Experimental Procedure

Estimated time to complete the experiment: 1.5 h.

Materials Reagents

3 10-mL volumetric flasks NH4CI

1 1-mL graduated pipet lOMNaOH

2 Beral pipets 3 M HC1

3 spectrophotometer cuvettes 6% NaOCl

3 5-mL screw-capped conical bottom vials 0.01 M H2SO4

with rubber septa 3 glass capillary tubes 1 10-mL syringe 1 spectrophotometer 1 pH meter

Caution: All the reactions for the production of chloramines must be done under a fume hood (see the hazards listed below). Note: A "portable" fume hood, made by packing a 20-mL syringe with activated charcoal, could be appropriate for this microscale production (i.e., the Obendrauf trap, see Corral, 2005, although its efficiency with chloramines remains to be analyzed).

In a 10-mL volumetric flask prepare an approximately 0.1 M NH4CI solution and adjust the pH to 10-11 witha lOMNaOH solution. This can be done, for example, by dissolving 53.5 mg of NH4C1 in





: nhci2 ;



Figure 1. Approximate chloramine distribution with pH (for a chlorine (1+) to nitrogen (3—) ratio, R < 1). If R > 1 the predominance zones are displaced towards higher pH values; for example, NCI3 under such conditions can in principle be stable even at pH = 8. (Data taken from Colin, 1987).

5 mL of water, adding the NaOH solution as needed and using the necessary amount of water to reach the 10-mL mark. With a pipet or a syringe, transfer 3-mL aliquots of this solution into three spectrophotometer cuvettes. Add to the first cuvette five drops of deionized (D.I.) water; add to second cuvette three drops of DI water and two drops of 3 M HC1, and add to the third cuvette five drops of 3 M HC1. Stir. Measure the pH of each one (these should be in the approximate ranges 10-11, 6-7, and 2-3, respectively).

Prepare chlorine gas in a 5-mL screw-capped vial, equipped with a septum and a U-glass capillary exiting through the septum so as to convey gases from the vial into the spectrophotometric cuvette. See Figure 3. Chlorine gas preparation can be done for example by adding 1 mL of 3 M HC1 to the vial, followed by injection with a 10-mL syringe of one mL of commercial hypochlorite solution (typically 6%) plus 9 mL of air (see Mattson, 2003). Air is used here as the carrier so as to direct the chlorine gas that forms in the vial into the first spectrophotometric cuvette containing NH4CI solution. Allow this process of pushing the chlorine gas into the reaction mixture to last for 3 min. Measure the pH of the resulting solution and immediately take the corresponding absorbance spectrum in the region 200-400 nm. Repeat this procedure with the other two spectrophotometer cuvettes.

The solutions remaining in the vials and in the spectrophotometer cuvettes can be neutralized under a fume hood with HC1 or NaOH as needed, and discarded according to local regulations.

Hazards (see Delalu, 2001; Tanen, 1999; Pepi, 2003):

• The reaction of monochloramine with excess NH3 in alkaline media produces hydrazine, a known carcinogen. [Fortunately, at the working pH (about 10) the amount of NH3 in excess of NH4 is small and it may react with chlorine gas to produce the chloramine].


Figure 2. Species distribution diagram for the chloramines as a function of pH (at the arbitrary conditions: [NH3]tot = 0.40 mM and [C10"],0, = 0.50 mM). (Reproduced from The Chemical Educator, see Ibanez, 2006, figure 3. Experimental set-up. (Adapted from Ibanez,

a with permission).

• Accidental mixing of household cleaners containing hypochlorite and ammonia may result in severe lung injury.

• NCI3 is the most unstable chloramine in the pure form and it is highly explosive, a strong irritant, and a lacrimator.

• Concentrated NaOH is corrosive to human tissue (eye protection is especially important).

• Avoid mixing hypochlorite with any acid since this may result in the production of hazardous gaseous chlorine.

186 18. Production and Analysis of Chloramines Name_Section_Date_


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