pH and Solubility

Relationship between pH and Solubility

Let's start by defining pH.

To show whether a solution is considered acidic or basic, we use a pH scale that ranges from 0 to 14. Although sometimes pH might run outside this range, it is very rare for that to happen. Acidic solutions have a pH of less than 7, whereas basic solutions possess a pH greater than 7. A neutral solution is a solution that has equal concentrations of H⁺ and OH^- ions.

Fig. 1: The pH Scale, StudySmarter Originals.

Now, let's look at the definition of solubility.

The solubility and separation of mixtures are influenced by interactions between intermolecular forces. The general rule is that substances with similar intermolecular forces tend to be miscible in one another.

To be soluble, the attractive forces formed between solute and solvent during solution formation have to be comparable to those broken in the separation of solvent molecules and those broken in the separation of solute molecules.

Ionic Solids and K_sp

Ionic solids are solids that are held together by ionic bonds. Ionic solids are usually more soluble at higher temperatures. Salts are considered ionic compounds. When salts are dissolved in water, they completely dissociate into ions. An example of an ionic solid is table salt, NaCl.

$\mathrm{NaCl}\left(\mathrm{s}\right)\stackrel{}{\to }{\mathrm{Na}}^{+}+{\mathrm{Cl}}^{-}$

When dealing with ionic solids, there is a new constant that you need to be familiar with. The solubility product constant (K_sp) is referred to as the equilibrium constant for an ionic solid dissociating into ions in water.

Let's look at an example!

Salt solubility is pH sensitive when one of the constituent ions is a weak acid or base. Let's take a look at the pH effects on the solubility of salts!

• Acidic salts are more soluble in basic solutions (pH > 7), and less soluble in acidic solutions.

• Basic salts are more soluble in acidic solutions (pH < 7), and less soluble in basic solutions.

• The solubility of neutral salts is unaffected by a change in pH.

Molar Solubility and pH

We just learned about the solubility product constant, K_sp. Now, let's look at how to calculate molar solubility from K_sp.

Practice makes perfect, right? So, let's solve a problem!

The example above involved the calculation of molar solubility from K_sp. But, what if we were asked to calculate K_sp from molar solubility instead? Let's look at an example.

Solubility and pH Calculations

Now, when salts react with water, they produce a cation and an anion. This process is called hydrolysis.

Generally, the cations tend to have the potential to act as an acid, whereas the anion has the potential to act as a base. In other words, most cations are considered acidic, whereas most anions are said to be basic.

However, not all ions have the potential to act as an acid or base. Some of them possess no acid/base character and, thus, cannot change or influence the pH of water. These ions are said to have a negligible acidity or basicity, and can be thought of as neutral ions, or negligible ions.

Before diving into the solubility of salts, we need to review some rules used to identify negligible cations and anions.

• Most cations are said to be acidic. But, group 1 and 2 metal ions (or any transition metal with a charge of +1) are considered negligible.
• Most anions, on the other hand, tend to be basic (except for the anion HSO₄^-, which is acidic). Other exceptions include the conjugate bases of the strong acids, which are all mostly negligible.

We saw above that acidic salts are more soluble in basic solutions (pH > 7), whereas basic salts are more soluble in acidic solutions (pH < 7). Let's take barium fluoride (BaF₂) as an example.

Let's learn in more detail about the pH effects on solubility. We can start by looking at why basic salts are more soluble in acidic solutions. Let's use BaF₂ as our basic salt.

${\mathrm{BaF}}_{2\left(\mathrm{s}\right)}\stackrel{}{\rightleftharpoons }{\mathrm{Ba}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{F}}^{-}\left(\mathrm{aq}\right)$

According to the common ion effect, adding any strong electrolyte containing barium or fluoride ions will cause the equilibrium to shift to the left-hand side, reducing the solubility of BaF₂.

But, if we wanted to increase its solubility, what could we do? Let's say that we prepare a BaF₂ solution that is saturated (or has the maximum amount of BaF₂ dissolved in it) and that is also at equilibrium. If we add any acidic solution (pH < 7) to it, the added H⁺ ions will react with the fluoride ions, making HF. This will cause the equilibrium to shift to the right, forcing more BaF₂ to dissolve in that acidic solution, creating more F^- ions and getting the reaction back to equilibrium!

Now, let's see why acidic salts are more soluble in basic solutions. For example, Al(NO₃)₃ is an acidic salt (Al³⁺ is an acidic cation, and NO₃^- is a negligible anion). If you add it into a basic solution (pH > 7), Al³⁺ ions will react with the added OH^- ions to form Al(OH)₃. Similarly, this will cause the equilibrium to shift to the right-hand side, forcing more Al(NO₃) to dissolve in that basic solution (become more soluble), creating more Al³⁺ ions and getting the reaction back to equilibrium!

Let's look at a problem involving solubility.

pH and Protein Solubility

When the pH of a protein's environment changes, so does its solubility. Let's look at the protein casein. Casein is the principal protein in milk, and it is a phosphoprotein. This means that it has a phosphate group attached to the amino acid chain in its polypeptide structure.

At an intermediate pH (pH 2-pH 10), casein is pretty much insoluble because it would have an equal number of positive and negatively charged ions. So, casein would have a net charge of zero.

Now, if we added it to a dilute NaOH solution (high pH solution), casein would have a net negative charge because of the ionization of its acidic side chains. This would cause it to become soluble.

Casein is also soluble in strongly acidic solutions. When added to a solution of low pH, the protonation of its basic side chains would occur, causing casein to have a net positive charge.

Fig. 4: Structures of Casein at different pH, Isadora Santos - StudySmarter Originals.

Acid pH and Hemicellulose Solubility

Hemicelluloses are a structural component of plant cells. They are cell wall polysaccharides, capable of strongly binding to cellulose microfibrils by hydrogen bonds.

Xyloglucan is a type of hemicellulose found in the primary walls of all land plants. Xyloglucans are said to be hydrogen-bonded to cellulose and covalently bonded to pectins. They assist cellulose in load bearing.

Most hemicelluloses are considered more soluble in alkaline solutions (pH > 7) than in solutions with low pH. In water, most hemicelluloses are insoluble.

Fig. 5: Plant cell wall diagram, Wikimedia Commons.