Nature of Dissolved Species

In the formation of a solution the attractions among the particles in the original phases (solvent-to-solvent and solute-to-solute attractions) are broken up and replaced, at least in part, by new solvent-to-solute attractions. Unlike a compound, a solution has its components present in variable proportions and cannot be represented by a chemical formula. Equations for dissolution reactions do not include the solvent as a reactant. They indicate the original state of the solute in parentheses on the left side of the equation and identify the solvent in parentheses on the right side. For example, solid (s) sucrose dissolves in water to give an aqueous (aq) solution of sucrose  

Although the solute and solvent can be any combination of solid, liquid, and gas phases, liquid water is indisputably the best known and most important solvent. Consequently, we emphasize aqueous solutions in this chapter, but you should always remember that dissolution also occurs in many other solvents. We describe formation of aqueous solutions by considering the intermolecular forces between the solute and water molecules. Because these forces can be quite different for molecular solutes and ionic solutes, we discuss these two cases separately. 

FIGURE 11.2 A molecule of fructose In aqueous solution. Note the attractions between the hydroxyl (O —H) groups of the fructose and molecules of water. The fructose molecule Is equated the exact number and arrangement of the attached water molecules fluctuate. Also shown Is one hydrogen bond between a water molecule and an oxygen atom In the fructose ring. 

FIGURE 11.3 When an Ionic solid (In this case, K2SO4) dissolves In water, the Ions move away from their sites In the solid, where they were attracted strongly by Ions of opposite electrical charge. New strong attractions replace those lost as each Ion Is surrounded by a group of water molecules. In a precipitation reaction, the process Is reversed. 

Potassium sulfate is an ionic solid that dissolves in water up to 120 g at 25°C this maximum mass that can be dissolved in 1 L at 25°C is called the solubility in water. The chemical equation for this dissolution reaction is written as 

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In addition to the procedures and precautions required for aqueous systems, others inherent to non-aqueous systems must be observed. These are often ignored by the research worker unfamiliar with the nature of organic solvent systems. They can be divided into five categories (1) solvent purity, (2) solubility of solutes and nature of dissolved species, (3) rate of dissolution of solutes, (4) solvent vapour pressure and (5) measurements in dilute solution. While these same items arise in work with aqueous solutions, they do not occur with such frequency and cause such practical difficulties as with non-aqueous solvents. 

It does so by making a specific assumption about the nature of the species H+(aq). It is considered to have the molecular formula H30+(ag). Thus, when HC1 dissolves in water, the reaction is written... 

The amphiphilic nature of dissolved humic substances (DHS) lends them the ability to associate with both hydrophobic organics and polar or ionic species [62-64]. Inorganic ions or mineral colloids in solution will interact with the electrically active surface of humic material in solution or in the solid phase according to the same bonding forces which lead to the association between SP0M and the solid mineral matrix. Humic matter in water is associated with... 

Clusters of dissolved species combine to form unit cells. To form si coordination numbers of 20 and 24 are required for 512 and 51262 cavities, while sll requires coordination numbers of 20 and 28 for the 512 and 51264 cavities. Nucleation is facilitated if labile clusters are available with both types of coordination numbers for either si (e.g., CH4 + CO2 mixtures) or sll(e.g., CH4+C3H8 or most unprocessed natural gases). If the liquid phase has clusters of only one coordination number, nucleation is inhibited until the clusters can transform to the other size, by making and breaking hydrogen bonds. 

V2O5 is slightly soluble in water, producing a pale yellow, acidic solution that readily becomes colloidal, hi alkalis, it dissolves to form colorless solutions, the nature of the species present depending upon the pH. At high pH, the orthovanadate ion [V04] is formed, and at intermediate... 

Table 2.1. 1 The sources of major ions to rivers The average concentration of dissolved species in rivers is naturally somewhat uncertain. ...

Such a solution is said to be saturated. Saturation occurs at very low concentrations of dissolved species for slightly soluble substances and at high concentrations for very soluble substances. When imperfect crystals are placed in saturated solutions of their ions, surface defects on the crystals are slowly patched with no net increase in mass of the solid. Often, after some time has passed, we see fewer but larger crystals. These observations provide evidence of the dynamic nature of the solubility equilibrium. After equilibrium is established, no more solid dissolves without the simultaneous crystallization of an equal mass of dissolved ions. 

Beer s law generally holds good over a wide range of concentration if the structure of the coloured non-electrolyte in the dissolved state does not change with concentration. Small amount of electrolytes, which do not react chemically with the coloured components, do not usually affect the light absorption, large amounts of electrolytes may result in a shift of the maximum absorption and may also change the value of extinction coefficient. Discrepancies are normally observed when the coloured solute ionises, dissociates or associates in solution as because the nature of the species in solution will vary with the concentration. The law also fails if the... 

Many different complexes of elements in a given oxidation state may exist in water. The amphoteric nature of Al(III) and Fe(III) results from the formation of a series of dissolved species, MOH, M(OH)J, M(0H)3, and other forms, in addition to the more common M ". The speciation of soluble A1 and Fe is thus a sensitive function of pH. 

A Ithough it is the most abundant of the metallic elements in the outer crust of the earth, aluminum usually occurs in natural waters in concentrations below 100 micrograms per liter. High concentrations occur rarely and usually are associated with water having a low pH. The chemical properties of aluminum which control its behavior in water have been studied extensively. This paper is based on current research by the U.S. Geological Survey and on published literature. The principal topics considered here are the processes by which aluminum combines with hydroxide ions to form complexes and polymers, the influence of these processes on solubility of aluminum and the forms of dissolved species to be expected in natural water, and the relative importance of fluoride and sulfate complexes of aluminum. The experimental work is briefly summarized here. Details are published elsewhere (6). 

We also know that analytes ionise at pH values above or below the pKa, depending on the nature of the species of interest (i.e., acidic or basic). Our choice of column, therefore, is dependent on the ionised state of the analytes at the time of analysis. Amphetamine has a weak base with a pKa of 10.1. In order to retain the analyte in the non-ionised form, the pH of the mobile phase would need to be approximately 11 or 12 (see Figure 6.5). This would provide an ideal condition for separation because there would be maximum retention of the hydrophobic amphetamine. However, in reality, traditional silica-based columns are not stable at such extremes of pH, so it is not practical to operate above pH 11. The silanol linkages are cleaved below pH 2 and the silica may dissolve at pH levels greater than pH 8. 

When we adopt the idea that water was carried by icy ammonia hydrate bodies to the earth not only at the very beginning of the earth s formation around 4.6 Gy ago but also during the LHB 4 Gyr ago - when the oceans had already been recycled by the hot surface together with evaporation of dissolved species - there was competition between NH3 photolysis, an irreversible transformation process into N2 (no abiotic process is known on earth that produces NH3 and CH4 under natural conditions), and NH3 scavenging by rain. It also remains open to speculation how much of the ammonia was probably produced from nitrides. 

Figure 1.1 shows chemical equilibrium model of the natural system. Variables which determine the thermochemical feature of this system include temperature, total pressure, activities of dissolved species in aqueous solution (ions, ion pairs, complexes etc.), gaseous fugacity, activities of components in solid phases and dissolved species in aqueous solution where activity of i species, is equal to Yi mj (mi is molality of i species in aqueous solution and mole fractiOTi of i component in solid solution and yi is activity coefficient of i species in aqueous solution and of each component in solid phase). 

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