edited by
DONNABOGNER
insights The Preparation of Halogen Waters Damon Diemente Trinity School
101 West 91st Street New York, NY 10024 Aqueous solutions of chlorine, bromine, and iodine are useful in both demonstrations and experiments meant to illustrate periodicity in the chemical behavior of the halogens. However, many teachers are understandably reluctant to handle these elements in their pure form because of their vigorous reactivity. If pure, gaseous chlorine is used for chorine water, the element first has to be bled from a cylinder of the comDressed eas or DreDared chemicallv. Either Drocess must be Hpproa&ed with caution. chlorine rapidly corrodes nearly all metals and organic materials; thus it is very dangerous to inhale it. Its sharp, noxious odoris detectible at 2 ppm; 30 mmofchlorine causes couehiueandimtates the throat, - lu&, and eyes.' Liouid bromine. d e e red. ~ fuming, and volatile, is nearly as hazardous as 'chlorine. ~ i k cGlorine, e it attacks most metals and organic tissues. Its suffocating, odorous fumes irritate the eyes and lungs. The liquid can burn and blister the skin. Iodine is tamer. Its characteristic sharp odor is reminiscent of the more reactive halogens, but momentary cantact with skin results only in stains. Iodine, a solid at mom temperature, can be weighed, and its spills can be cleaned up with relative safety. Clearly, the hazards of the halogens are formidable, but there are quite safe ways to prepare aqueous solutions of all three of them that do not require handling gaseous chlorine or liquid bromine. Chlorine water can be prepared from household bleach with sodium hypochlorite as the active ingredient. These bleaches are usually 5.25% sodium hypochlorite by weight, the rest being water and buffer to keep the pH high. The hypochlorite in these bleaches is almost completely converted to chlorine by addition of chloride and acidification: 2 H++ oc1- + c1-+ C12+ HZ0 Commercial bleaches with 5.25% NaOCl have densities in the vicinity of 1.08 gImL. This means they contain 56.7 g of NaOCl (formula mass 74.5 amu) per liter, and that the concentration of hvoochlorite is 0.759 M. Therefore. to prepare a p p r o x i m ~ ~0.1 l y M chlorine water, dilute 20 mL offresh bleach with 128 mL of water, then stir in 2.5 mL of 6 M HC1 and 1.3 mL of 6 M H2S04.The resultant solution of chlorine water is clear, pale green in color, and smells strongly of chlorine. If precise pH is important, it can be adjusted with a few drops of 3 M HzSOa or 3 M NaOH. The density of liquid bromine is 3.1 g/mL, and the saturated aqueous solution of bromine is nearly 0.2 M. Therefore, one could make an approximately 0.1 M solution of Br2 by dissolving 0.5 mL of bromine in 100mLof water. To avoid
932
Journal of Chemical Education
WichitaState Universiw Wichita. KS67208
handling liquid bromine, the element is made in warm, acidic solution by the reaction between bromate and bromide: 6 H ' + B r 0 3 + 5 B r + 3Br2+3H20 For a 0.1 M solution of bromine, dissolve 1.67 g of KBr03 (formula mass 167 amu) and 6.19 g of KBr (formula mass 119 amu) in a little water, add 5.0 mL of 6 M HzS04, dilute with water to 100 mL, and warm for a few minutes to about 60 "C. Adjust the pH if necessary as described above. There is little danger involved in making iodine water simply by dissolving crystals of iodine in water, so there is no real advantage to an indirect method. Of course, ventilation is necessarv because iodine. whether solid or in solution is volatile: and its vapors &e imitating. Furthermore. iodine is not verv soluble and it is slow to dissolve. I t is, therefore, necessaryclthcr tousca littleorganicsolvent, or tod~ssolvethe iodme in lodide such as acrtoneofulcoh~~l. solution. Iodine reacts with iodide to produce triiodide: 12+r+13For a 0.1 M solution of iodine, dissolve 2.45 g of solid 12 in 100 mL of 0.1 M KI. This solution behaves chemically like a 1:l mixture of iodine and iodide, and the iodide may interface with some intended uses. One answer to this problem is to dissolve the iodine first in a little organic solvent (unless this also interferes), then add water (and more organic solvent if necessary) to bring the volume to 100 mL. All three halogen waters must be used with ventilation. In case of spills, the elements (Xz)are rapidly neutralized with excess base solution, such as soap solution or sodium bicarbonate paste: &+20K+XO+X+Hz0 3 XO- -r XO< + 2 X The neutralized spill can then be cleaned up with water. Solutions of haloeens should be made UD fresh as thev are needed, since shklf life can be short 'due to hydrolytic disproportionation: This reaction is suppressed if the halogen water is kept slightly acidic. All the reactions and equations mentioned above are well-known. They may be found in many high school texts and are described fully in the standard descriptive inorganic l i t e ~ a t u r e . ' ~ 'Stecher, P., Ed. TheMercklndexof ChemicalsandDrugs,7th ed.; Merck: Rahway. NJ, 1960. 'Sidgwick, N. The Chemical Elements and Their Compounds; Oxford:Oxford,1962;Vol. 2, p 1139 ff. 3Cotton. F.; Wilkinson, G. Advancedlnorganic Chemistry 2nd ed.; Interscience: New York, 1966;Chapter 22. 4Jolly,W. The Chemistry of the Non-Metals; Prentice-Hall: Englewood Cliffs, NJ, 1966.
