Sulfite solubility

Neutralization.—The preparation of reducing salts by neutralization requires no special comment. The most familiar examples are the soluble sulfites and sulfides. Since these are salts of weak, dibasic acids, it is not satisfactory to rely upon indicators for neutrality. The usual procedure is to prepare a solution of the base and divide it into two equal parts. One of these is saturated with the acid, producing the acid salt, and the other portion is then added, with the formation of the normal salt (example, preparation of (NH4)2S as a laboratory reagent). 

The best bases in liquid S02 are sulfites of organic amines, for example, tetramethyl ammonium sulfite, [(CH NhSOg (for most of the inorganic sulfites are quite insoluble in this solvent). As expected, the conductivity of liquid S02 increases when either S03 (an acid) or a soluble sulfite (a base) is dissolved in it, but the conductivity drops if one of these solutions is mixed with the other (neutralization). Thionyl chloride, SOCL, also exhibits acidic reactions when dissolved in S02 but does not, as is sometimes supposed, yield the S02+ ion (Exercise 1). It is interesting that aluminum sulfite, Als(SOs)s, is amphoteric in liquid S02, just as aluminum... 

There is a preparative advantage in hydrolysing the osmic ester by aqueous-alcoholic sodium sulfite solution, for then the osmium is precipitated as sparingly soluble sulfite complex. 

Soluble sulfites precipitate Ba as barium sulfite, BaSOs, white, insoluble in water but soluble in HCl (distinetionfrom the sulfate). 

Sulfur dioxide reacts with OH to form 803 and then, with more SO2, HS03 in solution or [8205] in solids. (The HS03 does not persist in solids, nor [8205] " in solution.) Soluble sulfites must be quite dry or air-free to be preserved. Even air-free, wet HSO3 slowly decomposes ... 

Soluble sulfites and H2Se precipitate selenium and sulfur together. 

The higjily water-soluble dienophiles 2.4f and2.4g have been synthesised as outlined in Scheme 2.5. Both compounds were prepared from p-(bromomethyl)benzaldehyde (2.8) which was synthesised by reducing p-(bromomethyl)benzonitrile (2.7) with diisobutyl aluminium hydride following a literature procedure2.4f was obtained in two steps by conversion of 2.8 to the corresponding sodium sulfonate (2.9), followed by an aldol reaction with 2-acetylpyridine. In the preparation of 2.4g the sequence of steps had to be reversed Here, the aldol condensation of 2.8 with 2-acetylpyridine was followed by nucleophilic substitution of the bromide of 2.10 by trimethylamine. Attempts to prepare 2.4f from 2.10 by treatment with sodium sulfite failed, due to decomposition of 2.10 under the conditions required for the substitution by sulfite anion. 

Another group of compounds called oxygen scavengers retard oxidation by reducing the available molecular oxygen. Products in this group are water soluble and include erythorbic acid [89-65-6] C HgO, and its salt sodium erythorbate [6381-77-7] C HgO Na, ascorbyl pahnitate [137-66-6] 22 38 7 ascorbic acid [50-81-7] C HgO, glucose oxidase [9001-37-0] and sulfites (23). 

Lead shows excellent resistance to phosphoric and sulfuric acid in almost all concentrations and at elevated temperatures, as well as to sulfide, sulfite, and sulfate solutions. The corrosion film is insoluble lead sulfate which rapidly reforms if it is damaged. Lead is also resistant to chlorides, fluorides, and bromates at low concentrations and low temperatures. However, because lead is soluble in nitric and acetic acids, it is not resistant to these acids. 

Physical Properties. Anhydrous sodium sulfite [7757-83-7] Na2S02, is an odorless, crystalline soHd and most commercial grades other than by-product materials are colorless or off-white (331—334). It melts only with decomposition. The specific gravity of the pure soHd is 2.633 (15.4°C). Sodium sulfite is quite soluble in water. It has a maximum solubiHty of 28 g/100 g sol at 33.4°C at higher and lower temperatures, it is less soluble in water. Below this temperature, the heptahydrate crystallizes above this temperature, the anhydrous salt crystallizes. Sodium sulfite is soluble in glycerol but insoluble in alcohol, acetone, and most other organic solvents. 

Physical Properties. Sodium metabisulfite (sodium pyrosulfite, sodium bisulfite (a misnomer)), Na2S20, is a white granular or powdered salt (specific gravity 1.48) and is storable when kept dry and protected from air. In the presence of traces of water it develops an odor of sulfur dioxide and in moist air it decomposes with loss of part of its SO2 content and by oxidation to sodium sulfate. Dry sodium metabisulfite is more stable to oxidation than dry sodium sulfite. At low temperatures, sodium metabisulfite forms hydrates with 6 and 7 moles of water. The solubiHty of sodium metabisulfite in water is 39.5 wt % at 20°C, 41.6 wt % at 40°C, and 44.6 wt % at 60°C (340). Sodium metabisulfite is fairly soluble in glycerol and slightly soluble in alcohol. 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

Dilution with water reverses the reaction, and heating the solution Hberates sulfur dioxide. Upon being added to a solution of teUurides, teUurium forms colored polyteUurides. Unlike selenium, teUurium is not soluble in aqueous sodium sulfite. This difference offers a method of separating the two elements. Like selenium, teUurium is soluble in hot alkaline solutions except for ammonium hydroxide solutions. Cooling reverses the reaction. Because teUurium forms solutions of anions, Te , and cations, Te" ", teUurium films can be deposited on inert electrodes of either sign. 

Thiosulfates. The ammonium, alkaU metal, and aLkaline-earth thiosulfates are soluble in water. Neutral or slightly alkaline solutions containing excess base or the corresponding sulfite are more stable than acid solutions. Thiosulfate solutions of other metal ions can be prepared, but their stabiUty depends on the presence of excess thiosulfate, the formation of complexes, and the prevention of insoluble sulfide precipitates. 

Chloride. Chloride is common in freshwater because almost all chloride salts are very soluble in water. Its concentration is generally lO " to 10 M. Chloride can be titrated with mercuric nitrate. Diphenylcarbazone, which forms a purple complex with the excess mercuric ions at pH 2.3—2.8, is used as the indicator. The pH should be controlled to 0.1 pH unit. Bromide and iodide are the principal interferences, whereas chromate, ferric, and sulfite ions interfere at levels greater than 10 mg/L. Chloride can also be deterrnined by a colorimetric method based on the displacement of thiocyanate ion from mercuric thiocyanate by chloride ion. The Hberated SCN reacts with ferric ion to form the colored complex of ferric thiocyanate. The method is suitable for chloride concentrations from 10 to 10 M. 

In the dual or double alkaU process, an alkaU salt that is considerably more soluble ia water than limestone is used. The alkaU salt is then regenerated usiag a second alkaU, CaCO. There are several alkaUes used ia the absorber the most common are magnesium sulfite, sodium sulfite, and ammonium sulfite. A typical process usiag magnesium sulfite iSjAbsorption... 

Compare the solubilities in water of calcium carbonate, calcium sulfite, calcium sulfate, magnesium sulfate, and dolomite. 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

Figure 10-6 continues this pictorial presentation of solubilities. Figure 10-6A shows the positive ions that form hydroxides of low solubility. Figure 10-6B shows the positive ions that have low solubility when combined with phosphate ion, PO 3, carbonate ion, CO 2, and sulfite ion,... 

Modern purification methods employ Na sulfite solns to react with the 0 and 7 -TNT isomers to form water soluble substances which can then be washed out of the desired a-TNT-However, these water washes form a blood-red soln — the bothersome red-water of TNT plants. Modern methods of disposing of red-water will be described in Section X... 

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