Harold T. McKone and Gloria J. Nelson' Saint Joseph College West Hartford. Connecticut 061 17
I I
I
Separation and Identification ofsome F D ~ Cyes ~ ~ T L C An undergraduate laboratory experiment
Several recent articles in this Journal have dealt with introducing thin layer chromatography (tlc) t o the undergraduate laboratory (1-3). While these articles involve interesting and relevant experiments, they usually include a visualization step which can complicate matters and confuse the heginning student. We would like to report a facile tlc experiment involving the separation and identification of three FD&C colors: Red ff2. ,, , Yellow 115 and Yellow lt6. Since this experiment involves neithe;'chemical nor light visualization, students can ohserve the chromatogram develop. In addition, because of the renewed controversy over these FD&C dyes (4, 5 ) , there is a high degree of relevancy in this a n a l y s i ~ . ~ The experiment necessitates the extraction of the FD&C color from the food. In order to simplify this extraction we have chosen from among the following "foods": non-alcoholic beverages, juices from cherries and pickles, "JELL0"-type products and certified red and yellow food coloring. Other ~ r o d u c t ssuch , as sausages, jams and icings can be used, hut the preliminary food treatment is more difficult. The problem of obtaining standards of Red #2, Yellow #5 and -~ ello ow ff 6 can be circumvented asfollows: we determined that certifild red food coloring has Red # 2 as one of its components (by comparison of its tlc with that of an authentic sample) and in the same manner determined that certified yellow food color is a mixture of Yellow #5 and Yellow #6. As is seen in the table, the Rf values of Yellow #5 and Yellow #6 are markedly different. Chromatograms of two other common FD&C food colors (Red #3 and Red #4) were also run. Again the Rf values were sufficiently different to allow facile s&arstionand characterization (see table). The analysis can he completed in one two-hour laboratory session. ~ t " d e n t s are asked to compare their results and conclusions with the information found (or not found) on the label of the food that they analyzed. We have found this ex~
Rf Values for Food Dyes FD&C Dye
nea #2
Red #3 Red #4
yellow
Rf 0.63
#s
Vellow #6
0.92 0.80 0.58
0.73
~
~
'Abstracted in part from the M.A. thesis of G. Nelson, Saint Joseph
-. .... .... .
periment to be a good introduction to tlc. In addition, this experiment generates further interest in the structures, chemical properties, etc., of other types of additives such as preservatives, stabilizers and artificial sweeteners.
Experimental The samples were prepared for color extraction as follows (6):soft drinks and juices: acidify a 50-ml portion with dil. acetic acid; " J E L L 0 products: add a small amount to 50 ml of water and acidify; food coloring: no preparation needed, can he spotted directly an slide. The coloring was extracted from the food as follows (6): a 20-cm strip of white knitting wool (purified in advance by the instructor by first boiling in very dilute NaOH and then by boiling in water) is added to the acidified sample and the mixture is boiled until the wool has absorbed as muehcalor as possible. The wool is then washed with cold water, transferred to asmall beaker and boiled gently with -10 ml of dil. ammonia. After the color is released. the wool is discarded and the solution is evaporated to near dryness. (CAUTION!The solution may spatter during this evaporation.) A few microliters of the solution are chromatagraphedon Silica Gel plates (2) using 50:25:25:10 n-hutanol-ethanol-waterIconceammonia as the developer. Acknowledgment The authors wish to thank Mr. A1 Wickroski of the Connecticut State Agricultural Experiment Station in New Haven, Connecticut, for supplying personal research data and for the giving of his time and knowledge. We would also like to exmess our ao~reciationto Mr. David Sweenev for determining the combbsition of the certified food colok.
f i l l n c a , 1976. . \ r t r r rhls article *ar arrcprcd for puhlrcation, the V1)A hanned lur~hrrussofFD&C Red ~ 2 Howe~er,xinrelhe . ban did noc involve the r r m d irf Red 0 2 ronulinme marerds fnm store rhrlvn. 11 nlll still be easv to obtak. for some time. hath Red ff 2 containing foods
to contact the authors
722 / Journal of Chemical Education
Literature Cited Ill 12) (3) 141 (51 16)
Chasar,D. W.andToth,G. B., J.CHEM.EDUC.,51,22(19741. Chasar,D. W.andToth,G. B., J.CHEM.EDUC.,51.487 11970. M8dren.B. C . J . CHEM. EDUC..50.852 11973). Consum~rRrporis,38, 131 11973). Feingold, B. F.. ChemTeeh. 264 (1975). Cox. H. E., and Pearson, D.. "Chemical Analyais of Foada". Chemical Publishing Co., New York. 1962. p. 1W.
