive and readily available in commercial products. Common rubbing alcohol is a convenient source of 70% isopropyl alcohol. Ammonium sulfate fertilizer (21-0-0), which is essentially pure, is a potential source of the salt for this demonstration. Food coloring is available in most grocery stores. If reagent grade isopropyl alcohol (99þ%) is used in place of rubbing alcohol, use 20 mL of water and 10 mL of isopropyl alcohol instead of the volumes listed above. Hazards As with all laboratory experiments and instructional demonstrations utilizing chemicals and other hazardous materials, proper personal safety equipment including protective eyewear should always be used. Ammonium sulfate (CAS # 778320-2) is recognized as a mild irritant to human eyes and skin. Caution should be taken when exposed because it can be absorbed through the skin. Inhalation of the compound may lead to respiratory tract infection. Isopropyl alcohol, also known as isopropanol, does have some notable hazards associated with its use. Isopropyl alcohol (CAS # 67-63-0) is a highly flammable liquid having a flash point of only 12 °C. Breathing of vapors should be avoided. Inhalation can cause drowsiness, dizziness, and respiratory infection. Isopropyl alcohol can also cause skin irritation, be absorbed through the skin, and may be harmful if swallowed. Target organs include the kidneys, liver, nervous, cardiovascular and gastrointestinal systems. Appropriate care should be taken to avoid fire hazards if using the flammability test on the separate layers. At a minimum, the area should be free of other combustible or flammable materials, the experiment should be performed in an area with adequate ventilation, and fire suppression equipment such as fire extinguishers should be available. Discussion In general, a solute will be soluble in a solvent if the strength of the intermolecular forces between the solute and solvent

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In the Classroom

particles is stronger than that of the interactions between the separate pure substances. In this case, hydrogen bonds between the alcohol and water are sufficiently strong to allow isopropyl alcohol and water to be miscible in all proportions. Most waterbased food coloring dyes contain polar functional groups that form strong dipole-dipole interactions with water including hydrogen bonds that allow them to be quite soluble in water. Take time before adding the salt to emphasize these solubility properties. Adding food coloring to water before adding the isopropyl alcohol demonstrates the solubility of the visible food coloring dye in both water and alcohol, creating an opportunity for a deeper discussion surrounding which layer is colored after salting. Ammonium sulfate is also soluble in water due to the strength of the ion-dipole attractions between ammonium and sulfate ions with the water molecules. Small quantities of ammonium sulfate (∼3 g for the 30 mL volume described) will dissolve in the 35% isopropyl alcohol solution formed in the demonstration without forming a second layer. As additional salt is added, there is insufficient water to completely solvate both the ammonium sulfate and the isopropyl alcohol. In this situation, water will favor the interactions with the lower potential energy resulting from stronger intermolecular forces. The displacement of isopropyl alcohol to form a separate layer while the ammonium sulfate dissolves completely illustrates that the ion-dipole interactions formed between water and ammonium sulfate are stronger than the hydrogen bonds formed between water and isopropyl alcohol. The significant reduction of solubility of nonelectrolytes such as isopropyl alcohol on the addition of high salt concentrations is typically referred to as salting. The solubility of the salt in the solution is also significantly reduced by the presence of isopropyl alcohol in the aqueous layer. Food coloring provides a convenient way to visualize the formation of these layers in part because it is also subject to the salting effects pushing it out of the aqueous layer. The isopropyl alcohol layer will have a density of approximately 0.87 g/mL causing it to sit on top of the aqueous layer with a density of approximately 1.16 g/mL. The addition of a second portion of water provides sufficient water to solvate both the ammonium sulfate and isopropyl alcohol allowing a single layer to form again. The quantity of ammonium sulfate recommended is below its solubility limit in the alcohol solution so that it will all dissolve quickly and so that a single layer can be formed upon addition of the second portion of water. If too much color is still evident in the aqueous layer for the instructor's preference, additional ammonium sulfate will essentially eliminate any detectable color in the aqueous layer. Blue food coloring will show more complete transfer to the alcohol fraction than the green coloring and significantly more than the red or yellow coloring in a typical box of McCormick brand food coloring. If more salt is added, more water may be required to reform a single layer and some solid ammonium sulfate may appear as a third layer at the bottom of the tube. The type of salt added to this demonstration is not critical and can have interesting effects on the partitioning of the color. All electrolytes may show salting effects to some degree. Ammonium sulfate was used in this demonstration because it is inexpensive, dissolves rapidly, and has the relatively strong salting effects necessary to displace the hydrogen bonding of isopropyl alcohol. The strength of the salting effects depends on the ionic strength of the electrolyte solution. This in turn depends on the charges of the ions and the overall solubility of the salt. As shown 1334


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Figure 4. The chemical structures of FD&C Yellow #5 and FD&C Blue #1, the two dyes used in McCormick green food coloring.

in the described variation, sodium chloride also shows salting effects and will form separate layers, but concentrations near saturation are necessary and do not form as quickly as the demonstration with ammonium sulfate. The described variation using sodium chloride takes slightly more time to form separate layers, but illustrates an interesting difference in the effect of salt on the two dyes that make up McCormick's green food coloring: FD&C Blue 1 and FD&C Yellow 5 (6). The chemical structures of these two dyes are shown in Figure 4. Whereas the sodium chloride is able to sufficiently displace the isopropyl alcohol to form two layers, it has different effects on these two dyes as observed in Figure 3. The top layer of the center tube appears blue instead of green because the alcohol layer contains significantly more of the blue dye than the yellow dye due to differences in their partitioning between the brine and alcohol layers, likely caused by a combination of differences in their relative hydrophobic character and hydrogen bonding potential. FD&C Yellow #5 has two charged sulfonate groups, two azo nitrogens, and a phenol with good potential to form hydrogen bonds with water due to minimal steric hindrance. FD&C Blue #1 shows increased hydrocarbon character with five benzene rings instead of only three. Although FD&C Blue #1 has four charged groups, the resonance structure with the positive charge on the central carbon atom is considerably stabilized as a tight ion pair with the neighboring ortho sulfonate group (supported by molecular modeling). Finally, the nitrogen atoms in FD&C Blue #1 are expected to have reduced hydrogen bonding potential due to steric hindrance and the resonance structures involving their lone pairs of electrons. Acknowledgment The authors would like to thank Angie Person for assistance with the photography and digital photo processing. Catherine Banks (Department of Chemistry, Peace College, Raleigh, NC) is thanked for checking this demonstration. Literature Cited 1. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1989; Vol. 3, p 313-316.


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In the Classroom 2. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1989; Vol. 3, p 301-304. 3. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1989; Vol. 3, p 242-248. 4. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1989; Vol. 3, p 266-268.


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5. Smith, E. T. Chem. Educator 1996, 1; http://chemeducator.org/ sbibs/samples/spapers/11smi897.htm (accessed May 2009). 6. McCormick Product Detail - Green Food Color. http://www. mccormickgourmet.com/ productdetail.cfm?id=6036 (accessed May 2009).

Supporting Information Available Video of the demonstration using green food coloring and ammonium sulfate. This material is available via the Internet at http://pubs.acs.org.


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Salting Effects as an Illustration of the Relative Strength of

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