Role of Hydroxide on Metal Solubility

When a salt is introduced to water (e.g., A1C13s), the charged metal (Al3+) has a strong tendency to react with H20 or OH and forms various Al-hydroxy species. Metal-hydroxide reactions in solution exert two types of influences on metal-hydroxide solubility, depending on the quantity of hydroxyl supplied. They either decrease or increase metal solubility. The solubility of a particular metal-hydroxide mineral depends on its Ksp, quantity of available hydroxyl, and solution pH of zero net charge. For example, aluminum (Al3+) forms a number of hydroxy species in water as shown below  

Commonly, different metals exhibit different solution pH of zero net charge. For this reason, different metals exhibit minimum solubility at different pH values, which makes it difficult to precipitate effectively two or more metals, as metal-hydroxides, simultaneously. Thus metal-hydroxide solubility as a function of pH displays a U-shaped behavior. The lowest point in the U-shaped figure signifies the solution pH of zero net charge and is demonstrated below. Consider the solid Fe(OH)2s, 

Rearranging and substituting Equations 2.45 and 2.46 into Equation 2.44 yields 

The pH of minimum solubility of Fe(OH)2s or solution pH of zero net charge can be obtained by differentiating Equation 2.47 and setting the derivative of FeT with respect to OH equal to zero. Therefore, 

The pH-dependent solubility behavior of metal-hydroxides and the corresponding solution pH of zero net charge can be demonstrated by deriving pH dependent solubility functions for all the metal-hydroxy species of a particular metal in solution. 

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