Methods for Preparing Aqueous Solutions of Chlorine and Bromine for Halogen Displacement Reactions Gene A. Hiegel, Miguel H. Abdala, and S. Vincent Burke California State University. Fullerton, Fullerton. CA 92634 Donald P. Beard Cypress High School, 9801 Valley View St., Cypress, CA 90630 Displacement reactions are commonly used to demonstrate that chlorine is more reactive or a stronger oxidizing agent than bromine and that bromine is more reactive than iodine. For example, bromide ion is converted into hromine in the presence of an aqueous solution of chlorine (eq 1) and iodide is converted into iodine in the presence of an aqueous solution of bromine (eq 2). The hromine or iodine formed can be concentrated for easier observation in afew drops of a hydrocarbon solvent such as hexane. C1,
+ 2 BrF
-
2 Cl-
+ Br,
(1)
The aqueous chlorine solution used for the displacement reaction is normally prepared by bubbling chlorine gas through water. The solubility of chlorine in water a t 25 OC is 0.092 mol/L (9.2 mmo1/100 mL or 0.65 g/100 mL).' Since chlorine is a gas, i t is sold in expensive steel cylinders, and because chlorine is toxic, i t is also dangerous t o use and store. The aqueous hromine solution used for the displacement reaction is normally prepared by adding liquid bromine to water. The solubility of bromine in water at 25 ' C is 0.214 mol/L (21.4 mmo1/100 mL or 3.4 g/100 mL).= Bromine i s toxic, causes burns a n d blisters on t h e skin, and has a low boiling point (hp 59.5 OC) and thus is dangerous to store and handle.2 We were interested in developing alternatives to handling and storing cylinders of chlorine gas and containers of liquid hromine. Several alternatives using generally available chemicals are presented. Chlorlne Solutlons from Sodium Hypochlorlte Sodium hypochlorite solution, available in grocery stores as chlorine bleach, can be used to generate chlorine. In the presence of hydrochloric acid, sodium hypochlorite is converted into chlorine and water (eq 3). An aqueous solution of chlorine, which is useful for the displacement reaction, can be prepared by combining 13 mL (9.2 mmol; equivalent to 0.65 g chlorine) of 5.25% sodium hypochlorite and 69 mL of water with 18 mL (18 mmol) of 1 N HC1. Since chlorine gas will escape, this procedure and similar procedures involving chlorine or bromine should be carried out in a hood or outdoors. NaOCl + 2 HCI
-
C1,
+ H,O + NaCl
a s trichloroisocyanuric acid because a violent reaction could occur. The oxidizing agent should be added to the water. Add 9 mL (9 mmol) of 1N HC1 to the mixture and stir with a magnetic stirrer to release the chlorine and form cyanuric acid, 2 (eq 4). All solid should dissolve in about an hour when the reaction is complete.
A Dlrect Subslltute for Chlorine I t is also possible to substitute a saturated aqueous solution of trichloroisocyanuric acid (solubility 0.2 g in 100 mL water) for a chlorine solution.3 This solution will react with bromide ion to give bromine and provide a simpler and safer way of demonstrating the formation of hromine by means of a stronger oxidizing agent. Bromlne Solutlons from Sodlum Hypochlorlte Because the displacement reaction will generate an aqueous solution of bromine from bromide ion, it can be used to prepare the bromine solution required for the reaction with iodide ion eliminating the need for working with liquid hromine. Addition of 30 mL (21 mmol) of 5.25% sodium hypochlorite to a solution of 4.4 g. (43 mmol) of sodium bromide in 42 mL (42 mmol) of 1 N hydrochloric acid and 28 mL water will give an aqueous solution of bromine of 3.4 g or 0.021 mo1/100 mL (eq 5). 2NaBr + NaOCl + 2HCI
-
Brp + 3NaC1 H20
(5)
Bromlne Solutions from Trlchlorolsocyanurlc Acid Prepare a solution of 4.4 g (43 mmol) sodium bromide in 21 mL (21 mmol) 1 N hydrochloric acid and 79 mL water. Add 1.8 g (7.7 mmol, about 90% available chlorine; equivalent to 21 mmol chlorine) of trichloroisocyanuric acid and stir for about an hour with a magnetic stirrer (eq 6). Gravity filtration in a hood to remove the small amount of fine solid will give a hromine solution of about 3.4 g or 21 mmo1/100 mL water.
(3)
Chlorlne Solutlons from Trlchlorolsocyanurlc Acld A similar procedure using trichloroisocyanuric acid (1) [or trichloro-s-triazinetrione], a stable solid that is used as a disinfectant in swimming pools and is available in pool supply and hardware stores, will also generate an aqueous solution of chlorine. Trichloroisocyanuric acid, 0.80 g (3.4 mmol with about 90% available chlorine; equivalent to 9.3 mmol or 0.66 g chlorine), should be added to 91 mL water. It is not advisable to add w a t e r to strong oxidizing agents such
Acknowledgment Partial support was provided by the California State University Fullerton Foundation.
The Merck Index. 10th ed.; Merck: Rahway. NJ. 1983; p 294. Merck Index. p 192. Merck Index. p 1295.
