edited bv
Brother Thomas McCullwgh, C.S.C. and ~ e r n h i a Tyminski St. Edward's University, Austin, TX 78704 Available at any convenience store or supermarket, liquid household bleach (5' NaOCI) is a remarkably versatile and economical lahoratory reagent, though somewhat corrosive. It will produce chlorine gas when reacted with hydrochloric acid'; it will release oxygen gas when treated with aqueous CoC12.Qn interesting organic reaction occurs whcn acetone and NaOCl are mixed. the ~ r o d u c t sheine NaOH. sodium acetate, and chloroform. his latter reaccon is noticeably exothermic. T h e change in temperature can be used to compare the strengths of various brands of bleach, t o monitor t h e loss of reactivity a s time passes, or to evaluate standard diluted solutions of a given brand. This experiment shows t h a t a simple thermometer can b e used as a quantitative instrument by noting t h e change in temperature.
Procedure Nest a 50-mI. beaker in a 100-mL beaker with insulating paper around the smaller beaker. Do not use Styrofoam or plastic cups as the reaction vessel. Place a magnetic stir bar and 10.0 mL of bleach in the 50-mL beaker. Suspend a thermometer so the hulb is just under the liquid and away from the stir bar. If a magnetic stirrer is not available, gentle stirring with the thermometer will give satisfactory results. Observe and record the initial temperature (TI) of the system. Begin stirring at a slow rate, and add 0.40 mL of acetone (caution, flammable) from a graduated 1.0-mL pipet or a calibrated eyedropper. The temperature will rise almost immediately and should he ohserved until a maximum value (T2)is reached. Tz - TI is very nearly proportional to the strength of the NaOCl and will he about 25 T. To obtain completely quantitative results, the heat capacities of all reagents, beakers, and products would have to he known and used in a heat balance equation, a refinement not needed for this simple experiment. Althouehthe chloroform oroduced does not form a seoarate laver. . . itsodur is easily detected. Since theacetone molevulecontains three carbon atoms, it i.i conceivable that a CI, a C2, or a CI organic salt could be obtained from the reaction. The identity of the arid product can he verified as sodium acetate by paper chromatography as follows:3 Solvent: 80%phenol (Highly corrosive. Wash any spills on skin with large amounts of water.) Paper: 17 X 20-cm rectangle of Whatman # 1 paper
Spors: Original bleach, reaction solution as is, and 157 solutions of sodium formate, sodium acetate, and sodium propionate. (Five separate spots.) Detection: 0.40% dichlorofluorescein in ethanol viewed under ultraviolet light. (Protect eyes from this light.)
Procedure Place 10 mL of the 807% phenol in a liter Berzelius h d e r , and cover with aplastic sheet held in place with a rubber band. Swirl the beaker gently to wet the sides so as to hasten vapo-liquid equilihrium. Place soots of the solutions mentioned above alone the 20-cm length of thh paper using a microcapillary spotter. The :pots should he about 15 mm above the edge of the paper and marked with a pencil. When the spots are dry the paper is formed into a cylinder 17 cm high and held in place with a plastic clip? The seal is removed from the beaker, the cylinder is carefully placed inside (spots down), and the beaker resealed. The solvent will slowly rise up the paper and reach the top in several hours. The system can be left unattended during this phase of the experiment. When the solvent reaches the top, the paper is removed (do not touch, use tongs) and air dried for at least an hour, preferably in a h w d with fan running. When the paper is dry, it is dipped in or sprayed with the dichlorofluorescein solution and viewed under UV light. Salts of the weak acids will appear as yellow fluorescenceon a neutral background. Appropriate laboratory assignments for this experiment would be to halanee the equations for the reactions mentioned in paragraph one, to calculate R, values for paper chromatography, and to determine the number of droos of acetone in 1mL. Iflaboratory rlmr la Ilmltpd, the paperrhnrmatography portlon of the crpernnenr e m he imrtted m order LO concentrate more on thr cornpanson ofdifferent hrandsof hleachor tuprepare agraph based on known diluted bleach solutions.
'Bums. D. T.; Townshend. A,; Carter, A. H. inorganic Reaction Chemishy; Wiley: New York. 1981; Vol. 2. Part A; p 132. Thorne, P. C. L.; Roberts, E. R. inorganic Chemishy, 4th ed.; Nordeman: New York, 1943; p 366. McCullough, T. J. Chem. Educ. 1974,51,279. 'Frirlson, G. J., Chm-Anal. 1962, 51, 19.
Volume 66
Number 11
November 1989
973
