edited by
TIM CHAMPION Johnson C. Smith University Charlone, NC 28216
safety tips The Use of Erythrosin B in Undergraduate Spectrophotometry Experiments L. James Stock Ill Central Michigan University, Mt. Pleasant, MI 48859
Begining in 1970 with the passage of both the Clean Air Act and the Occupational Safety and Health Act, the United States began enacting major environmental and worker safety legislation. The Clean Water Act was passed in 1972. The Safe Drinking Water Act was passed in 1974. I n 1976 the Resource Conservation and Recovery Act (RCRA) was passed by Congress. This act established a cradle to grave control of hazardous wastes to ensure safe disposal and prevent environmental contamination. In 1979 CMU's disposal costs for hazardous wastes were 854L for any liquids. Now, costs range from $4.15L for nonhalogenated high BTU solvents up to $8.75L for heavy metal solutions. In response to these inrreasing regulations and disposal costs thc! C M V rhemistw" deoartment has made several changes such as using microscale experiments and redesigning experiments so that chemicals used are less toxic. One success story bas been the substitution of a nontoxic food color for an orieinallv " used toxic chromium(VI) salt in a general chemistry experiment. In 1978 sodium dichromate solutions in dilute sulfuric acid were used to teach solution preparation and the principles of spectrophotometric analysis. Students began by weighing approximately 0.4 g of sodium dichmmate and preparing a concentrated solution in dilute sulfuric acid. k r o k tb$, several dilutions were made to give final solutions in the ranEe of approximately 0.0005 mollL to 0.0015 rnofi. These f i a l soiuiions were-then placed in a singlebeam spectrophotometer (Spec 20) with a range of 340-600 nm and the percent transmission (% T ) recorded. From this, a Beer's Law plot was generated. Finally, unknowns of varying concentrations were assigned, their transmissions recorded, and students attempted to determine their concentrations from Beer's Law plots. Students were able to determine accurately unknown . the ex~erimenthad concentrations from their ~ l o t.s but several serious drawbacks. Sodium dichromate is a confirmed carcinoeen with emenmental tumorieenic data. It is a poison by Tngestion &d skin contact ( l ) r ~ l s othe , dilute sulfuric acid used as the solvent is corrosive. Undergraduate general chemistry students rarely have eficlent skills to handle these materials safelv and in a auantitative manner. The solid often is spilledwhen weighing and transfering into the volumetric flask. The dilute sulfuric acid is spilled when filling the flask. The spectrophotometer tubes are very difficult to fill without spilling. Even more diasterous, solutions are sometimes spilled in the s a m ~ l ec o m ~ a r t m e n tof the instrument. Even though gloves can be provided to protect the students'hands, ¬iced contamination on the floor and bench tops still is of great concern. An additional problem arises from the meat volume of poisonous and co-mosive waste generated from the experi~~~~~~
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926
Journal of Chemical Education
ment. Volume reductions can be made by neutralization of the sulfuric acid, reduction of the chromium(VI) to chromium(II1). n a hvdroxide (2. . .. and final ~ r e c i ~ i t a t i oas . , 3). . These operations are time consuming, potentially dangerous. and still result in the ~roductionof a hazardous waste. T;I modify this eqerimknt so that it teaches the same principles yet is more hazard-free, substituting a dye for the chromium(VI)salt was considered. There were several requirements of paramount importance: low toxicity, single large absorbance in the range of the instrument, a solid with a high molecular weight and a molar absorptivity (€7 similar to the previously used chromium(V1) salt. After much experimentation, Erythrosin B (Acid Red 51) was found to be a suitable alternative. This is the FDA-certified red food dve used to color maraschino cherries. It has a large absoibance maximum a t 525 nm and a molecular weight of 879.87 dm01 (4). O& students b&in the revised experiment by calculating and weighing out a mass that is required to prepare 500 mL of a solution t h a t is between 0.00082 M and 0.00084 M of Erythrosin B. This corresponds to approximately 0.365 g and is weighed on a milligram top-loading balance. Once this initial solution is prepared the student uses a 10-mLvolumetric ~ i ~and e ttransfers exactlv 10 mL of the solution to a 2 5 0 - h i volumetric flask. Thehask is filled to the mark with distilled water and is labeled as the "stock solution". It is from this stock solution that four final dilutions are made to eive solutions that will be measured in the spectrophotom~ter.From the stock solution are taken 5-. lo-. 15-. and 20- mL aliauota using separate volumetric pipets and all diluted up to 100 LLi n separate volumetric flasks to produce concentrations of 1.66 x lo4 M, 3 . 3 2 0 ~ M, 4.98 x lo4 M, and 6.64 x M, respectively The %Tat hmiwof these four solutions is measured and a Beer's Law plot constructed. AB a fmal exercise, several unknowns are given for concentration determination using the %T and the Beer's Law plot. These unknowns a r e conveniently pre ared a t 2 x lo4 M (0.0018 g Erythrosin BL), 3 x 101M (0.0026 g Erythrosin BL), and e Ervthrosin BL). 4 x lo4 M (0.0035 " " This revised experiment teaches' solution preparation and s~ectro~hotometrv. It is safe and eenerates no hazardous waste. The idea of quantitative transfer is emphasized because even a tiny speck of dye generates a red color when water is added due to the large molar absorptivity of the dye. A
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Literature Cited 1. Lewis, R. J. Hasonlous Chmiculs DeskRefemnca, 2nd ed.; Van Noatrand Reinhold: New York, 1991: p 1052. 2. Walton, W. A. J. C h . Edue 1987, M,A69.
3. Armour, M.J Chem. Edve 1988.65, A65 4. Available born Aldrieh Chemied Co., catalog Y 19,826-9.
