Precipitation of metals

Addition of excess H ions to this solution will cause the equilibrium to move towards undissociated acid thereby decreasing the concentration of Ac . This effect is known as the common-ion effect and is of considerable practical importance. Thus, e.g. in the precipitation of metal ions as insoluble sulphides. 

Reduction of ammoniacal silver nitrate. Add a few drops of a neutral solution of a formate to ammoniacal AgNO (see Test 4, p. 342). A silver mirror or more usually a grey precipitate of metallic sih er is produced on boiling. 

Oxidation, (a) Ammoniacal silver nitrate. To a few ml. of ammoniacal AgNOj (preparation, p. 525), add a few drops of cold aqueous benzo quinone solution a silver mirror or (more generally) a dark precipitate of metallic silver is formed in the cold. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Gravimetric methods more suitable for general use involve the precipitation of metallic gold from tetrachloraurate solutions by reduction with oxaUc acid, SO2, or hydroquinone. Formaldehyde, hydrazine, ferrous sulfate, and hypophosphorous acid also have been used but are considered less efficient (40). 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

Cobalt in steel Discussion. An alternative, but less sensitive, method utilises 2-nitroso-l-naphthol, and this can be used for the determination of cobalt in steel. The pink cobalt(III) complex is formed in a citrate medium at pH 2.5-5. Citrate serves as a buffer, prevents the precipitation of metallic hydroxides, and complexes iron(III) so that it does not form an extractable nitrosonaphtholate complex. The cobalt complex forms slowly (ca 30 minutes) and is extracted with chloroform. 

Add cautiously (FUME CUPBOARD), and with constant stirring, 10 mL of the ammonium citrate solution this will prevent the precipitation of metals when, at a later stage, the pH value of the solution is increased. Then add 10 mL of the 20 percent ascorbic acid, and adjust to pH 4(BDH narrow-range indicator paper), by the cautious addition of concentrated ammonia solution down the side of the beaker while stirring continuously. Then add lOmL of the 50 per cent potassium cyanide solution (CAUTION ) and immediately adjust to a PH of 9-10 (BDH indicator paper) by the addition of concentrated ammonia solution. 

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. 

Fofanov GM, Kitaev GA (1969) Analysis of the conditions for the precipitation of metal selenides from aqueous solutions with sodium selenosulphate. Russ J Inorg Chem 14 322-324... 

The appearance of CO2 in the gas phase before precipitation of metal implies the production of a soluble Pd(0) complex such as [Pd(CO)2Bt2] . This receives support from the observation that three times as much CO is consumed as CO2 liberated before deposition of Pd. The following reactions may be involved... 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

The solubility product (JCsp) for the precipitation of metal sulfide (MmSn) from a solution containing m Mz+ and n S2 ions is... 

F. A. Schaufelberger, Precipitation of Metals from Solution by Reduction with Hydrogen, J. Metals, Vol. 8, p. 695, 1956. 

Most of the pollutants may be effectively removed by precipitation of metal hydroxides or carbonates using a reaction with lime, sodium hydroxide, or sodium carbonate. For some, improved removals are provided by the use of sodium sulfide or ferrous sulfide to precipitate the pollutants as sulfide compounds with very low solubilities. After soluble metals are precipitated as insoluble floes, one of the water-solid separators (such as dissolved air flotation, sedimentation, centrifugation, membrane filtration, and so on) can be used for floes removal.911 The effectiveness of pollutant removal by several different precipitation methods is summarized in Tables 5.15-5.17. 

The objective of this monograph is to include all major studies of metal ions in their aqueous solutions as well as some other important studies in their zerovalent metallic state or in alloys, since the nanoparticles of many of these metals have become too important. Besides, the study of the precipitation of metal ions in aqueous solutions, upon sonication, which has been carried out in our laboratory, would also be discussed. Some of such data include unpublished work. The sequence of metallic ions in this chapter are as they come in the sequence of wet chemical analysis of basic radicals, besides the cationic charge has been kept in mind to make groups and sequences that follow the detailed description. 

One successful technique involves the addition of urea to metal salts in aqueous solution under conditions of pH which facilitate the decomposition of the urea and precipitation of metal hydroxycarbonates.89-100 This has led, for example, to the preparation of spherical particles of Y203 Eu particles of 100, 200, 300, 400, and 500 nm dimensions having a size distribution that can be controlled to within 5%.92-97 Gd203 Eu has been prepared similarly, giving particles of sizes between 70 nm and 250 nm108... 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

Selective precipitation of metal ions using hydrogen- or sodium sulphides, hydroxides or carbonates... 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

Among the questions that arise are the following, at least, is the metal present as a sorbed/hydrated species , were the conditions of metal incorporation such that precipitation of metal-containing species occurred , if a metal-organic compound was present, what is the chemical form of the sorbed complex Answers to such questions have been provided in part from thermodynamic studies (1-4,9) and from a variety of spectroscopic techniques (10-12 ). 

---