Precipitation exchange reactions

If the precipitate is washed by decantation and dilute ammonia is added, no visible change occurs [difference from mercury(I) or silver ions], though a precipitate-exchange reaction takes place and lead hydroxide is formed ... 

Introducing hydrogen sulphide gas into a mixture which contains white lead chloride precipitate, the latter is converted into (black) lead sulphide in a precipitate-exchange reaction ... 

Carbonates. Basic zirconium carbonate [37356-18-6] is produced in a two-step process in which zirconium is precipitated as a basic sulfate from an oxychloride solution. The carbonate is formed by an exchange reaction between a water slurry of basic zirconium sulfate and sodium carbonate or ammonium carbonate at 80°C (203). The particulate product is easily filtered. Freshly precipitated zirconium hydroxide, dispersed in water under carbon dioxide in a pressure vessel at ca 200—300 kPa (2—3 atm), absorbs carbon dioxide to form the basic zirconium carbonate (204). Washed free of other anions, it can be dissolved in organic acids such as lactic, acetic, citric, oxaUc, and tartaric to form zirconium oxy salts of these acids. 

The exchange reaction is exothermic and too rapid an addition of the iodide will cause a temperature rise that may lead to the formation of unwanted side-products. A white precipitate of Lil is observed when approximately two-thirds of the iodide solution has been added. 

Wolfgang and Dodson have since made a study of the exchange reaction between Hg and HgCN using ° Hg and lOJ precipitation of the species They have proposed a mechanism involving the steps... 

Dodson et al. ° have also studied in detail the effect of added bromide on the exchange reaction using precipitation and extraction separations and ° T1 and ° T1 as indicators. The variation in the rate of exchange was found to be governed by the law... 

Bonner, using the isotopic method ( Sb) and either precipitation of the oxinate of Sb(III) or an isopropyl ether extraction of Sb(V), obtained the first results on this exchange reaction. In media 6 M HCl a complex rate law, viz. 

Eichler and Wahl have attempted an isotopic study ( Os and Os) of the exchange reaction between Os(dipy)3 and Os(dipy)3 using a direct injection technique so that reaction times 7 x 10 sec were possible. With total osmium 10" M in aqueous sulphate media at 0 °C complete exchange was observed. The separation methods used were, (a) perchlorate precipitation (in presence of iron(II) carrier) and (6) extraction with p-toluenesulphonic acid in nitromethane, of the osmium(II) complex. A lower limit of 1 x 10 l.mole. sec was placed on the rate coefficient (0 °C, 3.0 M H2SO4). Dietrich and Wahl using the line broadening effect produced by Os(dipy)3 on the nmr spectrum of Os(dipy)3 have been able to propose a value of > 5x 10" l.mole . sec at 6 °C in D2O (0.14 M [Cr] and 5x10 M [D- ]). 

The earliest investigation of the exchange reaction between the aquated ions of Co(III) and Co(II) was carried out by Hoshowsky et al., using the isotopic method ( Co). When sulphate salts ( 10 M) were employed, complete exchange was observed between the two oxidation states of cobalt, in a time of less than two min. Two separation methods were employed (a) adsorption on an alumina column, and (b) precipitation of the Co(III) as the cobaltinitrite. 

Baker et al, have obtained some kinetic data on the exchange reactions between the 1,10-phenanthroline, 2,2 -dipyridine and 2,2, 2"-tripyridine complexes of Co(III) and Co(ll) in both H2O and D2O solution. The isotopic method ( Co) and separations involving either extraction of Co(II) with organic solvents (n-hexanol-ether or saturated sodium acetate in n-hexanol) or precipitation of Co(III) as Co(phen)3(l3)3 were used. For the 1,10-phenanthroline system, a rate law... 

Polyion complex technique [40] is a unique method for immobilization of bilayer membranes with polymers. Water-insoluble complex is precipitated as the polyion complex when an aqueous solution of the charged bilayer membrane is mixed with a water solution of the counter charged polyelectrolyte. Stoichiometric ion pair formation is often found. Aging of the precipitate in a hot mixture kept above phase transition temperature of the bilayer membrane completes the ion exchange reaction [41], Chloroform solution of the polyion complex is washed by water several times to remove water soluble components [42]. 

The solutions of the metal compounds M3X7 can be used (a) for a study of redox-reactions under mild conditions, and (b) to serve as aducts for cation exchange reactions. Thus KaaPv reacts with phi PCl or phi AsCl accompanied by quantitative precipitation of KaCl, to give solutions of salts without metal cations. The least soluble materials which crystallize out of these solutions are the compounds (phi P)2Pie or (ph As)2Pie They contain the new polyanion Pie (Figure 5)> which is formed in the solution by a complex redox reaction (J 8). 

Thus a quantitative ion-exchange reaction was confirmed to take place by a simple precipitation of a THF solution of lc and l c into aqueous solution containing a benzoate anion, and the subsequent ring-opening reaction of the pyrrolidinium salt groups was found to proceed by the heating treatment at 100°C. 

The most widely used method for nucleophilic iodination is the Cu+-assisted method, which was first described by Mertens et al [5]. This method can be used in a bromine for radioiodine or an iodine for radioiodine exchange reaction, and results in a regiospecific substitution (scheme 3). A limitation of this labelling method is that it has to be performed in water. If the substrate however is insoluble in water, a mixture of 10% ethanol in water is also possible, higher concentrations of ethanol lead to the precipitation of reagents. It is therefore difficult to iodinate lipophilic compounds with this method. 

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