Sulfites contact sensitivity

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

EXPLOSION and FIRE CONCERNS not combustible NFPA rating (not rated) mercurous chloride is ineompatible with bromides, iodides, alkali chlorides, sulfates, sulfites, carbonates, hydroxides, ammonia, silver salts, eopper salts, hydrogen peroxide, iodine, and iodoform mercuric oxide reacts explosively with acetyl nitrate, chlorine and hydrocarbons, butadiene and ethanol and iodine (at 35°C), and hydrogen peroxide and traces of nitric acid forms heat or shock-sensitive explosive mixtures with metals and non-metals contact with acetylene, acetylene products, or ammonia gases may from solid products that are sensitive to shock and which can initiate fires of combustible materials decomposition emits highly toxic fumes of Hg use water spray, fog, or foam for firefighting purposes. 

On adding an excess of sodium azide to neutral or slightly acid solutions of cobalt salts, a violet color appears which is due to complex cobalt -azide anions. In contact with air an oxidation to anionic azide-complexes of tervalent cobalt takes place, shown by a change of color from violet to yellow. This very slow autoxidation is enormously accelerated by sulfurous acid or sulfite ions. Probably the autoxidation of sulfite to sulfate induces the oxidation of the complex bounded cobalt. However, the color change violet yellow is not sufficiently sensitive to serve as a test for this induction effect and therefore, for sulfite. It is better to identify the tervalent cobalt through a color reaction with an acetic acid solution of o-tolidine (formation of a blue quinoidal oxidation product of the base). Compare page 209. 

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