JoanbSenyk Phoebus High School Hampton, Virginia 23663
The Determination of lodide in Seaweed A general chemistry research experience
Annreciable auantities of iodine occur i n certain seaweeds, p a r t k l a r l y ph;eophyceae, t h e Brown Algae. Accumulated from seawater a n d associated with meristematic activity i n the plilnt, the iodine is known to be present in the form of iodide. T h e amwtnt cleuends on many factors: speries; whether t h e plant is fruiting or sterile; locde; season of t h e year; t h e dearee of exuosure t o sea water a n d wave action; and tempe;ature (I, 2) T h e oarweeds and rockweeds were t h e initial economic source of elemental iodine. Recovery of t h e element included ashing t h e dried weed in beach kilns a n d extraction of t h e salts, followed by oxidation. Optimum yields depended upon avoiding loss of iodide by leaching action of rainwater during drying and loss of free iodine before (due t o oxygeu-induced enzymatic oxidation) and during ashing. T h e best seaweeds, Laminaria digitata, properly burned, contained 0.13-0.63% iodine i n t h e dried weed, h u t values of over 4% have been reported (3). The Research Experience T h e student research experience was designed t o stimulate interest in laboratory work, t o challenge t h e students, to develon other approaches t o classical experiments, and a t t h e samk time to-provide stimulation for t h e instructor. It was used with a class of 15 students in three consecutive 3 h r lab periods in t h e second'semester of t h e general chemistry sequence. Experimental 'l'lw prohlrm nseigned to the studenw was tu dererminr the hest procedorrs fur student analys~fiof i d & in lwall\ gathered seaweed. l'rdcrduws werc wtlinrd b! the instructor with rhc meof nva~lal,le references (4-6). A preliminary cheek was made to ensure that the extracted salts did indeed contain iodide. Initial procedures, performed by all students, included collecting and drying the weed, weighing, pulverizing and heating it to ash, boiling the ash in deionized water, and filtering to obtain the clear solution of dissolved salts. Four oxidizing agents were tried: hydrogen peroxide, sodium nitrite in sulfuric acid (nitrous acid), iron(II1) chloride, and bromine-water followed by addition of potassium iodide. Work-up procedures included extraction of the iodine with chloroform following hydrogen peroxide and iron(II1) chloride oxidations, in-situ destruction of the oxidizing agent with urea in the nitrous acid case and with barium hydroxide in a second hydrogen peroxide case, and removal of excess bromine by distillation. The aqueous iodine, converted to triiodide, was titrated to a starch endpoint with 0.1 N sodium thiosulfate solution standarized by dichromate. The chloroform solutions were titrated directly, the disappearance of the purple color indicating the endpoint. Procedures were outlined for completion in two periods. The third period was included for a second chance a t the determination. As much detail as possible was left for the students to determine, e.g., specific labware, and exact volumes of solvent and acid to use. Students worked in pairs, one report submitted by each pair. Each report included an outline of steps taken. This was evaluated on the basis of accomplishment of desired results and ease of performance. Evaluation was done as work progressed as well as more formally a t the end. The same erade was eiven to each student in any one pair. Furl). prrrenr of th;grsdr 1 ~ ftlr1 lal, wwk: technique, n;tsnti"n to d t w l , the makmcdg~mdnutrs,grnrml knouledgeofwhat wns h e w ~
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''l'lw ~ n ~ t i work n l wad done n t Oknhx,sa-Walton .Itmior College. Nicewllr. Florida. Suh~equrntwork wasdoneat Auburn l l n ~ v ~ r r i t y a t Montgomery, Montgomery, Alabama.
Student Results of Determination of lodide in Sargassurn and Two Unidentified Species Iodide as % o f dry Oxidation Method
weiahf
30% H,O,
%Ash Reported
%
in
Iodide the ash
0.0627
Average Nitrous ~ c i d b
0.150 0.150 0.121 0.0317' 0.229
Average
Bromine Water
Average Average of all methods
0.260 0.309 0.572 0.343 0.0339b 0.0617~ 0.196 0.0972 0.09aa
on Sorsossum
aUnidentifled specie% b ~ n d ~ o i uncertaintler nt encountered.
Student values of
%
ikdide in Ssrglssum ash.
done; 60% of the grade was for the report, of which 45%was for results and write-up and 15%for a summary of findings from the literature study assigned with the problem. Students were informed of the project at the beginning of the course and were asked to begin collecting and drying their seaweed. Two weeks prior to the first project period they were given the problem outline, and were asked to choose a partner and to select an oxidation procedure. They were assigned a literature study on the extraction of iodide from seaweed and the methods they would use, and were enwurag~dto submit procedural improvements. They were informed that the oossibilitv, of ~rocedural failure existed.. ~articularlv in the . . work.upsteps, but were a..iured that such u d a not hwr their grader. The imptrrtance of their work m the development of n new lnh wns stressed. Volume 54. Number 8, August 1977 1 511
there was great variance in heating times and intensities and in the student values of % ash in the dried weed. The 36% average value is low compared with the 52%value reported for Japanese Sorgassum (11). The value of ashing foy analytical purposes was questioned. A future student project might well be to compare results of the procedure used here with "wet ashine" as well as with "no ashine" .. oro. CNIIITW. 0 t h P~~ ~~ O I Z O I ~in~ addition P , to thoit. frmn weighin?: and ritrat~un.~rtcludelass uf icdde snlta during wmhm,: (to remove sand, and failure to extract them from the boiled ash, as well as small losses of elemental iodine during chloroform extraction and through volatilization. In addition, no iorrection was made for any iodate present
Experimental Results Seaweed samples were identified through the use of pictorial references such as Taylor (7). Most were Sargassum, a free floating soecies known commonlv as Gulf weed. Two samples brought in hy &ha enthusiasts were dot oositivelv identified Qnnntitntive results are rloorted ih the table. Althoueh hieh ~.~ ~-~ .. when compared with t h 0.054, ~ iodine recowred from Japaneir Snrgnreum (XI, these results wereyrdced plausiblein vrewoithegrrnt variability in iodide content of weed and the probability of loss during commercial recovery procedures. Literature values from quantitative laboratory assays were not available. All oxidation attemots with iron (111) chloride were unsuccessful, the reason undetermined. The nitrous acid figures are ouestianahle. Although the oxidations ~uweeded,the ureaadditlond followed by chlurc,form extraction proved 110 Ire messy and rirmtim endpoints uncertnin. Although thr hromine-water oxidation procedures wurked well, minor difficulties were encountered in two endpoint determinations. Simple as these may have been to correct,the use of bromine in the general chemistry laboratory was judged risky due to the toxicity of bromine and its vapors, particularly in the case of minimal hoods. Initial attempts at the hydrogen peroxide oxidation were made with a purchased 3%solution and were unsuccessful. The switch to a 30% solution gave the desired results. Subsequent extraction with chlordform worked well. The work-up involving attempted in-situ destruction of the excess hydrogen peroxide with barium hydroxide, followed by addition of potassium iodide prior to titration, resulted in the formation of dark o. u r.~ l ce ~. s t a hThese . were filtered and were found w form n purple wlution when dirsohed in rhloruform. On rhir baiir, together with wlutil:ty data ( 9 1 the , students idenrifird the cr).stalu as rlemrntnl iodine, formrd due to the failure of the dpstruction procedure. Oxidation with 30% hydrogen peroxide followed by chloroform extraction and two phase titration was judged to constitute the best method for student determination of iodide. The procedures, adapted from the qualitative Nuffield Series experiment (10) follow. Reagent amounts are given for an estimated maximum iodide concentration of 2% in 5 g of dried weed. Collect the weed, directly from seawater, if possible. If beached weed is used it must he fresh and washed as little as possible in the surf to remove sand. Do not wash in fresh water. Dry the weed and pulverize it with a mortar and pestle. Weigh out 5-7 g and heat strongly to gray-white ash (flamed tuna eans may be used if available crucibles are tm small),and weigh. Add the ash to 30 ml of deionized water, add glass or corundum boiling chips, and boil for 5 min. Gravity filter into a 250-Erlenmeyer flask, using a small amount of deionized water to wash the solids, then discard paper and residue. (Whatman's # 4 filter paper works well. Student grade paper will not work.) Dilute the filtrate to 100 ml with deionized water. Fill a buret with the standardized 0.05-0.01 N NazSz03 solution. Pour the diluted filtrate into a 250-ml separatory funnel. Add 25 ml CHCL. 5 m13 M H2S01, and 5 ml 30% HzOz. CAUTION:CHC13 is toxic and 30% Hz02 is corrosive to skin. Work in a well ventilated area. Wear protective gloves or wash hands thoroughly following the extraction. Shake gently. Drain the purple solution into a 250-ml Erlenmeyer flask. Extract once or twice more with I s 1 5 ml CHCk until no purple color is visible in the CHCI, layer. Place white paper under the flask and titrate immediately as the free iodine is highly volatile. The endpoint is indicated by thedisappearance of the color from the CHCh layer. Record the volume of titrant used and its normality and calculate the percent iodide in the dried s e a ~ e e d . ~ Procedural improvements submitted were as follows: taking weed directly from sea water for optimum iodide content, increasing the amount of weed sample used, weighing the dried weed after grinding rather than before to avoid loss during transfer, and the use of boiling chips during ash extraction. In addition, it was decided that in view of the small amount of titrant used (6 ml average) a more dilute solution would increase the accuracy. The greatest source of error was judged to be the ashing process as
Guidelines for Use of This Approach In each application of this approach a new problem should he chosen, as the excitement of delving into the unknown, shared hv both students and instructor, is important to the experience. However, once the area of research is chosen and rhe rrmtndu,ork Imd, many prohlema ma\. tw sttrlitd. Much more, for example, could-hk done with lodine in seaweed: content of differing species, content a t different times of year, work to perfect the iron (111) chloride, or nitrous acid oxidations. In each case few variables should he chosen in order that the instructor he able to give each student optimum attention. Library work should he assigned well in advance t o provide adeauate time for its comoletion. and students should he assisted in the use of the literature. Grading percentages might well he adiusted t o 25% for t h e lihrarv work t o emohasize its import, leHving 35% credit for result; and write-up and 40% for lab work.
2This procedure was subsequently used in a classical laboratory sequence. Drying and ashing of Sargossurn weed provided by the instructor were done by lab assistants to enahle students to complete work in one 3-hr period. All students analyzed samples of the ash mixture and were asked to determine the % iodide in a 3-g sample of ash, the weight of the dried weed, unfortunately, being omitted. A 0.009501 N solution of the titrant was used. Values, reported in the bar graph in the figure, were judged against a determination made by a lab assistant under supervision of the instructor.
Summary This approach t o the lab stimulated interest in the course through its problem orientation and through its relevance to the local area. It is applicable t o other local vegetation or geographical circumstances. It is an excellent synthesis of lab techniques, exemplifying their use in the study of a problem. Students were challeneed t o think. t o evaluate. and to make decisions. They were exposed t o reaction failure and unexpected happenings a s part of research work. A new lab was
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512 1 Journal of Chemical Education
and no blanks were run for endpoint correction. The Educational Experience All students were deeply involved in the work and rated the experience highly. Asked for comments over half of them expressed t h a t the experience was very interesting and t h a t i t was a good way to learn the various techniques. Several stated that they liked the research approach, liked working with the natural product, and t h a t such experiences help one learn t o think. One student expressed frustration a t not having enough time to nerfect the chosen method. Most students who had worked 6n unsuccessful procedures felt quite defeated a t the end of the second oeriod. T h e second chance a t the determination was important to them. It was also expressed t h a t neither adequate time nor assistance was given in the use of the literature. This was horn out by the brevity of the literature studv summaries included in the reoorts. T h e experience provided a challenge to the students. Several who had done well on exams and the usual e x ~ e r i m e n t s were timid and unsure in this situation requiring more independent thought and decision, while some of the "poorer" a.t .~.~.--~. w l m t s h l o s s o m~.. d . From the instructor's point of view, too many variables had been chosen. Differing oxidation methods would have been sufficient without differing work-ups a s well. Too much was going on. Secondly, all students should have analyzed samples of one homogenous mixture of dried pulverized weed. This might have been done, and each student still analyze his or her own sample, by dividing each pulverized sample and placing half of it in a common container. Each student then could have analyzed a sample from the homogenous mixture for method determination, and then analyzed the personal sample with the chosen method.
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developed. And last but not least, it was a joy to prepare and teach. Acknowledgment
~h~ author is grateful to ill &Ipern, ~~h~ Hamilton, and Lawrence Sacks for their encouragement and constructive criticism in the planning of the research experience and in the preparation of this paper. Literature Cited ( I ) Chapman,V.J.,'~Seswold~andTheirU~s."2ndEd.. Methuen and Ca.. Lfd., London, 1970. ch. 2-3.
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131 Ref. lil.p.43. (41 Nufiield Foundstion. "Chemistry: Calleeted Experimantn." Longmans. Green. and Co., Lfd., 1 9 6 8 , ~46. . I51 Feimtein. H. I.. C h e m k t w 4613). 25 119781(61 Pierce, Willis Comusy, ef sl.."Qosntitatiue Anslyris,"4th Ed., John Wiley and Sans. I"
