Base-insoluble sulfides and hydroxides

Group 3. Base-insoluble sulfides and hydroxides After the solution is filtered to remove any acid-insoluble sulfides, it is made slightly basic, and (NH4)2S is added. In basic solutions the concentration of is higher than in acidic solutions. Thus, the ion products for many of the more soluble sulfides are made to exceed their K p values and precipitation occurs. The metal ions precipitated at this stage are Al +, Cr Fe +, Zn +, Ni +, Co +, and Mn + (The Al +, and Cr ions do not form insoluble sulfides instead they precipitate as insoluble hydroxides, as Figure 17.23 shows.)... 

In the section entitled Cleaning Up at the end of each experiment the goal is to reduce the volume of hazardous waste, to convert hazardous waste to less hazardous waste, or to convert it to nonhazardous waste. The simplest example is concentrated sulfuric acid. As a by-product from a reaction, it is obviously hazardous. But after careful dilution with water and neutralization with sodium carbonate, the sulfuric acid becomes a dilute solution of sodium sulfate, which in almost every locale can be flushed down the drain with a large excess of water. Anything flushed down the drain must be accompanied by a large excess of water. Similarly, concentrated base can be neutralized, oxidants such as Cr " can be reduced, and reductants such as hydrosulfite can be oxidized (by hypochlorite— household bleach). Dilute solutions of heavy metal ions can be precipitated as their insoluble sulfides or hydroxides. The precipitate may still be a hazardous waste, but it will have a much smaller volume. 

As a hard base, water prefers to coordinate to the harder acid Ba +, the softer base ammonia prefers to coordinate the softer acid Ag+ to form Ba(OH2) + and Ag(NH3)2+, respectively. Sulfates, sulfites, carbonates, phosphates, arsenates, oxides, and sulfides are virtually insoluble in ammonia. Hydroxides and amides are also very poorly soluble. 

Another class of compounds whose cations may not be precipitated by the addition of hydroxide ions are the most stable complexes of metal cations with Lewis bases, such as ammonia, amines, and tertiary phosphines. Because of the large number of these compounds and their wide range of properties, it is not possible to give a general procedure for separating the cations. In many cases, metal sulfides can be precipitated directly from aqueous solutions of the complexes by the addition of aqueous sodium sulfide. If a test-tube experiment shows that other measures are needed, the addition of hydrochloric acid to produce a slightly acidic solution will often decompose the complex by protonation of the basic ligand. Metal ions that form insoluble sulfides under acid conditions can then be precipitated by drop wise addition of aqueous sodium sulfide. 

To detect free alkali in the presence of alkali sulfides, which also give an alkaline reaction owing to hydrolysis, use is made of the fact that thallous sulfide is insoluble whereas thallous hydroxide is soluble in water and is almost as strong a base as potassium hydroxide. Free alkali may be detected by adding neutral thallous nitrate and then testing the solution for an alkaline reaction. 

The more common salts of aluminum, such as the nitrate, the sulfate, and the many double sulfates (alums) need not be redescribed here. It should be noted, however, that aluminum hydroxide is such an extremely weak and insoluble base that aluminum salts of weak acids (the acetate, cyanide, and sulfide) are almost completely hydrolyzed in contact with aqueous systems. 

Thus, when OH" is added to a solution of Hg +, a dark precipitate consisting of Hg and HgO is formed evidently mercurous hydroxide, if it could be isolated, would be a stronger base than HgO. Similarly, addition of sulfide ions to a solution of Hg + gives a mixture of Hg and the extremely insoluble HgS. Mercurous cyanide does not exist because Hg(CN)2 is so slightly dissociated though soluble. The reactions in these cited cases are... 

Most of the separation procedures are based on the precipitation [27, 68, 72, 76, 80, 89, 99, 110, 139, 158, 170] or extraction [11, 72, 81, 119, 122, 124, 139, 162] of the Pd dimethyl-glyoxime (DMG) complex. In combination with the commonly used scavenger precipitations of Fe, La, or Zr hydroxides and Ag halides quite high decontamination factors can be achieved (see for example procedure 1 below). The samples for counting are prepared using either Pd metal or insoluble Pd compounds, like DMG complex, sulfide, or iodide. 

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