In the Laboratory
In the Laboratory
Revisiting History: Encountering Iodine Then and Now A General Chemistry Laboratory To Observe Iodine from Seaweed M. Farooq Wahab† Chemistry, University of Sciences, Alberta, Edmonton, AB T6G 2G2, Canada; Department of Mathematics and Basic NED University of Engineering
[email protected] Technology, Karachi 75270, Pakistan;
[email protected] Many elements of the periodic table have fascinating histoMany elements of the periodic table have fascinating histories behind their discovery. The techniques used in discovering ries behind theirbediscovery. The techniques used in discovering elements might as sophisticated as the transuranic elements elements mightand be discovered as sophisticated as thereactors, transuranic elements being studied in nuclear or might be being studied and discovered in nuclear reactors, or might be as simple yet puzzling to chemists as “a new substance which as simpleayet puzzling to chemists as “a new substance which becomes violet coloured gas by heat” (1). becomes a violet coloured gas by heat” (1). A Beautiful New Substance from Humble Origins A Beautiful New Substance from Humble Origins In 1813, an article appeared in Annales de Chimie on the In 1813, an article appeared Annales de Chimie on the “Découverte d’une substance nouvelleindans le vareck” (Discovery of “Découverte d’une substance nouvelle dans le vareck” (Discovery of a new†Current substance in kelp)Department (2), relatingoftheChemistry, story of Bernard Couraddress: University of tois, French manufacturer saltpeter. This article describes the Alberta, Edmonton, AB T6G of 2G2, Canada
a new substance in kelp) (2), relating the story of Bernard Courdiscovery ofmanufacturer a new elementofthat is nowThis known to describes us as iodine. tois, French saltpeter. article the After extracting seaweed ash, Courtois noticedtothat when the discovery of a new element that is now known us as iodine. mother liquor was acidified concentrated acid, After extracting seaweed ash,with Courtois noticed sulfuric that when thea substance appeared as a vaporwith of beautiful violetsulfuric color and con-a mother liquor was acidified concentrated acid, densed as black crystals. supplied the substance to hisand friends, substance appeared as a He vapor of beautiful violet color conFrench as chemists NicolasHe Clément andthe Charles Desormes, so that densed black crystals. supplied substance to his friends, they could continue research on this French chemists Nicolas Clément and substance. Charles Desormes, so that Clément believed iodineon to this be a substance. substance similar to chlothey could continue research rine and showed it to Humphry other scientists (3). Clément believed iodine toDavy, be a among substance similar to chloIn 1814, Davy published his work in among Philosophical Transactions rine and showed it to Humphry Davy, other scientists (3). as 1814, “SomeDavy experiments on a new substance In publishedand his observations work in Philosophical Transactions which becomes a violet and coloured gas by heat” aftersubstance carrying as “Some experiments observations on a(1)new out analytical on this substance. Figure 1.) This new which becomestests a violet coloured gas by(See heat” (1) after carrying
[ 74 ] VI. Some Experiments and Observations on a new Substance which becomes a violet coloured Gas by Heat. By Sir Humphry Davy, Knt. LL.D. F.R.S. Read January 20, 1814 A new and a very curious substance has recently occupied the attention of chemists at Paris. This substance was accidentally discovered about two years ago by M. Courtois, a manufacturer of saltpetre at Paris. In his processes for procuring soda from the ashes of sea weeds, (cendres de vareck) he found the metallic vessels much corroded; and in searching for the cause of this effect, he made the discovery. The substance is procured from the ashes, after the extraction of the carbonate of soda, with great facility, and merely by the action of sulphuric acid :— when the acid is concentrated, so as to produce much heat, the substance appears as a vapour of a beautiful violet colour, which condenses in crystals having the colour and the lustre of plumbago. M. C ourtois soon after he had discovered it, gave specimens of it to M. M. D esormes and C lement for chemical examination ; and those gentlemen read a short memoir upon it, at a meeting of the Imperial Institute of France, on Nov. 29th. In this memoir, these able chemists have described its principal properties ; they mentioned that its specific gravity was about four times that of water, that it becomes a violet coloured gas at a temperature below that of boiling water, that it combines
Figure 1. The first page of Sir Humphry Davy’s paper on the discovery and characteristics of the new element, iodine (1). Reprinted with permission, © The Royal Society. © Division Journal of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No.• XX Month 2008 • Journal of Chemical 1 206 of Chemical Education • Vol. 86 No. 2 February 2009 www.JCE.DivCHED.org • © Division of Education Chemical Education
In the Laboratory
out analytical tests on this substance. (See Figure 1.) This new and strange substance attracted the attention of chemists in Paris. Further independent work by Davy and Joseph Louis GayLussac showed that the substance was an element that behaved very much like chlorine. It was named iodine for its beautiful violet vapor from the French word “iode”, derived from Greek “ioeidēs”, meaning “violet-colored” (4). Indeed, the color of gaseous iodine is very characteristic of the element and pleasing to see. These violet vapors still attract chemists, more than a century after its first description, for some novel uses, such as an iodine thermometer (5). A more detailed sketch of iodine’s discovery, along with the historical processes of extracting iodine from seaweeds, is described by Lyday (6). Iodine Today: Its Sources and Uses The element iodine is well known to most of us today as an antiseptic in the form of tincture of iodine or in iodized salt. Iodized salt was introduced in the United States as early as 1917 for its beneficial use in preventing goiter, a disease of the thyroid glands, after extensive trials (4). One of the motives behind the experiment outlined here, besides the “discovery” part, is to familiarize students with iodine and iodized salts, and to show them that iodine is found naturally in certain plants. An interesting classroom activity for testing for iodide in iodized salts has been published in this Journal (7). Naturally occurring, reliable sources of iodine include brown-colored seaweed (Figure 2) and algae from saltwater sources; these plants are known to concentrate iodine in variable amounts from seawater. The highest concentrations of iodine are found in marine plants in shallow parts of the oceans receiving the most intense sunlight (6). The plants used in previous trials of this experiment (Sargassum spp.) had I2 concentrations of roughly 0.04% in air-dried material. Using plants collected from lakes and rivers would not yield results in the experiment because the water these plants take up contains little or no salts. Chemistry students today may not be able to appreciate one of the most enjoyable parts of chemistry: integrating chemistry with their everyday life. A simple activity is therefore described that can involve students in “discovering” an element, in quite a similar way to the original observations of Courtois. Without requiring sophisticated apparatus or special skills, such an activity will increase the interest of students in chemistry. Perhaps no other element can be “discovered” as easily as iodine by following the original method of the discoverer. The chemistry involved in the experiment is extremely simple and no special reagents are required. Hazards Chemicals and reagents used in this experiment are strong oxidizing agents. Hexane is highly flammable and should be kept away from hot sources. Inhalation of hexane vapors should be avoided. Hot solutions and objects should be carefully handled. Seaweed ash remains hot for some time after being removed from the oven. Chemistry of the “Discovery” of Iodine The procedure outlined here follows the same general method that Courtois used for isolating iodine from seaweeds.
Figure 2. A stem of brown-colored seaweed. (Figure available in color in the table of contents and in the online PDF. Photo by the author.)
The main steps include: collecting (or otherwise obtaining) the seaweed; reducing the plant material to ashes; and extracting material from the ashes using water. The mother liquor is oxidized to generate iodine vapors. Courtois, being a commercial vendor, most likely had tons of seaweed ash available to him as a ton of ash yields only about 15 kg iodine (8). Because this procedure uses a much smaller amount of seaweed, variations of the procedure more suitable for smaller-scale experiments are offered. Further, because iodine content of the seaweed varies by species and local growing conditions (including the seasonal strength and duration of sunlight), procedures of different sensitivities are presented. In the first part of the experiment, A, a qualitative test is done to show the presence of iodine in the mother liquor obtained from seaweed ash. Two options follow this step, B and C: in both the oxidation of iodide in aqueous medium is carried out. In the simpler method, B, mother liquor containing liberated iodine is slightly warmed to produce iodine vapors that are observed against a white background. Alternatively, in the preconcentration method C, liberated iodine is first extracted into hexane and allowed to react with sodium hydroxide. In this way, all the extracted iodine is now concentrated in a very small volume of sodium hydroxide as iodide and iodate. This technique gives better results as compared to B because a smaller volume will have maximum quantity of iodine. The students can do part A followed by B or C. In particular, we avoid the use of hot, concentrated sulfuric acid as an oxidizing acid for safety reasons. A similar demonstration on generating violet vapors of iodine by treating dried skeletons of gorgonians (a soft coral filter feeder found primarily in shallow tropical or subtropical oceans) with concentrated sulfuric acid has been described (9). This is a simple, quick demonstration that shows the violet vapor of iodine, however gorgonians are unlikely to be found heaped along the shores as seaweed might. Instead of using concentrated sulfuric acid as an oxidant for iodides present in the seaweed ash, solid potassium peroxodi sulfate in 2 M sulfuric acid is employed. This approach avoids evolving noxious gases associated with oxidations with sulfuric acid. Laboratory instructions and more details are provided in the online supplement.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 2 February 2009 • Journal of Chemical Education
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In the Laboratory
Boiling seaweed ash in water dissolves a large amount of the iodide ion in water. Adding an oxidizing agent such as concentrated sulfuric acid, K2S2O8, or hydrogen peroxide oxidizes the iodide to free iodine according to the following equations (eqs 1–3). The reactions expressed in eq 1a and eq 1b were the essential reactions of the Courtois method of accidental observation of violet vapors. These reactions are exothermic. Thus the iodine in solution readily vaporizes to violet vapors. However, according to Lyday (6), the commercial method for producing iodine in Courtois’s time involved manganese dioxide as oxidant in the presence of concentrated acid. With manganese dioxide, there are no gaseous products of sulfuric acid and only pure iodine is generated. Also note that when concentrated sulfuric acid is used as an oxidant, the reduction products might be different (10). 2NaI + 2H2SO4 → Na2SO4 + I2 + SO2 + 2H2O (1a) 8NaI + 5H2SO4 → 4Na2SO4 + 4I2 + H2S + 4H2O (1b)
→ I2 + 2SO42−
2I − + S2O82−
2I − + H2O2 + 2H+
→ I2 + 2H2O
(2) (3)
As iodine is released in the solution, it turns the solution deep brown. Because elemental iodine is volatile and has the property of subliming, warming the solution releases violet vapors of iodine that then condense on the ice-cooled glass as fine black needles. As a result, the brown solution loses its color. Upon addition of hexane to the solution, iodine (which is more soluble in nonpolar hexane) dissolves rapidly and remains in solution with the hexane, creating a beautiful, violet-colored solution. When NaOH is added to the hexane solution (eq 4), elemental iodine disproportionates into iodine and iodate:
6NaOH + 3I2
→ 5NaI + NaIO3 + 3H2O
(4)
Sodium iodide on oxidation releases iodine as shown in the equations above. The advantage of this procedure is that sodium iodide is concentrated in a very small volume. Summary This exercise helps to engage students’ interest in the history of the elements while teaching applications of redox reactions.
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Students can learn about and balance redox equations employed in their experiments. The colored vapors generated in the Erlenmeyer flask and the colored layer extracted into hexane help explain the etymology of the name of iodine. Acknowledgments The author thanks Bob Bruner (University of California, Berkeley) for relating some historical material, and Noushaba Rafiq and Tehseen for help in collecting the seaweed. The author also thanks an anonymous referee for identifying some commercial seaweed vendors in the United States. This laboratory experiment was first conducted at Karachi University, Karachi, Pakistan. Literature Cited 1. Davy, H. Phil. Trans. 1814, 104, 74–93. 2. Courtois, B. Découverte d’une substance nouvelle dans le vareck. Annales de Chimie de Physique 1813, 88, 304–310. Cited in Davis, H. M. The Chemical Elements, 2nd ed.; Science Service: Washington, DC, 1959; pp 139–140. 3. Weeks, M. E. Discovery of the Elements, 6th ed.; Journal of Chemical Education: Easton, PA, 1956; pp 736–740. 4. Rosenfeld, L. J. Chem. Educ. 2000, 77, 984–987. 5. Ramette, R. W. J. Chem. Educ. 2003, 80, 878. 6. Lyday, P. A. J. Chem. Educ. 1987, 64, 152–153. 7. Wright, S. W. J. Chem. Educ. 2007, 84, 1616A. 8. Emsley, J. Nature’s Building Blocks: An A–Z Guide to the Elements; Oxford University Press: New York, 2001; p 198. 9. Ciereszko, L. S.; Chang, C. W. J. J. Chem. Educ. 1978, 55, 744. 10. Alexeyev, V. N. Qualitative Chemical Semimicroanalysis; CBS Publishers: New Delhi, India, 1994; p 508.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Feb/abs206.html Abstract and keywords Full text (PDF) Links to cited URLs and JCE articles Figure 1 in color Supplement Instructions for students; notes for instructors; commercial sources for kelp and other brown-colored seaweed
Journal of Chemical Education • Vol. 86 No. 2 February 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
