Precipitative methods

The focus of this article is on the preparation of stable isolated particles and methods suitable for the preparation of such materials will be emphasised. A sound synthetic method will lead to crystalline nanoparticulates of high purity, with a narrow size distribution, that are suitably surface derivatized. 

This type of method was used by Brus and co-workers in studies on CdS and ZnS [12, 13], which have led to the explosion of interest in solid samples of semiconductors which show quantum confinement effects. They [12] prepared CdS nanoparticles by a process involving the controlled nucleation of CdS in dilute aqueous solutions of cadmium sulfate and ammonium sulfide. The dynamic equilibrium between solvated ions and solid CdS in acetonitrile, as a solvent in the presence of a styrene or maleic anhydride copolymer, allowed the preparation of 

Henglein and Weller made significant progress [14-16] using CdS colloids prepared by controlled predpitation methods [17-22]. However, to obtain highly monodispersed nanopartides, post-preparative separation techniques such as size exdusion chromatography [20] and gel electrophoresis have been employed [21]. Gel electrophoresis was found to be superior to other separation techniques. 

The relative solubility of inorganic salts can be used to prepare more complex structures by such methods and examples indude CdS/ZnS [24], CdSe/AgS [25] HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe [31] particles. The main constraints on the production of such structures involve the relative solubility of the solids and lattice mismatches between the phases. The preparation of quantum dot quantum well systems such as CdS/HgS/CdS [32, 33], has also been reported, in which a HgS quantum well of 1-3 monolayers is capped by 1-5 monolayers of CdS. The synthesis grows less soluble HgS on CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are 5 X 10 and 1.6 x 10 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower energy values with increasing thickness of the HgS well [33]. 

Other examples of the preparation of semiconductor nanocrystallites by solution methods can be found in the literature [1-3]. Solution methods provide a cheap route to many nanoparticle materials. However, a lack of reaction control can be problematic when larger scale preparations are necessary. Also several important semiconductors are not easily obtained by this preparative method, with some being air and/or moisture sensitive, e.g. GaAs and InSb. 

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Organic Analysis Several organic functional groups or heteroatoms can be determined using gravimetric precipitation methods examples are outlined in Table 8.5. Note that the procedures for the alkoxy and alkimide functional groups are examples of indirect analyses. 

In the hot precipitation method, sodium carbonate solution is added slowly to a refluxing solution of mercuric chloride, followed by an additional reflux period of 1 to 2 h. The washed precipitate is then dried. A variation allows the substitution of mercuric nitrate for the chloride if substantial quantities of sodium chloride are used. Sodium hydroxide, ia the presence of sodium carbonate, is the precipitant. 

Color None Decaying organic material and metallic ions causing color may cause foaming in boilers hinders precipitation methods such as iron removal, hot phosphate softening can stain product in process use Coagulation, filtration, chlorination, adsorption by activated carbon... 

FUll-methode, /, precipitation method, -mittel, n. precipitating agent, precipitant. 

The separation of the element or of the compound containing it may be effected in a number of ways, the most important of which are (a) precipitation methods (b) volatilisation or evolution methods (c) electroanalytical methods and (d) extraction and chromatographic methods. Only (a) and (b) will be discussed in this chapter (c) is considered in Part E, and (d) in Part C. 

Diverse thermogravimetric results can be obtained from samples with different pre-histories for example, TG and DTG curves showed that magnesium hydroxide prepared by precipitation methods has a different temperature of decomposition from that for the naturally occurring material.32 It follows that the source and/or the method of formation of the sample should be ascertained. 

QUANTITATIVE SEPARATIONS BASED UPON PRECIPITATION METHODS... 

Precipitation methods 418 Precipitation reactions 340 theory of, 340, 342, 579 Precision 13, 129 Preparation for analysis 109 solution of sample, 110 Preventive solution 368 Primary amines see Amines Primary standard substances requirements of, 261... 

Delmas and his co-workers have done extensive work on pyroaurite-type materials which has recently been reviewed [73], In addition to precipitation methods, they have prepared the materials by mild oxidative hydrolysis of nickelates that were prepared by thermal methods similar to those used for the preparation of LiNiOz [74]. A cobalt-substituted material NaCoA ( Ni( A02) was prepared by the reaction of Na20, Co304 and NiO at 800 °C under a stream of oxygen. The material was then treated with a 10 molL-1 NaCIO +4 molL 1 KOH solution for 15h to form the oxidized y -oxyhydroxide. The pyroau-... 

A ferrite waste treatment process is being investigated to determine if it can more effectively remove actinides from waste solution with less solid waste generation than the flocculant precipitation method presently used (18). 

It was reported that various catalysts and reductants have been utilized in DSRP for the SO2 reduction [1-5]. In this study, appropriate catalysts were developed for DSRP. The Sn02-Zr02 catalyst was prepared by co-precipitation method and used for the catalysts in DSRP. The characteristics of SO2 reduction were investigated with CO as a reducing gas. 

Sn02 and Zr02 catalysts were prepared by a precipitation method, while Sn02-Zr02 catalysts with Sn/Zr molar ratios corresponding to 2/8, 3/5, 5/5 and 2/1 were prepared by a copreparation method. 

CO oxidation over Au/CeOi prepared by a co-precipitation method... 

The precipitation method of separation involves the addition of salts such as ammonium sulfate or solvents such as polyethylene glycol to the reagent mixture to cause precipitation of the large molecular weight bound species. These methods of precipitation lack specificity and work well only when there is a large difference between the molecular weight of the material being measured and that of the bound complex of it. 

Brubaker and Mickel later reported results, obtained in a more detailed study for sulphate media of ionic strength of 3.68 M with the species Tl(III), T1(I), and S04 in the concentration ranges 1.8x10 to 1.08x10 M, 1.0x10 to 2.0X 10 M, 4.5 X 10 to 2.90 M and 7 x 10 to 7x 10 M, respectively. The chromate precipitation method was used. The experimental data at 24.9 °C were found to be consistent with a detailed rate equation... 

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

The activity of the Au/metal oxide catalysts is extremely sensitive to the method of preparation. The Au/metal oxide catalysts were prepared by the co-precipitating method [1]. During the course of this study, we have determined that the activity and the stability of the catalyst for room temperature CO oxidation were a function of Ph of the solution, temperature of precipitation, aging temperature and time, catalyst wash procedure, and calcination. 

The activity of gold catalyst is normally strongly size dependent and the control as well as the narrowest possible distribution of particle size represent the main goal for the production of an active gold catalyst. From a catalytic point of view, several preparation methods have been proposed for obtaining highly dispersed gold catalyst, most of them derived from deposition-precipitation method proposed by Haruta et al. [3]. 

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