The Ideal Solvent for Paper Chromatography of Food Dyes Peter G. Markow St. Joseph College, West Hartford, CT 061 17
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Thesenaration and identification of food dves usine paper . chromatography is a simple and inexpensive experiment for teaching the basics of chromatoera~hv( 1 4 ) . The range of colors and the variety of food ;rodu& that contain Ayes seem naturallv to interest students. In addition, there are several controbersial topics related to the safety of food dyes that can he presented hv teachers d u r i n ~ pre-lab discussions to interest i n d motivate students. example, there is some evidence which indicates that F D and C Yellow No. 5 and FD and C Yellow No. 6 may cause mild cases of skin rashes and asthma in asmall portion of the population (7-9). This is whv the Food and Drue Administration (FDA) now requires these two dyes to he speciiirally identified on food vmduct lal~cls.Also. althourh the L'nited States banned use bf F, D, and C Red NO. 2 in 7976, this dye is still widely used in Canada (10-12). The solvent systems currently used by students to separate food dves via paper chromatogra~hvare given in Tahle 1. While the solv& are not p&&larly expensive nor hazardous, high school teachers may not have access to the various organic liquids (some of which are somewhat flammable), and the use of many of these mixtures without a fume hood is undesirahle due to the strong ammonialalcohol smell. In addition, these solvent systems require long develonment times We have f m n d that a 0.10 wt 'salt (NaCI) solution allows better senaration of theseven certified food dvcs 113).in one third the-time, when compared to the solvent system^ given in Tahle 1. Experlrnental All chromatograms were run on 1.5- X 13.5-cm Whatman No. 1 chromatography paper, with the origin and solvent front lines being 1cm from the long edges. The concentration of all F, D, and C dyes was 0.50 wt %.I Pure dyes and mixtures were spotted using wooden toothpicks. The spots were 1cm apart on the origin line. The developing chambers were 250-mL glass beakers with Petri dish lids, and 10 mL of developing solvent was used. After spotting the dyes, each chromatogram was rolled into the sbapeof a cylinder and stapled at the top and bottom so the edges did oat overlap. To find an optimum developing solvent, 29 solvent systems were investieated includine those listed in Table 1. These raneed from pure water up to 2.0 i t % salt, and included several solvenisystems using rubbing alcohol, a readily available alcohol. Rlvalues were determined by drawing a circle or oval around each spot and using the center of this circle or oval to determine the distance each spot moved. Initially, the pure food dyes were spotted on chromatography paper and developed in each solvent to determine Rl values. Then several mixtures of the pure dyes were preparedas given in Table 2. The solvents listed in Table 1, aqueous 0.10 wt % salt, and several systems using rubbing alcohol were then used to separate the above mixtures after they bad been spotted. Results Ri values and development times for the majority of solvents tested are given in Tahle 2. Additional salt concentrations (0.010, 0.20, 0.30, 0.40, 0.60, 0.70, 0.80, and 0.90 wt %)
Table 1. Solvents lor Paoer Chromatoaraohv of Food Dves Miller (l)
David (2)
95% ethanol
abs. elhand
1-bmnol
I-pentanal
2 M NH,OH 1:l:l
2 M NHIOH 1:l:l
Thompson (3 Kroschwitz (4) +propano1 water 2:l
Roberts el al. (5)
acetone conc NH. water 1:1:8
Table 2. Food Dye Mlxtures Used To Compare Solvent Separating Ability
were tested, hut not listed to simplify Tahle 2. The Rivalues for these solvent systems followed the trend apparent in Tahle 3: as the wt % salt increases, Rf values decrease. Discussion
As the concentration of salt increases from 0-2.0 wt %, the movements of all dyes (except B 1 and G3), are dramatically reduced (see Tahle 3). A solution of 0.10 wt % salt gave the best seoaration. Water and 0.050 wt % salt eave the poorest separation and the most tailing of the dyes. solveits with 0.20 or greater wt % salt had smaller Ri values, more overlap of dyes, and poor separation. No simificant differences in Rr values were evident when ordinary table salt and l a h ~ r a t & ~ - ~ r asodium de chloride were compared. As the concentration of alcohol increased, the development times also increased, hut spot spreading decreased ' salt: there was no when compared to 0.10 w t 3 improvement in separationof thedyes Nosignificant difference in K . values was noticed when ruhhing alcohvl and 95% ethanol were compared. The four solvent svstems listed in Tahle 1rewired about 30 minutes to develop (except the acetone-containing system), giving minimal streaking and more circular spots. The solvent systems given in Table 1 and 0.10 wt % salt were identified as producing the best separation. This was determined after c&nparincthe chromatbgrams of dye mixtures in the second part of the experiment. Of these best solvents, only 0.10 wt % salt allowed complete separation of Blue No. 2 and Yellow No. 5. The same is true for Blue No. 1 and Red No. 40 (waterlconc NHJace-
o ow ever,
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Twc-gram samples of each of the seven certified F, D, and C dyes can be purchased for $15.00 from Rainbow Colors. RR 1 Box 126D. Brookfield, MA 01506 (617-867-6428). Volume 65 Number 10 October 1988
899
Table 3.
~~l~ent deionlzd water 0.050 wt % salP 0.10 wt % salt 0.50 wt % salt 1.0 wt % salt 2.0 wt % salt 0.50 wt % NaCl 1.0 wt % NaCl 2.0 wt % NaCl ~propanoI/waler2:1.0.50 wt % salt rubbing alcohola 0.50 wt % sail 10%rubbing alcohol 0.10 wt % sari 20% rubbing alcohol 0.10 wt % sail 30% rubbing alcohol 0.10 wt % salt 10% 95% elhanol0.10 wt % salt 20% 95% ethanol 95% EtOH/l-BuOH/2M NHIOH 1:l:l ab. EtOH/l-pentmoll2M NHIOH 1:l:l ~pr0pan01/~~1er 21 water/conc NHdacetone 8:l:l impropy1 alcohol/water 3 9 0.10wt % salt
Solvent Syrtem Comparlson
81
82
Y5
R, Y6
R3
R40
Ge
Development Time (min)
0.97 0.95 0.95 0.92 0.94 0.90 0.93 0.91 0.92 0.65 0.57 0.97 0.97 0.97 0.97 0.97 0.70 0.60 0.64 0.95 0.79 0.95
0.65 0.32 0.27 0.17 0.16 0.083 0.15 0.14 0.10 0.041 0.048 0.28 0.35 0.52 0.31 0.35 0.36 0.31 0.38
0.88 0.61 0.58 0.40 0.37 0.30 0.35 0.33 0.27 0.12 0.072 0.64 0.67 0.74 0.60 0.72 0.29 0.20 0.28 0.93 0.43 0.58
0.75 0.46 0.44 0.32 0.24 0.20 0.31 0.23 0.20 0.47 0.42 0.46 0.58 0.70 0.46 0.60 0.62 0.58 0.49 0.85 0.65 0.44
0.21 0.083 0.065 0.032 0.033 0.033 0.034 0.034 0.034 0.94 0.95 0.10 0.13 0.30 0.10 0.16 0.83 0.78 0.67 0.47 0.93 0.065
0.68 0.31 0.27 0.19 0.15 0.11 0.20 0.15 0.14 0.51 0.49 0.36 0.45 0.64 0.37 0.52 0.70 0.62 0.63 0.78 0.67 0.31
0.96 0.93 0.93 0.94 0.93 0.90 0.93 0.93 0.89 0.59 0.59 0.97 0.95 0.96 0.95 0.97 0.63 0.49 0.67 0.96 0.70 0.93
9 9 10 10 10 10 10 10 10 31" 3Id 14 21 29 12 21 30 30 31" 12 30d 10
-r
0.45 0.27
' S t w & Shop Iodized table sal
tone) gives fairly complete separation). This latter separation is of great importance since Blue No. 1and Red No. 40 a r e the most commonly used blue and red food dyes (14). Blue No. 1and Yellow No. 6 also are best separated using 0.10 wt % salt. None of these solvents separates Blue No. 1 from Green No. 3. This is not very important since Green No. 3 is the least used food dye. T h e onlv advantaee the solvents listed in Table 1 eniov over 0.10 u;t % salt is their ability to separate clearly Blue I ? ; . 2 and Red No. 40. However, this is of little value since Blue No. 2 is not oftenused due to the fact that i t is light sensitive and breaks down relatively rapidly in solution.
Summary Of the 29 solvent systems tested, aqueous 0.10 wt % table salt gave the best separation of the seven certified food dyes. I n addition, chromatogram development t i m e for 0.10 wt % salt was one-third that of the solvents containing alcohols. Cost, availability of solvent components, and safety considerations, as well as superior separating ability, all make 0.10 An experiment titled "Separation and Identification of Food Dyes Using Paper Chromatography" (ANAL 372) will be available January 1989 from Chemical Education Resources, Box 357, Palmyra, PA 17078 (717-838-3165).
900
Journal of Chemical Education
wt % table salt the solvent of choice for separating food dyes by paper c h r o m a t ~ g r a p h y . ~ Acknowledgment
The author wishes to thank Harold T. McKone for providing the dyes for this research as well as information on food dye history and use. Thanks also to Richard Steiner for having sent m e an elementary school activity written by Dr. and Mrs. Koehn where a solution of table salt was used to separate food colors.
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