Precipitating reagents properties

In coprecipitation, the type of precipitation reagent used also affects the acidic properties of the resulting binary oxide. An example is shown in Fig. 3.49 for Ti(32 - ZnO. One Ti02 — ZnO is prepared using urea as the precipitation reagent while another is prepared using aqueous ammonia. As urea is heated, it decomposes into ammonia and carbon dioxide ... 

The second principal advantage is that CYANEX 272 is the only one of the three above-mentioned compounds that extracts cobalt in preference to calcium (52). This property can minimise or eliminate the solvent losses that are associated with calcium extraction and the subsequent precipitation of gypsum cmds in the scmbbing or stripping circuits. This is illustrated in Eigure 1 where calcium extraction is shown as a function of pH for the three subject reagents. 

It was assumed throughout that the compound which separated out from the solution was chemically pure, but this is not always the case. The purity of the precipitate depends inter alia upon the substances present in solution both before and after the addition of the reagent, and also upon the exact experimental conditions of precipitation. In order to understand the influence of these and other factors, it will be necessary to give a short account of the properties of colloids. 

Urea possesses negligible basic properties (Kb = 1.5 x 10 l4), is soluble in water and its hydrolysis rate can be easily controlled. It hydrolyses rapidly at 90-100 °C, and hydrolysis can be quickly terminated at a desired pH by cooling the reaction mixture to room temperature. The use of a hydrolytic reagent alone does not result in the formation of a compact precipitate the physical character of the precipitate will be very much affected by the presence of certain anions. Thus in the precipitation of aluminium by the urea process, a dense precipitate is obtained in the presence of succinate, sulphate, formate, oxalate, and benzoate ions, but not in the presence of chloride, chlorate, perchlorate, nitrate, sulphate, chromate, and acetate ions. The preferred anion for the precipitation of aluminium is succinate. It would appear that the main function of the suitable anion is the formation of a basic salt which seems responsible for the production of a compact precipitate. The pH of the initial solution must be appropriately adjusted. 

From the practical point of view, reuse of the deposition solution after filtering out the precipitated CdS and addition of fresh reagent was shown to have no effect on the device properties of CdS/ClGS cells (see Sec. 4.1.6.10 for more details of the deposition) [11]. 

The fact that most triphosphates of polyvalent cations which are difficultly soluble in water are soluble in excess of triphosphate, and that such polyvalent cations (e.g. Ca++) are either not precipitated or not quantitatively precipitated by the usual reagents from solutions of their salts containing triphosphate, has become of great commercial importance in water softening (90). The term sequestration is used in this connection. Numerous publications have appeared which are not mentioned in detail here, and which attempt to determine this property quantitatively. In general, the effect is attributed to the formation of relatively stable ion pairs or complexes, the stability of which is defined by the formation constants K or K. ... 

Tin(II) and Tm(IV) Hydroxides. Prepare tin (I I) and tin(IV) hydroxides in separate test tubes from solutions of tin chlorides. What reagent should be used to precipitate the tin hydroxides Do tin hydroxides exhibit amphoteric properties What tin compounds are known as a- and P-stannic acids How are they prepared Write the equations of the reactions. 

The terchlorido is the usual and most convenient form of obtaining a solution of gold, and examining its properties in that state. The different reagents by which1 pulverulent gold may be precipitated from this solution, have been already mentioned. It is by this means that gold is obtained in a state of chemical purity. 

---