In the Laboratory
An Easy Way To Make Chlorine Water L. H. Holmes, Jr. Department of Chemistry and Physics, Southeastern Louisiana University, Hammond, LA 70402
Chlorine can be prepared on a laboratory scale by reaction of MnO2 with HCl: MnO2 + 2Cl { + 4H3O+ → Cl2 + Mn+2 + 6H2O
(1)
The chlorine produced can be bubbled into water, which is a standard method used to prepare chlorine water (Cl2 in water) for laboratory use. In water, chlorine reacts as Cl2 + H2O
HCl + HOCl
(2)
but since the equilibrium favors Cl 2 + H2 O, not much HCl and HOCl are formed. The same equilibrium mixture could be obtained by mixing equal mole amounts of HCl and HOCl in water. HOCl is not readily available; but its sodium salt (while not stable in the pure form) is available in the form of aqueous solutions of sodium hypochlorite. Mixing HCl solutions with sodium hypochlorite solutions gives the familiar greenish color of chlorine in water. The reaction of HCl and sodium hypochlorite can be written as NaOCl + 2HCl → Cl2 + H 2O + NaCl
(3)
or written in ionic form OCl{ + Cl { + 2H3O+ → Cl2 + 3H 2O
(4)
In aqueous solution, OCl{ hydrolyses as OCl{ + H 2O
HOCl + OH{
(5)
Calculations based on the hydrolysis constant for eq 5 (obtained from KW for water and Ka for HOCl) indicate that the solution has a theoretical pH near 11. One HCl in eq 3 will react with the hydroxide ions in solution, driving the equilibrium in eq 5 to the right. The other HCl pushes the equilibrium in eq 2 to the left, forming chlorine water. The overall effect is that one of the HCl molecules (in eq 3) converts the NaOCl (present ionically as OCl{ and Na+) to HOCl and NaCl; the HOCl and the other HCl then reacts
1326
as in eq 2. Mixing hypochlorite and HCl so that there are 2 moles of HCl for each mole of NaOCl will yield a solution of chlorine water that contains one mole of NaCl for each mole of hypochlorite used. If the presence of the NaCl does not interfere in the use of the chlorine water, then this method is a simpler way to prepare chlorine water than using eqs 1 and 2 above. One can use sodium hypochlorite solutions obtained from a chemical supply company, but in many cases household bleach will work just as well. The ratio of volumes used will vary depending on the concentration of the HCl and the hypochlorite solutions, but assuming that household bleach is 5% NaOCl and concentrated HCl is 12 M, one adds about 180 mL of household bleach and 20 mL of concentrated HCl to 800 mL of water to give about a liter of chlorine water. By eq 3, this should lead to the production of about 0.12 mol (8.5 g) of Cl2 . This should be less than the solubility of chlorine in a liter of water at room temperature (1). If the concentrated reagents are mixed without dilution, chlorine gas is evolved from the solution, since the solubility of chlorine is too low for the solution to dissolve all the chlorine produced. Solutions prepared in this way have been used successfully in our undergraduate laboratories as chlorine water for many years. If the chlorine water is being used simply as an oxidant (as it often is), then the ratio does not have to be exactly stoichiometric because a small excess of hypochlorite or acid would not have an adverse affect. Standard safety precautions as indicated on MSDS sheets for working with hypochlorite, chlorine, and hydrochloric acid should be followed when carrying out the above procedure. Literature Cited 1. Lange’s Handbook of Chemistry, Rev. 10th ed.; N. A. Lange, Ed.; McGraw-Hill: New York, 1967; pp 256–257. 2. A discussion of chlorine chemistry is presented in: Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon: New York, 1984.
Journal of Chemical Education • Vol. 74 No. 11 November 1997
