Determining the pH of Salt Solutions

Salt solutions can be acidic, basic, or neutral. The individual ions can be examined to determine if the pH of its solution will be below, above, or equal to seven.

Salts that contain pH-neutral cations and anions form neutral solutions. These salts are composed of the counterions of a strong acid and a strong base. A pure aqueous solution of potassium nitrate will have a pH of 7, as neither potassium nor nitrate has acidic or basic properties.

Salts that contain a cation that is the counterion of a weak base and an anion that is the counterion of a strong acid will have a pH less than 7. In a solution of ammonium bromide, the bromide ions are pH neutral, whereas the ammonium ions are acidic and donate protons to the water molecules.

Salts that contain a cation that is the counterion of a strong base and an anion that is the counterion of a weak acid will have a pH greater than 7. In a sodium acetate solution, the sodium ions are pH neutral, whereas the acetate ions are basic and accept protons from the water molecules.

Some salts contain a cation and anion that are counterions of a weak acid and weak base, respectively. The solution will be acidic if the K_a of the cation is larger than the K_b of the anion; basic if the K_b of the anion is larger than the K_a of the cation; or neutral if the K_a and the K_b are equal.

In a solution of ammonium nitrite, the K_a for ammonium is higher than the K_b for nitrite ions; therefore, the solution will be acidic.

The exact pH of a salt solution containing an acidic or basic ion can be determined using the dissociation constant for the conjugate acid or base of that ion.

The pH of 0.35 M sodium cyanide can be determined by calculating the K_b of cyanide from the K_a of its conjugate acid - hydrocyanic acid - and setting up an ICE table to determine the equilibrium concentration.

As sodium cations do not react with water, they do not affect the pH and can be omitted. In contrast, cyanide ions accept protons from water molecules and generate hydroxide ions.

The K_b for cyanide ions can be determined using the equation: K_a × K_b = K_W. As the K_a for hydrocyanic acid is 4.90 × 10⁻¹⁰, the K_b for cyanide ions is 2.04 × 10⁻⁵.

The K_b is equal to the concentration of hydrocyanic acid times the concentration of hydroxide ions divided by the concentration of cyanide ions.

An ICE table can be constructed to express the initial and equilibrium concentrations. Substituting the equilibrium concentrations into the expression, K_b equals x times x divided by 0.35, which can be verified later by the 5% rule.

Upon solving the equation, x equals 2.7 × 10⁻³ molar. As x is only 0.77% of 0.35 M, the approximation 0.35 minus x is equal to 0.35 is valid.

The pOH of the solution can be calculated by taking the negative log of 2.7 × 10⁻³ M, which equals 2.57.

The pH of the solution is determined using the equation pH plus pOH is equal to 14; therefore, the pH of this sodium cyanide solution is 11.43.