You have probably heard that pH is very important in life, such as in the digestion of food! The solubility of compounds depends on the pH of the solution it is being dissolved in. So, let's dive into pH and solubility!
- This article is about pH and solubility.
- First, we will talk about pH, solubility, and their relationship.
- Next, we will look at molar solubility.
- Then, we will learn more in-depth about pH and solubility.
- Lastly, we will look at protein solubility and also hemicellulose solubility.
Relationship between pH and Solubility
Let's start by defining pH.
pH is a measure of the concentration of protons (H+ ions) in a solution.
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, Vaia Originals.
Now, let's look at the definition of solubility.
The amount of a substance that can dissolve in a given volume of solvent, at a specified temperature, is referred to as the solubility of that substance.
The solvent is the liquid that compounds (solutes) are dissolved in.
The solute is a substance(compound) that is dissolved into a solvent.
A solution is the combination of the solvent and solute.
A compound that is miscible with something will able to be mixed with it to form a solution.
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.
Did you know that mixing leads to an increase in entropy? Entropy is related to the disorder or randomness of a system! You can learn more about this by reading "Entropy".
Ionic Solids and Ksp
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.
When dealing with ionic solids, there is a new constant that you need to be familiar with. The solubility product constant (Ksp) is referred to as the equilibrium constant for an ionic solid dissociating into ions in water.
Let's look at an example!
Write down the Ksp expression for MgCO3 in water.
This question asks us to write a solubility equilibrium expression for MgCO3 in water. First, we need to know how MgCO3 dissociates in water. This is given by the balanced equation below.
Remember that solids and liquids (like the solvent) are not included in the solubility equilibrium expression. So, the Ksp expression for MgCO3 in water would be:
If you are unsure about what Ksp is, check out "Solubility Product Constant". You can also learn more in-depth about ionic solids by reading "Ionic Solids"!
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, Ksp. Now, let's look at how to calculate molar solubility from Ksp.
Molar solubility is the molar concentration of the solid that dissociates in water. In an ICE table, molar solubility is represented as "x".
Practice makes perfect, right? So, let's solve a problem!
Calculate the molar solubility of AgCl. (Ksp = 1.8 x 10-10)
In water, AgCl dissociates into Ag+ and Cl- ions. The Ksp equilibrium expression is written as: .
The first thing we need to do is set up an ICE chart for this process. The acronym "ICE" stands for "Initial concentration, "Change in concentration" and "Equation for concentration".
So, Initially, we have no ions in the solution, so the concentration of Ag+ and Cl- is zero. As AgCl dissociates, we are going to have a change (a gain) of +x for both ions, Ag+ and Cl-. Finally, at equilibrium, we add the terms in lines "I" and "C" and then substitute the terms in line "E" into the equation for Ksp.
Since AgCl is a solid that dissolves over time, the only thing we need to add about it in the ICE chart is the change. To show that AgCl decreases we use a change (a loss) of " -x " because the molar concentration of the solid decreases as it dissolves in water! In other words, the variable x, itself, in effect, represents the molar solubility of AgCl.
Fig. 2: ICE chart for the dissociation of AgCl, Vaia Originals.
Now, we have to solve for the value of x using the equilibrium expression.
$$K_{sp}=[Ag^{+}]\cdot [Cl^{-}]\cdot 1.8\cdot 10^{-10}=x^{2}\sqrt{1.8\cdot 10^{-10}}=x$$
$$x=1.3\cdot 10^{-5}$$
So, the molecular solubility of AgCl is 1.3 · 10-5 M.
The example above involved the calculation of molar solubility from Ksp. But, what if we were asked to calculate Ksp from molar solubility instead? Let's look at an example.
BiI3 has a molar solubility of 1.32 x 10-5 M. Calculate the solubility product constant, Ksp.
Here, the Ksp equilibrium expression would be: . Now, if we decided to make an ICE chart, we would have:
Fig. 3: ICE chart for the dissociation of Bismuth (III) Iodide, Vaia Originals.
The goal of this problem is to calculate Ksp. So, we can use the equilibrium expression and the molar solubility value given by the question to solve for Ksp.
$$K_{sp}=[Bi^{3+}][I^{-}]^{3}=x\cdot x^{3}=27x^{4}$$
$$K_{sp}=27(1.32\cdot 10^{-5})^{4}=8.20\cdot 10^{-19}$$
Notice, that in this case, we substitute the given molar solubility value for x and then solve for the solubility product constant, Ksp.
This way of solving for Ksp from molar solubility only works when common ions are not present. Check out the "Common Ion Effect" to learn more!
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.
A negligible ion is an ionic species that has negligible hydrolytic (water-splitting) strength.
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.
Negligible Cations | Negligible Anions |
| Group 1 metal ions (Li+, Na+, K+, Rb+, Cs+) | Conjugate bases of strong acids (Cl-, Br-, I-, NO3-, ClO4-, ClO3-) |
| Group 2 metal ions (Mg2+, Ca2+, Sr2+, Ba2+) | Almost all other anions are basic anions (except for HSO4-, which is acidic) |
| Transition metal ions with a +1 charge (Ex. Ag+) | |
| All other cations are considered acidic cations | |
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.
How can we know whether BaF2 is an acidic, basic or neutral salt?
First, look at the formula for barium fluoride and identify if any negligible ions are present. Ba2+ is a negligible acidic cation and therefore is considered negligible and does not count. F- is not on the list, so it is not negligible.
F- ions (as the anion) is considered to be basic. Therefore, we can say that BaF2 is a basic salt!
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.
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.
What would happen if you added a strong acid to lead oxalate, PbC2O4? (Ksp = 4.8·10-10)
If you added a strong acid to a saturated solution of lead oxalate, the H+ ions from the added acid would react with the oxalate ion to form HO2CCO2H (aq).
This decrease in oxalate ions will cause more lead oxalate to dissolve in the acidic solution to replenish the oxalate ions and put the system back to equilibrium!
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.
The pH at which a protein contains a zero net charge is called the isoelectric point.
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 - Vaia 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.
Hemicelluloses are a group of cell wall polysaccharides.
A hydrogen bond is a relatively weak bond involving two electronegative atoms and hydrogen in between them. For example, water is a liquid that forms a high degree of hydrogen bonds within its structure.
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.
pH and solubility - Key takeaways
- The solubility of a substance is the amount of substance that can be dissolved in a given quantity of solvent at a given temperature.
- pH is a measure of the concentration of protons (H+ ions) in a solution.
- 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 is the molar concentration of the solid that dissociates in water.
References
- “Chad’s General Chemistry Course: PH Effects on Solubility.” Chad’s Prep -- DAT, MCAT, OAT & Science Prep, courses.chadsprep.com/courses/take/general-chemistry-1-and-2/quizzes/2570261-17-6-ph-effects-on-solubility-4-questions.
- PH and Protein Solubility. https://www.flinnsci.com/api/library/Download/25c26f81fcae4cccadbe40045f38a646.
- “Hemicellulose - an Overview | ScienceDirect Topics.” Sciencedirect.com, 2009, www.sciencedirect.com/topics/agricultural-and-biological-sciences/hemicellulose.
- Cheng, Heli, et al. “Alkali Extraction of Stover.” BioResources, vol. 11, no. 1, 2010, pp. 196–206, bioresources.cnr.ncsu.edu/BioRes_06/BioRes_06_Unsecured/BioRes_06_1_0196_Cheng_ZFL_Alk_Etrac_Hemicel_Stover_Pulping_1289.pdf. Accessed 26 June 2022.
- “18.7: Solubility and PH.” Chemistry LibreTexts, 26 Nov. 2013, chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/18%3A_Solubility_and_Complex-Ion_Equilibria/18.7%3A_Solubility_and_pH.
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