PH of salts

From WikiLectures


When we talk about the pH of salts, we mean the pH of aqueous solutions of soluble salts. Such salts dissociate in solution according to Eq

Where is a salt of an acid and policies which arises, for example, by neutralisation according to the equation

pH of salts of strong acids and strong bases[edit | edit source]

In the case of a salt of a strong acid and a strong base, we consider that

  • Cation — because it is strong — it will remain dissociated:

  • Anion — because it is strong — it will also remain dissociated:

Therefore, not one of the salt ions will react with the water molecules and the only source and will be the autoprotolysis of water itself. I mean

and at 25 °C the pH will be equal to 7.

pH of salts of weak acids and strong bases[edit | edit source]

In the case of a salt of a weak acid and a strong base, we consider that

  • cation — because it is strong — it will remain dissociated:
  • anion — because it is weak — it will react with water, i.e. undergo so-called hydrolysis, according to the equation:
  • will hydrolyze very few anions, the amount of hydrolyzed anions will be negligible, i.e
  • ,or otherwise
  • the only source is the above-mentioned hydrolysis reaction, we neglect other sources of hydroxide anions and oxonium cations, i.e. the amount and will be the same according to her equation.

The above hydrolysis reaction has an equilibrium constant

We will consider the concentration of water in water as constant and introduce a new constant, the so-called hydrolytic constant:

If we adjust the (slightly imprecise) notation of water in chemical equations, it will be easier to see that the hydrolysis equation is de facto just the opposite equation to dissociation:


  • dissociation:
  • hydrolysis:

It is therefore natural that the hydrolytic constant and the dissociation constant they will be related:

For a constant so we get the formula

If to the definition of the hydrolytic constant we substitute for according to our assumptions and for we will get

When we express the dependence of the concentration of oxonium cations on the concentration of hydroxide anions from the definition of the ionic product of water , we obtain

We take the square root (concentrations are always positive), logarithmize and multiply by −1:

At 25 °C we get the formula  

The resulting pH will be alkaline . This is due to the fact that the anion of the acid draws hydrons from the system.

The pH of a salt of a strong acid and a weak base[edit | edit source]

In the case of a salt of a strong acid and a weak base, we consider that

  • cation —because it is weak — it will hydrolyse according to the reaction

  • anion — because it is strong — it will not hydrolyse, that is

  • will hydrolyze very few cations, and the amount of hydrolyzed cations will thus be negligible, i.e
  • , or not
  • hydrolysis of cations is the only source of oxonium cations, we neglect other sources, so according to the hydrolysis equation

For hydrolysis, we introduce a hydrolytic constant like

for which it can again be proved to hold

According to the assumptions, we substitute in the hydrolytic constant za and for :

We take the square root (concentrations are always positive numbers), logarithmize and multiply by −1:

So at 25 °C we get the formula:

The resulting pH will be acidic . This is because the base cation adds hydrons to the system.

pH of salts of weak acids and weak bases[edit | edit source]

In the case of a salt of a weak acid and a weak base, we consider that

  • Cation will hydrolyse according to Eq

  • anion will react with the resulting oxonium cations and then possibly further hydrolyse according to the equation

  • both ions will hydrolyse in negligible amounts, ie

  • there is no other source of hydroxide anions and oxonium cations in the system, therefore
    • oxonium cations formed by hydrolysis of the basecation we mark and according to the chemical equation of hydrolysis it applies to them
    • oxonium cations destroyed by acid anion hydrolysis we mark and according to the chemical equation of hydrolysis it applies to them
  • the equilibrium concentration of oxonium cations is calculated as the difference between the concentration of oxonium cations formed by the hydrolysis of the base cation and the concentration of oxonium cations consumed by the hydrolysis of the acid anion , i.e.

For the hydrolysis of the cation, we have the hydrolysis constant:

and for the hydrolysis of the anion it is better to express the concentration of oxonium cations using another constant describing the equilibrium, namely the dissociation constant:

By adding the penultimate assumption to the last assumption, we get

We express and from the equations for the equilibrium constants:

We will edit:

We express the concentration of oxonium cations:

Because it is we can neglect the denominator and approximate the formula to

By supplementing by assumptions we get

We take the square root (these are positive constants), take the logarithm, multiply by −1 and get

Thus, the pH of the salt of a weak acid and a weak base (after approximation) does not depend on the concentration of the salt .

At 25 °C we get


Links[edit | edit source]

related articles[edit | edit source]

related articles[edit | edit source]

References[edit | edit source]

  • BERKA, Antonín – FETL, Ladislav – NĚMEC, Ivan.  Practitioner's Guide to Quantitative Analytical Chemistry.. 1. edition. SNTL, 1985. 228 pp. pp. 56–66. 


recommended literature[edit | edit source]