reaction under control. It is usually more convenient to add solid halides, such as AlCl3 and ZrCl4, to stirring water and crushed ice in a flask or beaker. The acidic solution can be neutralized and, depending on the metal and local regulations, discarded in the sanitary sewer or with other hazardous or nonhazardous solid waste.
Halides and acid halides such as PCl3, PCl5, SiCl4, SOCl2, SO2Cl2, and POCl3 are water-reactive. The liquids can be hydrolyzed conveniently using 2.5 M sodium hydroxide by the procedure described earlier for acyl halides and anhydrides. These compounds are irritating to the skin and respiratory passages and, even more than most chemicals, require a good hood and skin protection when handling them. Moreover, PCl3 may give off small amounts of highly toxic phosphine (PH3) during hydrolysis.
Sulfur monochloride (S2Cl2) is a special case. It is hydrolyzed to a mixture of sodium sulfide and sodium sulfite, so that the hydrolyzate must be treated with hypochlorite as described earlier for sulfides before it can be flushed down the drain.
The solids of this class (e.g., PCl5) tend to cake and fume in moist air and therefore are not conveniently hydrolyzed in a three-necked flask. It is preferable to add them to a 50% excess of 2.5 M sodium hydroxide solution in a beaker or wide-mouth flask equipped with a stirrer and half-filled with crushed ice. If the solid has not all dissolved by the time the ice has melted and the stirred mixture has reached room temperature, the reaction can be completed by heating on a steam bath, and then the acidic solution neutralized and disposed of in the sanitary sewer.
Many inorganic wastes consist of a cation (metal or metalloid atom) and an anion (which may or may not contain a metalloid component). It is often helpful to examine the cationic and anionic parts of the substance separately to determine whether either possesses a hazard.
If a substance contains a ''heavy metal," it is often assumed that it is highly toxic. While salts of some heavy metals, such as lead, thallium, and mercury, are highly toxic, those of others, such as gold and tantalum, are not. On the other hand, compounds of beryllium, a "light metal," are highly toxic. In Table 7.1, cations of metals and metalloids are listed alphabetically in two groups: those whose toxic properties as described in the toxicological literature present a significant hazard, and those whose properties do not. The basis for separation is relative, and the separation does not imply that those in the second list are "nontoxic." Similarly, Table 7.2 lists anions according to their level of toxicity and other dangerous properties, such as strong oxidizing power (e.g., perchlorate), flammability (e.g., amide), water reactivity (e.g., hydride), and explosivity (e.g., azide).
Materials that pose a hazard because of significant radioactivity are outside the scope of this volume, although they may be chemically treated in a manner similar to the nonradioactive materials discussed in this chapter. Their handling and disposal are highly regulated in most countries. Low-level radioactive mixed waste is discussed in section 7.C above.
Chemicals in which neither the cation nor the anion presents a significant hazard consist of those chemicals composed of ions from the right-hand columns of Tables 7.1 and 7.2. Those that are soluble in water to the extent of a few percent can usually be disposed of in the sanitary sewer. Only laboratory quantities should be disposed of in this manner, and at least 100 parts of water per part of chemical should be used. Local regulations should be checked for possible restrictions. Dilute slurries of insoluble materials, such as calcium sulfate or aluminum oxide, also can be handled in this way, provided the material is finely divided and not contaminated with tar, which might clog the piping. Some incinerators can handle these chemicals. If time and space permit, dilute aqueous solutions can be boiled down or allowed to evaporate to leave only a sludge of the inorganic solid for landfill disposal. However, appropriate precautions, including the use of traps, must be considered to ensure that toxic or other prohibited materials are not released to the atmosphere.
An alternative procedure is to precipitate the metal ion by the agent recommended in Table 7.1. The precipitate can often be disposed of in a secure landfill. The most generally applicable procedure is to precipitate the cation as the hydroxide by adjusting the pH to the range shown in Table 7.3.
Because the pH range for precipitation varies greatly among metal ions, it is important to control it carefully. The aqueous solution of the metal ion is adjusted to the recommended pH (Table 7.3) by addition of a solu-