Experimental Procedure

Estimated time to complete the experiment: 1.5 h.

Materials

Reagents

3 10-mL Erlenmeyer flasks

5 M H2O2

3 10-mL beakers

30% H2O2

3 5-mL syringes

D.I. water

1 universal stand

0.4 M NaCI02

1 three-finger clamp

NaCIO

3 2-mL graduated pipets

2.5 M NaC103

3 Beral pipets

10 M H2S04

1 propipet bulb

0.01 M H2S04

1 spectrophotometer

0.2 M NaC102

3 spectrophotometer cuvettes

2 M H2S04

1 ice bath

1 10-mL graduated cylinder

1 pH meter

Each one of the three methods described below for the preparation of C102(g) utilizes the apparatus shown in Figure 1. The apparatus setup is as follows: First stretch the plastic stem of a 4-mL, narrow stem disposable transfer pipet (Beral pipet) by holding the tip in one hand and the bulb in the other hand and pulling in opposite directions slowly and firmly. This will make a long capillary-like tube (3045 cm) that allows for easy delivery of the gas. Submerge the tip of the pipet in a small Erlenmeyer flask

Experimental Procedure Chemistry
Figure 1. Experimental set-up. (Adapted from Ibanez, 2006).

(or preferably, in a spectrophotometer cuvette) filled with distilled or deionized water. The reactants are then introduced sequentially in each experiment, as described below. As chlorine dioxide gas is evolved from the reaction in the bulb, it travels through the pipet stem and into the flask or cell, where it readily dissolves in water. The solubility of chlorine dioxide in water increases with decreasing temperature; therefore, it is helpful to place the receiving flask or cell in an ice bath during the reaction. Injecting several mL of air into the pipet bulb serves to push the product CIO2 out of the bulb and into the receiving flask or cell. The aqueous solution can then be used for characterization and/or further experiments. Owing to the high solubility of this gas, the analytical test described below can be applied either to the gas or to the collection solution.

Caution: All the preparations described below must be performed under a fume hood, since inhalation of CIO2 may cause respiratory irritation, pulmonary edema and even death. Do not wear contact lenses during its preparation or use. Whenever production of C102 is to be stopped (for example, in a chemical spill) make the solution alkaline so as to convert it to CIO4" and CI02", thus avoiding further gas release. In case of contact with skin or eyes, flush immediately and abundantly with lukewarm water. Before voluntary release to the atmosphere, CIO2 should be reacted or decomposed; appropriate media for CIO2 removal include a solid soda-lime fine mixture, a basic thiosulfate solution, the Witches brew (i.e., a mixture of sodium hydroxide and potassium salts), etc. Do not allow C102 pressure to build up, since at p >150 mm Hg it will likely decompose explosively! It decomposes thermally and photochemically with expansion.

Preparation method #1: Chlorate reduction by hydrogen peroxide

Caution: Sodium chlorate is a strong, explosive oxidizer. Do not allow contact with organic matter or other oxidizable substances.

In a 10-mL beaker, place 7.5 mL of distilled water, 0.1 mL of a freshly prepared 2.5 M sodium chlorate solution, 0.2 mL of a freshly prepared 5 M hydrogen peroxide solution (this can be prepared, for example, by diluting 5.7 mL of 30% H202 with D.I. water to a total of 10 mL), and 1.0 mL of a 0.01 M sodium chloride solution. Caution: Concentrated H2O2 is harmful to human tissue. Use skin and eye protection.

Draw approx. 2 mL of this mixture into the Beral pipet through its stem (before stretching it as described above); then, stretch it and puncture a small hole on one of its sides and introduce very slowly through it (e.g., with a 5-mL syringe) 1 -2 mL of 10 M sulfuric acid to initiate the reaction. See Figure 1. Caution: Sulfuric acid is corrosive, especially when concentrated; use skin and eye protection. Syringe needles are very sharp, handle with care. (If possible, unsharpen the needles with sandpaper or a fine metal file to convert them into "steel tubes"). Push the gas product out of the pipet and collect it for analysis in the manner described above. A yellow color (indicative of the production of CIO2) appears immediately. Pump more air with the syringe by injecting several syringes-full of air into the pipet so as to displace the gas. Alternatively, an aquarium air pump or a compressed inert gas (e.g., nitrogen) can provide the necessary carrier gas.

An aqueous solution of chlorine dioxide has a distinctly yellowish cast, whereas chlorine gas dissolved in water is more green. Because Cl2(g) is often a by-product in the production of C102(g), it may be

CI02 UV/Vis absorbance spectrum

0.55

0.55

-260

466-

Wavelength figure 2. Gas-phase chlorine dioxide absorbance. (Adapted from Ibanez, 2006).

-4666--1500

difficult to distinguish by eye if the solution is more yellow or green, and therein whether the reaction produced the desired product. UV-Vis spectra may be taken (of either the aqueous solution or the gas) to confirm the presence of CIO2. A peak for chlorine dioxide gas is observed at 360 nm with distinctive vibrational fine structure (see Figure 2). This electronic spectrum is discussed in detail elsewhere (see Esposito, 1999). It may be helpful to take a scan of chlorine gas to compare it to the chlorine dioxide spectrum.

Preparation method #2: Chlorite oxidation by hypochlorite

Caution: Sodium chlorite is an oxidizing, corrosive material. Combustibles wetted with its solution and subsequently dried are easily ignited and burn vigorously (e.g., paper). It is incompatible with all combustibles, reducing agents (including reactive metals) and acids. It is a severe (corrosive) irritant. In case of spill of the solid material, collect into a clean metal or high-density polyethylene container. Wash away residues with a large amount of water. Do not use rags, sawdust or other combustible absorbents. If contact with the skin occurs, wash with soap and water.

Pour a few mL of a 0.4 M NaC102 solution into a 10-mL beaker. Neutralize it to pH 7 with 0.01 M H2S04. In a separate container, prepare a working solution of sodium hypochlorite by diluting 0.2 mL of a commercial (6% w/v) solution (bleach) to 10 mL with D.I. water and neutralize to pH 7 with 0.01 M H2SO4. Draw approximately 1.5 mL of this dilute bleach solution into a new Beral pipet through its stem (before stretching it as described above); then, stretch it and puncture a small hole on one of its sides and introduce through it (with a syringe) 1.5 mL of the neutralized NaCIO solution. Proceed as in the preparation method #1 to displace, collect, and analyze the CIO2. See the cautionary notes given above.

Preparation method #3: Disproportionate of chlorous acid

Draw approximately 1.5 mL of a 0.2 M NaClCh solution into a Beral pipet through its stem (before stretching it as described above); then, stretch it and puncture a small hole on one of its sides and introduce through it (with a syringe) approximately 1.5 mL of a 2 M sulfuric acid solution. Proceed as in the preparation method #1 to displace, collect, and analyze the CIO2. See the cautionary notes given above.

Experimental Procedure Name_

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