Introduction

As discussed in Section 10.1 of the companion book, chlorine gas is a potent and relatively cheap oxidizer and disinfectant. Unfortunately, its reaction products (disinfection by-products, or DBP) can be quite dangerous. A chlorine-based alternative to chlorine itself is chlorine dioxide, CIO2 (also called chlorine peroxide) that can act as an extremely effective biocide, disinfectant, and oxidizer. It is active against some chlorine-resistant pathogens. Additional advantages include that its oxidizing and disinfecting properties remain essentially constant over a wide pH range (from 4 to 10), and that its DBPs are substantially fewer than those produced by chlorine. A summary of its applications, economic perspectives, molecular geometry and properties is given in Section 10.1.

Chlorine dioxide can be produced in many different ways, mainly through the reduction of C1(V) or the oxidation of Cl(III). The following equations in aqueous solution are only indicative of the main reactions:

a) Chemical reduction of C1(V)

b) Chemical oxidation or disproportionation of Cl(III)

C102- + V2CIO- + 1/2H20(1) ^ C102(g)+0H- + i/2Cr (7)

The production of C102 by chlorate reduction or by chlorite oxidation can also be performed elec-trochemically. Several of the routes described in the reactions above also entail the production of Cl2 as a by-product, which is undesirable for certain applications; it can be removed by contacting the gas mixture with oxides, hydroxides, and various carbonates of the alkali and alkaline earth metals. In addition, the newer processes (e.g., reduction of chlorate with hydrogen peroxide, reaction 1) are designed to minimize the amount of CI2 generated as by-product.

Analytical methods for chlorine dioxide include:

• Spectrophotometry at 360 nm (extinction coefficient = 1150 M"1 cm"1)

• Titrimetry with iodine

• Polarography

• Amperometry

• Chromatography

In the present experiment we describe three mi-croscale procedures to obtain chlorine dioxide: a) C1(V) reduction, b) Cl(III) oxidation, and c) Cl(III) disproportionation. The gas produced is analyzed spectrophotometrically either in the gas phase or dissolved in water.

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