0.4 pH of a salt solution

We’ve already seen that ions can act as acids or bases. For example, through the practice problems in the previous sections we've seen that NH₄⁺ is a weak acid and CH₃COO⁻ is a weak base. Such ions are also associated with a counterion, which forms an ionic compound or salt. For example, CH₃COO⁻ may be paired with Na⁺ to form the salt CH₃COONa. Soluble salts like these dissolve in water and dissociate into their anionic (e.g. CH₃COO⁻) and cationic (e.g. Na⁺) components. If either of these ionic components are acidic or basic, they will react with water to affect the pH of the resulting solution, just as we saw for any weak acid or weak base. For example, since CH₃COO⁻ is a weak base and Na⁺ is neither an acid nor a base, dissolving CH₃COONa in water results in a basic solution. The pH of such a solution can be determined using the same ICE table method as for any weak acid/base.
 
So how can we know whether any given salt would dissolve in water to give an acidic or basic solution? One simple approach is to consider the acid-base reaction that could have happened to form the salt (acid + base → salt) by following the steps below:

1. Split the salt into its anionic and cationic components.
    
2. Add H⁺ to the anion to form the acid that reacted to form the salt.
    
3. If possible, remove H⁺ from the cation to form the base that reacted to form the salt. If there is no H⁺ to remove, instead consider the hydroxide (HO⁻) base as Xⁿ⁺(⁻OH)_n, where X is the cation.
    
4. Consider the strength of the acid (step 2) and base (step 3) that reacted to make the salt to determine the qualitative pH of the salt solution.
   • Strong acid + strong base → neutral salt
   • Strong acid + weak base → acidic salt
   • Weak acid + strong base → basic salt
   • Weak acid + weak base → either acidic or basic salt depending on the relative strengths

Apply these steps to solve each of the examples below yourself before expanding to see the full solution.

 
An alternate approach:
While the above approach is straight forward, it may not be intuitively clear why it works. Let’s consider the four steps outlined above from a different perspective:

1. Splitting the acid into its anion and cation components allows us to analyze each of these components separately to see how they each affect the pH.
    
2. Adding H⁺ to the anion forms the conjugate acid of the anion, which allows us to assess the strength of the anion as a base. If the resulting conjugate acid is weak, then the original anion is a weak base. If the resulting conjugate acid is strong, then the original anion does not act as a base.
    
   For example, SO₄²⁻ is a weak base (its conjugate acid, HSO₄⁻, is weak) whereas Cl⁻ does not act as a base (its conjugate acid, HCl, is strong).
   Why don’t anions act as bases when they have strong conjugate acids?

   Strong acids, such as HCl, react essentially completely in water to form H₃O⁺ and Cl⁻. The fact that essentially every molecule of HCl releases H⁺ in water (which forms H₃O⁺) demonstrates that the remaining Cl⁻ has a very low affinity for H⁺. That is, Cl⁻ does not act as a base. Conversely, weak acids such as HSO₄⁻ only react partly in water, which demonstrates that their conjugate bases (e.g. SO₄²⁻) are weak since they have some affinity for H⁺. The stronger an acid is (i.e. the more it releases H⁺) the weaker its conjugate base will be (i.e. the less affinity the conjugate base has for H⁺).
   

    

3. Removing H⁺ from the cation (if possible) forms the conjugate base of the cation, which allows us to assess the strength of the cation as an acid. If the resulting conjugate base is weak, then the original cation is a weak acid. If the resulting conjugate base is strong, then the original cation does not act as an acid. If H⁺ cannot be removed, then the species cannot act as an acid.
    
   For example, NH₄⁺ is a weak acid (its conjugate base, NH₃, is weak) whereas Na⁺ does not act as an acid (there is no H⁺ to remove).
    
4. To determine the overall pH of a salt solution, consider the acidity/basicity of the individual anion and cation components:
   • If the anion does not act as a base and the cation does not act as an acid, then the resulting salt solution will be neutral.
   • If the anion does not act as a base and the cation is a weak acid, then the resulting salt solution will be acidic.
   • If the anion is a weak base and the cation does not act as an acid, then the resulting salt solution will be basic.
   • If the anion is a weak base and the cation is a weak acid, then the pH of the resulting solution will depend on the relative strength and quantities of the acid vs base components.

 
The approaches above show how to qualitatively classify a solution as acidic, basic, or neutral. To calculate the pH of a salt solution quantitatively, use the same ICE table method introduced in the previous sections to assess how an acidic or basic cation or anion reacts with water to affect the pH. An example is given below.

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