Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
The Oxidation of Iron in a Gel Using Consumer Chemicals submitted by:
Stephen W. Wright* Pfizer Global Research and Development, Groton, CT 06340; *
[email protected] Marsha R. Folger and Ryan P. Quinn† Lyme–Old Lyme High School, Old Lyme, CT 06371
checked by:
Frederick C. Sauls Department of Chemistry and Physics, King's College, Wilkes-Barre, PA 18711 Diane Krone Northern Highlands Regional High School, 97 Lexington, Dumont, NJ 07628
The corrosion of a steel nail in an agar gel is a classic demonstration that has also been a popular student experiment for decades and has been published in many collections of chemical demonstrations (1). As a hands-on experiment for students, it has great appeal, particularly as the oxidation of iron is such a familiar phenomenon with profound economic consequences. The experiment is pertinent to the students’ understanding of redox chemistry and of the relative oxidation potentials of various metals. We wanted to adapt this experiment so that it could be carried out with consumer chemicals that might be purchased at a supermarket or pharmacy and commonly found in the home in order to make this enjoyable and instructive experiment available to those who may be without access to laboratory chemicals. As generally published, the experiment uses an agar gel containing potassium hexacyanoferrate(II), K4Fe(CN)6, which reacts with iron(III) ion formed by oxidation of iron to yield a blue precipitate, commonly known as Prussian blue. When the experiment is carried out with a nail as the source of iron, the blue color typically appears first at the point and the head of the nail, which are those areas most stressed during the machining of the nail. These areas are the anodes of the electrochemical cell. Occasionally the directions call for the inclusion of phenolphthalein in the gel mixture. When phenolphthalein is included in the gel mixture, the cathode appears as a pink coloration as oxygen is reduced with formation of hydroxide ion. This typically occurs along the shank of the nail. Of course, these chemicals are not available on the consumer market.
dues. We needed to develop a gelatin formula that would gel as quickly at room temperature as the agar gel formula did. This was accomplished by preparing a more concentrated gelatin solution than those used for food uses. A suitable indicator for iron(III) ion proved to be somewhat more problematic. The reaction of iron(III) ion with salicylic acid (HOC6H4CO2H) in aqueous solution affords a deep red–purple color. This reaction forms the basis of qualitative analysis tests for both aspirin (acetylsalicylic acid, CH3CO2C6H4CO2H) and for salicylic acid (Figure 1) (2). The product has been isolated as garnet–red crystals and is thought to be a solvated chelate containing two molecules of salicylic acid coordinated to each iron(III) ion (3), although chelates containing one molecule of salicylic acid coordinated to each iron(III) ion or three molecules of salicylic acid coordinated to each iron(III) ion may also be present (4). Initial experiments carried out using laboratory sodium salicylate (HOC6H4CO2Na) gave excellent results, which confirmed our belief that a salicylate salt could be used as a satisfactory reagent for the detection of iron(III) ion in this experiment. In contrast to potassium hexacyanoferrate(II), which produced a blue color close to nail, the sodium salicylate gave a reddish–purple color that was more diffuse than the color produced with potassium hexacyanoferrate(II). We then directed our attention towards the preparation of a satisfactory solution of a salicylic acid salt from consumer chemicals. Attempts to generate a suitable solution of sodium or ammonium salicylate from acetylsalicylic acid (aspirin) by hydrolysis with sodium carbonate (washing soda)1 or ammonia
Commonly Available Chemicals
O
O
Gelatin appeared to be an obvious choice as a substitute for agar, although their chemical properties are very different: agar is a polysaccharide, while gelatin is a polypeptide with a high content of polar and charged amino acid resi-
OH
OH OH
O O
CH3
†
Current address: 410 Sousa, University of Connecticut, Storrs Campus, Storrs-Mansfield, CT 06269.
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Figure 1. Structures of aspirin (left) and salicylic acid (right).
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(household ammonia)2 were unproductive as we found that hydrolysis was generally incomplete. Moreover, the excess base required to generate such solutions subsequently interfered with both gel and color formation. Neutralization of the excess base with dilute acetic acid (white vinegar) was tedious, difficult to monitor and reproduce, and resulted in the precipitation of salicylic acid. The direct use of aspirin as the indicator was likewise unsuitable owing to the precipitation of aspirin as the gel cooled, resulting in a milky, opaque gel. At a loss for a means to produce a satisfactory salicylate salt solution, we searched the literature for any salicylate salts that might be found in over-the-counter medicines. We were delighted to find that magnesium salicylate is the principal ingredient in backache pain medications. Experiments with laboratory magnesium salicylate showed that this salt was as suitable as sodium salicylate had been, and subsequent experiments with backache pain medications showed that they would serve as a satisfactory source of a salicylate salt. Experimentation identified a mixture that would set to a transparent gel in approximately 20 minutes at room temperature and which would produce a color indicative of the formation of iron(III) ion around the nail within about 60 minutes at room temperature. The color continued to intensify over the next 24 hours. To accomplish this, we found that including a small quantity of sodium chloride (table salt) resulted in a noticeable increase in the rate of color formation, as did precleaning the nails to remove oil remaining from their manufacture. Any flat-bottomed disposable plastic container can serve in place of the customary Petri dish. We found that white polypropylene containers, such as those sold with margarine and soft cream cheese, worked well. When common nails were used in this experiment, the garnet–red color of the iron(III) ion–salicylate complex formed around the head and the point of the nail first. When a common nail was bent sharply in the middle of the shank, the garnet–red color formed at the bend rather than at the head and point. Galvanized nails failed to produce a garnet– red color, illustrating the protection of the iron by the more chemically reactive zinc coating, which acts as a sacrificial anode. Unlike the experiment using potassium ferrocyanide, which produces a white color with the zinc ions produced in this process, no color is produced between the salicylate and zinc ion. A piece of aluminum wire or aluminum foil wrapped tightly about the shank of the nail likewise failed to produce a garnet–red color. Galvanic corrosion of the nail could be demonstrated by wrapping a few turns of copper wire tightly around the shank of a common nail. In this case, the steel nail becomes the sacrificial anode and the rate of oxidation of iron is increased, as shown by the visibly faster formation of the garnet–red color around the head and point of the nail.
tablets containing 580 mg of magnesium salicylate tetrahydrate were purchased from a local pharmacy. Liquid volumes were measured using Nalgene polypropylene student-grade graduated cylinders or kitchen measuring implements.
Experimental Section
1. Washing soda is a hydrated sodium carbonate. 2. Household ammonia is a dilute solution of ammonia in water. The composition is not standardized but typically contains 1 M to 2 M ammonia by acidimetric titration. 3. Six-penny nails are two-inches long; boxes are commonly marked as 6d. 4. Disposable containers made of white polypropylene plastic (recycle code 5), such as clean margarine or cream cheese con-
Six-penny3 common and galvanized steel nails were purchased from a local hardware store. Nails must be washed prior to use; this may be accomplished by shaking them with mineral spirits (paint thinner) in a clean can or washing with hot water and detergent, followed by immediate drying with a paper towel. Generic or name-brand backache pain reliever
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Materials Four precleaned six-penny nails 90 mL (6 tablespoons) of water in a small pan that can be heated on a hot plate or stove 30 mL (2 tablespoons) of water in small glass jar Two 7-g packets of unflavored gelatin Half of a backache pain reliever tablet 0.6 g (about 1/4 teaspoon) of sodium chloride (table salt) Spoon for stirring Two containers for the gel mixture4
Procedure Place two nails into each of the containers. If desired, one nail may be bent sharply in the middle with pliers or a galvanized nail may be used. Other nails may be wrapped about their shanks with bare copper wire or aluminum foil. The nails should be placed in the containers before the gelatin mixture is poured over them. Add the gelatin to the 30-mL portion of water with stirring to make as evenly wetted a mixture as possible. This mixture will quickly become a rather stiff paste. Add the sodium chloride (table salt) and the half backache pill to the 90-mL portion of water. Heat the 90-mL portion of water to boiling, and add the gelatin–water paste to the 90 mL of boiling water slowly enough to keep the mixture at a gentle boil. Once the gelatin appears to have dissolved, remove the mixture from heat. Note that a small quantity of the backache pill will not dissolve. Gently pour half of the solution into each of the two dishes and allow to cool undisturbed. The gelatin mixture should just cover the nails, which should not be moved or allowed to come into contact with each other as the gelatin mixture is poured. The dishes will cool and gel more quickly if they are placed on a smooth stone or metal surface, such as granite counter or metal baking sheet. The gel will typically set in about 20 minutes at room temperature. The color development becomes discernible after 1 hour and is clearly visible after 2 hours. After 16 hours the color development becomes so extensive that the nail is entirely surrounded by color. Notes
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Chemistry for Everyone tainers, work well. Heat resistant glass containers, such as custard cups, may be used if they are large enough. Disposable glass containers should not be used as they may break when the hot gel mixture is added. Metal containers are not suitable as they will participate in the reaction.
Literature Cited 1. See, for example: Tested Demonstrations in Chemistry; Alyea, Hubert N., Dutton, Frederic B., Eds.; Journal of Chemical Education: Easton, PA, 1965; p 134. Summerlin, Lee R.;
Borgford, Christie L.; Ealy, Julie B. Chemical Demonstrations: A Sourcebook for Teachers, Volume 2, 2nd ed.; American Chemical Society: Washington DC, 1988; p 191. 2. The United States Pharmacopeia USP 25; National Publishing: Philadelphia, PA, 2002; pp 165, 1548. 3. Hantzsch, A.; Desch, Cecil H. Justus Liebigs Ann. Chem. 1902, 323, 1–31. Hopfgartner, Karl. Monatsh. Chem. 1909, 29, 689– 712. 4. Ogawa, Kinya; Tobe, Nobuko. Bull. Chem. Soc. Japan 1966, 39, 227–232. Ernst, Z. L.; Menashi, J. Trans. Faraday Soc. 1963, 59, 1794–1802.
This article has been developed into a JCE Classroom Activity: “Trusty or Rusty? Oxidation Rates of Nails” by Stephen W. Wright See pages 1648A–B in this issue.
JCE Classroom Activities
Find all 75 Activities at JCE Online
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