Precipitation with metal ions

The mobility of arsenic compounds in soils is affected by sorp-tion/desorption on/from soil components or co-precipitation with metal ions. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of arsenic in natural environments has been studied for a long time (Livesey and Huang 1981 Frankenberger 2002 and references there in Smedley and Kinniburgh 2002). Because the elements which correlate best with arsenic in soils and sediments are iron, aluminum and manganese, the use of Fe salts (as well as Al and Mn salts) is a common practice in water treatment for the removal of arsenic. The coprecipitation of arsenic with ferric or aluminum hydroxide has been a practical and effective technique to remove this toxic element from polluted waters... 

Hydroxyl ions tend to form far more stable complexes or precipitates with metal ions (Fe"", Mn and SP ) considered present in... 

Fractionation of the material is recommended because certain substances interfere with some detection methods. Percolation with petroleum ether removes lipophilic substances. Proteins and mucilages can be removed by precipitation with metal ions or 5% (w/v) trichloroacetic acid (TCA). Discoloration of biological materials is achieved with charcoal. Owing to the insolubility of benzylidine derivatives of sugar alcohols in water, they can easily be crystallized and purified from adhering matter by washing with water. 

In type A reaction soluble sulfide ions combine with metal ions to form a precipitate of insoluble metal sulfide. Sulfur s oxidation state of minus two does not change in this reaction. The reaction is... 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

Anthocyanins can form complexes with metal ions such as tin, iron and aluminium. The formation of a complex, as expected, alters the colour, usually from red to blue. Complex formation can be minimised by adding a chelating agent such as citrate ions. Another problem with anthocyanins is the formation of complexes with proteins. This can lead to precipitation in extreme cases. This problem is normally minimised by careful selection of the anthocyanin. 

FA can interact with clay minerals and are known to form stable complexes with metal ions and hydrous oxides [59,61]. The operational technique for isolation of HA involves a pH-induced precipitation and it is likely that accessory minerals may be associated with the precipitation process. Complexes of HA and clay minerals are also formed, the increased ash content of HA suggesting that amorphous silica, iron hydroxides, and clay may aggregate with the HA fraction [58,60,61]. 

For hydroxamic acids, it is generally assumed that it is the Af-hydroxyamide/keto form, as opposed to the hydroximic/hydroxyoxime form, that predominates in acid medium, the environment usually required for most precipitates or colors to form . It is in general unknown what is the stoichiometry and structure of most metal hydroxamate complexes in solution. Nevertheless, the reaction of the majority of hydroxamic acids with metal ions can be written schematically as shown in equation 2. 

Anions such as sulphate and phosphate form involatile salts with metal ions and reduce the reading of the sample solution. These anions may be removed by the addition of lanthanum chloride which precipitates them out and replaces them with the chloride anion. 

The authors assumed that the precipitation of these metal sulfides was controlled by the hydrolysis reaction of TAA promoted by proton in the acidic conditions. However, the hydrolysis of TAA observed in acidic and alkaline ranges is a much slower process than observed in the precipitation of these metal sulfides (7-12), and it may not be accelerated by consumption of S2 ions because of its irreversible nature. In addition, the reaction virtually finished with a great part of the starting metal ions and TAA left unreacted. Also, it has already been verified that the probability of direct reaction of TAA with metal ions is zero or at least negligible from its strong dependence of pH in reactivity (7). Thus, it seems reasonable to consider that the main path is the release of S2 ions from TAA according to the following reaction scheme with the production of acetonitrile (7,13) as found in the reaction of TAA with Cd2+ ions in an alkaline media ... 

When undesirable metal ions generally need to be removed from a solution without disturbing the other metal ions present, the process of selective precipitation comes into play. This step involves the precipitation of metal ion by using another solution whose anion forms an insoluble salt with the undesirable ion in the mixture. For example, if a NaCl solution is added to a solution containing both Ag" " and Mg2+ ions, AgCl will be precipitated as a white solid. Since MgCl2 is soluble, Mg2+ ions will remain in the solution. 

A method based on a metal chelate affinity chromatography (MCAC) was described by Carson (24). Generally, it consists of extraction/precipitation with succinate buffer followed by cleanup on a Chelating Sepharose column preloaded with copper(II) sulphate when TCs are specifically adsorbed by chelatation with metal ions bound. Elution of TCs was achieved using EDTA-containing buffer. 

The influence of the dose was quite low (see Figure 8.8), and the efficiency of the solvent for the extraction of actinides (III, IV, and VI) and back-extraction of divalent cations was maintained up to 0.6-0.7 MGy. No precipitation was observed after contact of radiolyzed solutions with metallic ions. 

Bipolar membrane synthesis also holds promise for regenerating spent pickling liquors in stainless steel manufacture. As shown in Fig. 43(b), waste acid laden with metal ions can be continuously neutralized, filtered to remove the precipitated metal oxide, and the clarified salt solution split into its acid and base components in a bipolar membrane unit [Type (IV) shown in Table IX], As much as 95% of the hydrofluoric and nitric acid used are returned to the pickling bath, thereby solving a waste... 

Effecting deposition-precipitation by decreasing the pH level is interesting with metal ions present in the stable state in aqueous solution as anions [35]. With silica no interaction is observed, which has led to the development of the electrochemical reduction procedure. To apply metal ions, such as, molybdenum or vanadium, on alumina, a homogeneous decrease in pH level is interesting. The pH level has been decreased by injection of nitric acid or perchloric acid and electro-chemically. However, the rate of crystallization of the hydrated oxides of vanadium(V) and molybdenum(VI) was observed to be fairly low. To prevent dissolution of the alumina supports the pH cannot be decreased to levels below about 3, at which the crystallization of the hydrated metal oxides does not proceed rapidly. 

Hypophosphites are all soluble in water and therefore the solutions give no precipitates with the ions of the alkaline earths, silver, etc. Silver salts, however, are rapidly reduced to metallic silver, and phosphoric acid is found in the solution —... 

Summary Ammonium chlorate is prepared in a similar manner as other inorganic ammonium salts, by the precipitation of metal ions with carbon dioxide. In thiscase, sodium chlorate istreated with ammonia, and carbon dioxide isthen passed there into. The carbon dioxide reacts with the water forming carbonic add. The carbonic add reacts with the sodium chlorate forming the water insoluble sodium bicarbonate. The ammonia molecules in solution take up an addic hydrogen from the carbonic add, and then unite with the chi orate ions forming ammonium chlorate in solution. The salt isthen easily recovered by recrystallization. 

Metal deactivator. Metal deactivator prevents precipitation of metal ion oxidation reactions and precipitation of insoluble metal compounds. Metal deactivator in combination with other antioxidants, shows strong synergistic effects. Oxygen and moisture present, diffuse through oil film and cause corrosion. Amine derivative, used in the additive has good water-displacing properties. They impede sludge formulation, disperse sediments and reduce corrosion in various fuel systems. 

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