The Effects of pH on Solubility

The pH of a solution can affect the solubility of a compound in that solution. For example, consider the dissociation of Mg(OH)2, the active ingredient in milk of magnesia  

The solubility of this compound is highly dependent on the pH of the solution into which it dissolves. If the pH is high, then the concentration of OH in the solution is high. In accordance with the common ion effect, this shifts the equilibrium to the left, lowering the solubility  

If the pH is low, then the concentration of H30 (a ) in the solution is high. As the Mg(OH)2 dissolves, these H30 ions neutralize the newly dissolved OH ions, driving the reaction to the right  

Consequently, the solubihty of Mg(OH 2 in an acidic solution is higher than in a pH-neutral or basic solution. 

In general, the solubility of an ionic compound with a strongly basic or weakly basic anion increases with increasing acidity (decreasing pH). 

The hydronium ion concentration can have a profound effect on the solubility of an ionic compound. If the compound contains the anion of a weak acid, addition of HjO (from a strong acid) increases its solubility. Once again, Le Chatelier s principle explains why. An especially interesting case occurs with calcium carbonate. In a saturated solution of CaC03, we have 

Adding some strong acid introduces a large amount of H3O , which immediately reacts with to form the weak acid HCOs  

more CaCOs dissolves. In this particular case, the effect is increased by gas formation. If enough H30 is added, further reaction occurs to form carbonic acid, which decomposes immediately to H2O and CO2, and the gas escapes the container  

As this sequence of changes shows, the net effect of added HsO is a shift in the equilibrium position to the right  

In contrast, adding to a saturated solution of a compound with a strong-acid anion, such as silver chloride, has no effect on the equilibrium position AgCl(j) Ag iaq) + Cr(aq) 

This section will integrate material from this chapter with material from the previous chapter. In Chapter 14, we looked at salts composed of strong and weak acids and bases. We saw that these salts have characteristic behaviors in solution. So far in this chapter, we have not considered the pH of the solutions. We re going to connect these two ideas to consider the solubilities of salts in non-neutral solutions. 

Let s begin with a conceptual example by considering the salt magnesium hydroxide, Mg (OH) 2, which is a common ingredient in many over-the-counter antacids. Magnesium 

If a strong acid, such as HC1 is added to the solution, the hydrogen ions will react with the hydroxide ions in solution to form water molecules. Therefore, the addition of a strong acid will decrease the concentration of hydroxide ions. The ion product, Q, will decrease. Our friend Le Chatelier would tell us that at this point the equilibrium will shift to the right to compensate for the loss of the hydroxide ions. The Mg(OH)2 will continue to dissociate until equilibrium is restored. If additional HC1 is added to the solution, it will drive the equilibrium to the right again. If sufficient acid is added, all of the Mg(OH)2 will dissolve. 

This process will occur with any salt whose anion is basic. Anions that are the most basic are affected the most by changes in pH. Salts whose anions are from strong acids have virtually no basicity and will therefore be unaffected by changes in the pH. 

Sample For which of the following is pH likely to affect the solubility  

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Another important parameter that may affect a precipitate s solubility is the pH of the solution in which the precipitate forms. For example, hydroxide precipitates, such as Fe(OH)3, are more soluble at lower pH levels at which the concentration of OH is small. The effect of pH on solubility is not limited to hydroxide precipitates, but also affects precipitates containing basic or acidic ions. The solubility of Ca3(P04)2 is pH-dependent because phosphate is a weak base. The following four reactions, therefore, govern the solubility of Ca3(P04)2. 

An important example of the effect of pH on solubility is tooth decay. Tooth enamel contains the mineral hydroxyapatite, which is insoluble near neutral pH, but dissolves in acid because both phosphate and hydroxide in the hydroxyapatite react with H+ ... 

The effect of pH on solubility is also important in understanding how fluoride ion reduces tooth decay. When tooth enamel comes in contact with F ions in drinking water or fluoride-containing toothpaste, OH- ions in hydroxyapatite, Ca5(PC>4)30H, are replaced by F- ions, giving the mineral fluorapatite, Ca5(PC>4)3F. Because F- is a much weaker base than OH-, Ca5(P04)3F is much more resistant than Cas(P04)30H to dissolving in acids. 

Some solids are only weakly soluble in water but dissolve readily in acidic solutions. Copper and nickel sulfides from ores, for example, can be brought into solution with strong acids, a fact that aids greatly in the separation and recovery of these valuable metals in their elemental forms. The effect of pH on solubility is shown dramatically in the damage done to buildings and monuments by acid precipitation (Fig. 16.8). Both marble and limestone are made up of small crystals of calcite (CaCOs), which dissolves to only a limited extent in natural rain (with a pH of about 5.6) but dissolves much more extensively as the rainwater becomes more acidic. The reaction... 

Unfortunately, the predicted values for solubility and those experimentally determined do not always correlate well because the effect of pH on solubility is often complicated by other factors. This was clearly illustrated during a study on the solubility of salts of a poorly soluble and weakly basic experimental antimalarial dmg. The solubility of the DL-lactate salt was shown to be 200 times greater than the hydrochloride (see Table 37.5) and twice that of the L-lactate, suggesting that the OL-lactate could provide a route to a parenteral formulation. 

Many organizations use colon adenocarcinoma (Caco-2) for detailed study of permeability however, this method can be resource intensive. Parallel artificial-membrane permeability (PAMPA) [19] has proven to be a reliable predictor of passive transcellular permeability for intestinal absorption prediction. It is also useful to interpret results of cell-based discovery assays, in which cell-membrane permeability is limiting. Finally, pTf provides insight into the pH dependence of solubility and permeability. It can be measured [20] or calculated to get an understanding of the regions of the intestine in which the compound will be best absorbed, as well as to anticipate the effect of pH on solubility and pemieability. Permeability at the blood-brain barrier (BBB) also can be rapidly profiled [21]. 

Figure 12.8 The effect of pH on solubility (S) of 3-lactoglobulin. The pi of this protein is 5.2. (Data from Gronwell 1942.)...

The effect of pH on solubility of silica at temperatures from 0 to 200 C was measured by Goto (167a) and is shown in Figure 1.7. At 22-100 C the solubility was about 30% higher than for most powders and gels, as shown in Figure 1.7. This is... 

Detailed studies of the effect of pH on solubility of amino acids have been canied out by Hitchcock (103) for tyrosine and by Sano (180) for cystine. The simple relations given by them may readily be generalised so as to apply to a complex molecule such as a protmn with a very large number of ionising groups. 

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