Aqueous solution precipitation reactions

Three major t)q)es of chemical processes occur in aqueous solution precipitation reactions, acid-base reactions, and oxidation-reduction reactions. 

As we discussed in Chapter 7, many chemical reactions take place in aqueous solutions. Precipitation reactions, neutralization reactions, and gas evolution reactions, for example, all occur in aqueous solutions. Chapter 8 describes how we use the coefficients in chemical equations as conversion factors between moles of reactants and moles of products in stoichiometric calculations. These conversion factors are often used to determine, for example, the amount of product obtained in a chemical reaction based on a given amount of reactant or the amount of one reactant needed to completely react with a given amount of another reactant. The general solution map for these kinds of calculations is ... 

In aqueous solution the reactions are those of NH and SO4 ions. For example, addition of barium chloride, BaCb precipitates out barium sulfate, BaS04. The filtrate on evaporation yields ammonium chloride, NH4CI. 

Fluorination with fluorine produces copper(ll) fluoride, CuF2. Adding potassium ferrocyanide to CuCb aqueous solution precipitates out reddish brown cupric ferrocyanide. Reaction with caustic soda forms blue cupric... 

In addition, at very low water contents, ampicillin accumulation curves do not exhibit a clear-cut maximum, inherent in the enzymatic acyl transfer reactions in aqueous medium (including quite concentrated heterogeneous aqueous solution-precipitate systems), because of the secondary hydrolysis of the target product by penicillin acylase (Figure 12.6) [84]. 

The addition of phosphorus trichloride to an aqueous solution precipitates brown amorphous arsenic the reaction does not occur if the trichloride is first dissolved in water, nor when phosphorous acid is used.5 The reaction, which accords with the equation... 

Naphthol (100 g., 0.70 mole) is dissolved in an aqueous solution containing 360 g. (6.3 moles) of potassium hydroxide. To this solution is added an aqueous solution of 95 g. (1.0 mole) of chloroacetic acid. Water is added to the mixture until a total volume of 2 1. is reached, and the resulting solution is stirred and heated on a water bath for 4 hours. The reaction mixture is treated with 100 ml. of concentrated hydrochloric acid and cooled the precipitated solid is filtered off, washed with water, and dried. There is obtained a 95 yield of crude product melting at 145-150 . The crude acid is treated with excess aqueous ammonium carbonate solution, and the resulting mixture is extracted with ether to remove the unreacted /8-naphthol. Acidification of the aqueous solution precipitates the yj-naphthoxy-acetic acid, which is collected by filtration and recrystallized from hot water to give white prisms melting at 156°. 

This arsenical is obtained by the Schotteii-Baumann reaction from benzene- n-disuIphonic chloride and Salvarsan. It is a yellow or yellowish-grey powder, yielding a yellowish-brown solution in sodium carbonate, but is insoluble in water. Addition of sodium chloride or alcohol to its aqueous solution precipitates the sodium salt, which has a neutral reaction in water and may be kept imchanged for several days if air is excluded. ... 

One common type of reaction that occnrs in aqneons solntion is the precipitation reaction, which results in the formation of an insoluble product, or precipitate. A precipitate is an insoluble solid that separates from the solution. Precipitation reactions usually involve ionic compounds. For example, when an aqneons solution of lead nitrate [Pb(N03)2l is added to an aqueous solution of sodinm iodide (Nal), a yellow precipitate of lead iodide (Pbl2) is formed ... 

Sulfonium salt monomer, I, is obtained by reacting a,a -dichloro-/)-xylene with excess tetrahydrothiophene at 50°C in methanol for 20 h. The monomer is purified by precipitation in cooled acetone. A polymerization reaction to form the water-soluble precursor polymer, II, is effected in aqueous solution by reaction of the monomer with an equimolar quantity of sodium hydroxide at 0 C for I h under rigorous anaerobic conditions. The reaction is quenched by neutralization with HCI solution. Polyelectrolyte should be separated from the residual monomers and low-... 

On heating, this compound begins to decompose at 225°C with evolution of gases smelling strongh of hydrogen cyanide. As in the case of formal-hydrazine, aqueous solutions precipitate formalazine on warming, a reaction which is accelerated by the presence of traces of acid. 

This is an acid-base reaction, in which the base is the oxide ion (p. 89) the acidic oxide SiOj displaces the weaker acidic oxide CO2 in the fused mixture. But in aqueous solution, where the 0 ion cannot function as a strong basefp. 89),carbon dioxide displaces silica, which, therefore, precipitates when the gas is passed through the aqueous silicate solution. In a fused mixture of silica and a nitrate or phosphate, the silica again displaces the weaker acidic oxides N2O5 and P4OJ0 ... 

This reaction proceeds slowly in aqueous solution, so that the basic salt. Sn(OH)Cl, is slowly precipitated. Addition of excess hydrochloric acid gives the acids of formulae HSnCl3 and H2SnCl4. Salts of these acids containing the ions SnCl J and SnCl (chloro-stannates(II)) are known. 

How ever, the Mn(II) ion forms a variety of complexes in solution, some of which may be more easily oxidised these complexes can be either tetrahedral, for example [MnClJ , or octahedral, for example [Mn(CN)f,] Addition of ammonia to an aqueous solution of a manganese(II) salt precipitates Mn(OH)2 reaction of ammonia with anhydrous manganese(II) salts can yield the ion [MnfNH y T... 

Copper(II) ions in aqueous solution are readily obtained from any copper-containing material. The reactions with (a) alkali (p. 430), (b) concentrated ammonia (p 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained ... 

Dissolve 0 01 g. equivalent of the amino acid in 0 03 g. equivalent of N sodium hydroxide solution and cool to 5° in a bath of ice. Add, with rapid stirring, 0 -01 g. equivalent of 2 4-dichlorophenoxyacetyl chloride dissolved in 5 ml. of dry benzene at such a rate (5-10 minutes) that the temperature of the mixture does not rise above 15° if the reaction mixture gels after the addition of the acid chloride, add water to thin it. Remove the ice bath and stir for 2-3 hours. Extract the resulting mixture with ether, and acidify the aqueous solution to Congo red with dilute hydrochloric acid. Collect the precipitate by filtration and recrystallise it from dilute alcohol. 

To hydrolyse an ester of a phenol (e.g., phenyl acetate), proceed as above but cool the alkaline reaction mixture and treat it with carbon dioxide until saturated (sohd carbon dioxide may also be used). Whether a solid phenol separates or not, remove it by extraction with ether. Acidify the aqueous bicarbonate solution with dilute sulphuric acid and isolate the acid as detailed for the ester of an alcohol. An alternative method, which is not so time-consuming, may be employed. Cool the alkaline reaction mixture in ice water, and add dilute sulphuric acid with stirring until the solution is acidic to Congo red paper and the acid, if aromatic or otherwise insoluble in the medium, commences to separate as a faint but permanent precipitate. Now add 5 per cent, sodium carbonate solution with vigorous stirring until the solution is alkaline to litmus paper and the precipitate redissolves completely. Remove the phenol by extraction with ether. Acidify the residual aqueous solution and investigate the organic acid as above. 

Aminothiazole is efficiently purified by treating an aqueous solution of 2-aminothiazole hydrochloride with H2SO4 at 25 to 105°C, filtering the precipitated reaction product and transforming it back to 2-aminothiazole... 

MetaUic ions are precipitated as their hydroxides from aqueous caustic solutions. The reactions of importance in chlor—alkali operations are removal of magnesium as Mg(OH)2 during primary purification and of other impurities for pollution control. Organic acids react with NaOH to form soluble salts. Saponification of esters to form the organic acid salt and an alcohol and internal coupling reactions involve NaOH, as exemplified by reaction with triglycerides to form soap and glycerol,... 

Rosin sizing usually involves the addition of dilute aqueous solutions or dispersions of rosin soap size and alum to a pulp slurry (44—46). Although beater addition of either coreactant is permissable, addition of both before final pulp refining is unwise because subsequently exposed ceUulose surfaces may not be properly sized. The size and alum should be added sufficiendy eady to provide uniform distribution in the slurry, and adequate time for the formation and retention of aluminum resinates, commonly referred to as size precipitate. Free rosin emulsion sizes, however, do not react to a significant degree with alum in the pulp slurry, and addition of a cationic starch or resin is recommended to maximize retention of size to fiber. Subsequent reaction with aluminum occurs principally in the machine drier sections (47). 

Analysis for Poly(Ethylene Oxide). Another special analytical method takes advantage of the fact that poly(ethylene oxide) forms a water-insoluble association compound with poly(acryhc acid). This reaction can be used in the analysis of the concentration of poly(ethylene oxide) in a dilute aqueous solution. Ereshly prepared poly(acryhc acid) is added to a solution of unknown poly(ethylene oxide) concentration. A precipitate forms, and its concentration can be measured turbidimetricaHy. Using appropriate caUbration standards, the precipitate concentration can then be converted to concentration of poly(ethylene oxide). The optimum resin concentration in the unknown sample is 0.2—0.4 ppm. Therefore, it is necessary to dilute more concentrated solutions to this range before analysis (97). Low concentrations of poly(ethylene oxide) in water may also be determined by viscometry (98) or by complexation with KI and then titration with Na2S202 (99). 

Aluminum iodide [7884-23-8] AIL, is a crystalline soHd with a melting poiat of 191°C. The presence of free iodine ia the anhydrous form causes the platelets to be yellow or brown. The specific gravity of this soHd is 3.98 at 25°C. Aluminum iodide hexahydrate [10090-53-6] AIL -6H20, and aluminum iodide pentadecahydrate [65016-30-0], AIL -15H20, are precipitated from aqueous solution. They may be prepared by the reaction of hydroiodic acid [10034-85-2], HI, with aluminum or aluminum hydroxide. 

Iron Reduction. The reduction of nitrophenols with iron filings or turnings takes place in weakly acidic solution or suspension (30). The aminophenol formed is converted to the water soluble sodium aminopheno1 ate by adding sodium hydroxide before the iron-iron oxide sludge is separated from the reaction mixture (31). Adjustment of the solution pH leads to the precipitation of aminophenols, a procedure performed in the absence of air because the salts are very susceptible to oxidation in aqueous solution. 

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