Solubility of precipitates

26 SOLUBILITY OF PRECIPITATES A large number of reactions employed in qualitative inorganic analysis involve the formation of precipitates. A precipitate is a substance which separates as a solid phase out of the solution. The precipitate may be crystalline or colloidal, and can be removed from the solution by filtration or by centrifuging. A precipitate is formed if the solution becomes oversaturated with the particular substance. The solubility (5) of a precipitate is by definition equal to the molar concentration of the saturated solution. Solubility depends on various circumstances, like temperature, pressure, concentration of other materials in the solution, and on the composition of the solvent. 

The variation of solubility with pressure has little practical importance in 

The variation of solubility with the composition of the solvent has some importance in inorganic qualitative analysis. Though most of the tests are carried out in aqueous solutions, in some cases it is advantageous to apply other substances (like alcohols, ethers, etc.) as solvents. The separation of alkali metals can for example be achieved by the selective extraction of their salts by various solvents. In other cases the reagent used in the test is dissolved in a non-aqueous solvent, and the addition of the reagent to the test solution in fact changes the composition of the medium. 

Broadly considered, solubilities depend in part on nonspecific electrolyte effects and in part on specific effects. The nonspecific effects can be considered in terms of activity coefficients (Chapter 2). But activity-coefficient effects often are negligible compared with the uncertainties arising from disregarded or unknown side reactions and also with uncertainties arising from the crystalline state, the state of hydration, the extent of aging of the precipitate, and intrinsic solubility, all of which may contribute to the solubility of the precipitate. To the extent that each can be identified and measured, each can be accounted for. Nevertheless, the magnitude of unsuspected effects makes it expedient to assume activity coefficients of unity unless otherwise specifically indicated for relatively soluble salts or solutions containing moderate amounts of electrolytes. 

In this chapter the specific chemical and physical factors affecting the solubility of a precipitate are examined. Solubilities of precipitates are considered first, since an understanding of this aspect is basic to their properties generally. In Chapters 8 and 9, other properties of precipitates are described— their mode of formation, physical properties, and chemical purity. 

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Failure of reactions to proceed to completion, 1. Significant solubility of precipitates,... 

According to Trommel [15b] the cellulose dinitrate chains become more irregular after being dissolved in acetone and precipitated with water. Hence the solubility of precipitated cellulose dinitrate is increased [15c]. The proposed relationship between the solubility and regularity of the structure of nitrocellulose has been confirmed by experiments with nitrocellulose at different stages of swelling and... 

The effect of foreign ions on the solubility of precipitates is just the opposite the solubility increases slightly in the presence of foreign ions. 

To explain the effect of foreign ions on the solubility of precipitates, one has to bear in mind that the solubility product relation, in the strictest sense, has to be expressed in terms of activities. For the saturated solution of the electrolyte AVaBvb, which ionizes into vAAm+ and vbBb ions... 

Many cations of heavy metals act as acids and undergo hydrolysis, with a resultant increase in the solubility of precipitates involving these metals. 

Figure 7.19. Solubility of CdS (greenockite) as a function of total sulfide at various pH values. The solubility is governed in the near neutral and alkaline pH range by Cd complexes with sulfur ligands. Minimum solubility is achieved in mildly acidic solutions. The solubility of precipitated CdS(s) may be larger by one or two orders of magnitude than that of greenockite. In seawater competition of complex formation with Cl needs to be considered. The lines are calculated (Tableau 7.8) with the help of stability constants determined by Daskalakis and Hclz (1992).

The use of the systematic method is illustrated in the sections that follow with examples involving the solubility of precipitates under various conditions. In later chapters, we apply this method to other types of equilibria. 

The solubility of precipitates containing an anion with basic properties, a cation with acidic properties, or both will depend on pH. 

Concentration of 1.25 X 1Q 2 mol/dm3 Aqueous Silica. At this concentration the precipitation edge for silica is at pH 10.0 this result is confirmed by the blank run in Figure 9, as was the limiting solubility of precipitated silica of ca. 2.5 X 10-3 mol/dm3 at pH values less than 9. In the presence of titania, precipitation losses still dominate, although some extraction of silica occurs above the pH of precipitation, and the maximum in adsorption at pH ca. 9 is still evident. 

In Chapter 6 we defined the thermodynamic equilibrium constant written in terms of activities to account for the effects of inert electrolytes on equilibria. The presence of diverse salts will generally increase the solubility of precipitates due to the shielding of the dissociated ion species. (Their activity is decreased.) Consider the solubility of AgCl. The thermodynamic solubility product K p is... 

Diverse salts increase the solubility of precipitates and have more effect on precipitates with multiply charged ions. 

Before describing precipitation titrations, we shall consider the effects of competing equilibria on the solubility of precipitates. Before you read any further, you might want to review the discussion of polyprotic acid equilibria and the calculation of a s, the fractions of each acid species in equdibrium at a given pH, in Chapter 7. 

Figure A-1 Solubility of precipitated nickel carbonate. Experimental data [38SMU] solid curve experimental data fitted to logn, + BIT + C lnF dotted curve ...

The solubility of precipitated silver iodide in solutions of potassium cyanide at 25 °C was determined by Randall and Halford (J. Amer. Chem. Soc. 1930, 52, 187). The solutions were made up to 0.02 M in sodium hydroxide to prevent the formation of hydrogen cyanide. After several days, samples were acidified with nitric add, heated to remove hydrogen cyanide and to predpitate silver iodide which was collected on a filter, dried, and wdghed. Tj-pical results are given in table 1 where a denotes the stoichdometric concentration of potassium cyanide and b denotes that of dissolved silver iodide. 

As the name implies, a conditional solubility product is one whose value depends on solution conditions other than the presence of a common ion. Conditional solubility products are useful when determining the solubility of precipitates whose ions interact with solution constituents through reactions such as complexation or hydrolysis. This type of behavior is indeed the rule rather than the exception for the systems encountered in natural waters and treatment processes. Also, in many analytical measurements the total concentration of species is determined rather than the free ion concentration. 

The pH value influences the solubility of precipitates of hydroxides of metals, for which the coefficient a is >1. Using the concept of side reaction coefficients the conditional solubility products, fC o, can be calculated ... 

Man] Mandry, R, Domelas, W., Solubility of Precipitates ofNb in Austenite in the Case of low Construction Steels Containing Very Quantities of Nb (in French), Compt. Rend. Acad. Sci. Paris, Ser. C, 263C, 1118-1121 (1966) (Crys. Structure, Experimental, Phase Relations, Phys. Prop., 7)... 

Besides a large variety of metals, several chemical species of specific interest, e.g., cyclic. 3, 5-adenosine monophosphate, thyroxine and 3.5,3 -trioiodothyronine have been determined by sub- and superequivalence IDA and the results show higher accuracy of the method over that of radioimmunoassay [102], [103]. The principle of the method also allows the determination of certain physicochemical characteristics, e.g.. estimation of the stability constants of complexes and the solubility of precipitates. 

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