How Does the pH of Salts Affect Solutions?
Salt is a compound which is formed by the neutralisation reaction between an acid and a base. They ionise in water producing cations and anions, which either exist as hydrated ions in aqueous solutions or mix with water to regenerate the acids and bases. The three kinds of salts depending on their pH value are:
1. Neutral Salts: pH value 7.
2. Acidic Salts: The pH less than 7.
3. Basic Salts: The pH more than 7.
The pH value measures the level of acidity or alkalinity in a water-soluble solution. The scale measures from 0 to 14. Anything lower than 7 is acidic and anything higher than 7 is basic. The calculation of acidity or alkalinity of a solution is known as pH, the relative quantity is stated by pH. Acidity depends on the concentration of the hydrogen ions and if its concentration is more then more is the acidity of the solution. So the meaning of pH is justified as the power of hydrogen.
pH Value of Sodium Chloride
Sodium chloride is an ionic compound which represents a 1:1 ratio of sodium and chloride ions. Sodium chloride is responsible for the salinity of seawater. The pH of sodium chloride remains at seven due to the weak basicity of chloride ions. It is formed from the reaction of a strong acid and a strong base, in the formation hydrogen chloride is neutralised by sodium hydroxide.
When sodium chloride is mixed with water it will not show any acidic or basic properties, as both ions are not susceptible to hydrolysis. Brine is water saturated with salt or a solution of sodium chloride and water which occurs naturally on the earth's surface or in the benthic zone of oceans.
pH of Salt of Weak Acid and Strong Base
Salts from strong bases and weak acids have a pH greater than seven, the anion being from the weak acid, accepting the proton from the water in the chemical reaction.
KCN(s) → K+(aq) + CN−(aq)
CN−(aq) + H2O(l) ⇌ HCN(aq) + OH−(aq)
Ammonium chloride is an acidic salt as it is a salt of strong acid namely hydrochloric acid, and a weak base which is ammonium hydroxide, another example is sodium acetate which is salt in solid state and can't be used in liquid form as an acid or base.
Sodium hydroxide is a strong base and acetic acid is a weak acid, so the sodium salt is called sodium acetate. It is in white granular powder form, hygroscopic in nature as it easily absorbs water, medically it is used intravenously as an electrolyte for sodium level correction in hyponatremic patients.
pH of Salts List
Most of the salts have pH values less than or more than 7. This means they may be neutral, acidic or basic.
1. Sodium chloride, pH is 7 and the nature of the salt is neutral.
2. Sodium carbonate, pH is ~11 and nature is basic.
3. Ammonium chloride, pH is ~6 and nature is acidic.
4. Potassium sulphate, pH is 7 and nature is neutral.
5. Ammonium sulphate, pH is ~5.5 and the nature is acidic.
6. Potassium chloride, pH is 7 and nature is neutral.
7. Ammonium carbonate, pH is ~7.8, and nature is slightly basic.
8. Sodium sulphate, pH is 7, nature is neutral.
Interesting Facts
Basic salts such as sodium carbonate are used to maintain soil acidic pH.
Even if we put a teaspoon of table salt in a glass of pure water, it won't change its pH. Pure water has a pH value of 7.
Key Features of pH of Salts
Salt is a compound which is formed by the neutralisation reaction between an acid and a base.
The pH value measures the level of acidity or alkalinity in water-soluble solution.
Salts from strong bases and weak acids do not hydrolyze and they have a pH greater than 7.
FAQs on pH of Salts Explained: Key Concepts & Examples
1. What determines the pH of a salt solution in water?
The pH of a salt solution is determined by the strength of the acid and base from which the salt was formed. When a salt dissolves in water, its ions can react with water in a process called salt hydrolysis. This reaction can produce excess hydrogen ions (H⁺), making the solution acidic, or excess hydroxide ions (OH⁻), making it basic. If neither ion reacts with water, the solution remains neutral.
2. Why isn't the pH of all salt solutions neutral (pH 7)?
The common misconception that all salts are neutral arises from examples like table salt (NaCl). However, a salt solution is only neutral if it is formed from a strong acid and a strong base. In this case, neither the cation nor the anion hydrolyzes water. For all other combinations (weak acid/strong base, strong acid/weak base, weak acid/weak base), at least one of the ions reacts with water, producing H⁺ or OH⁻ ions and shifting the pH away from 7.
3. Can you give examples of acidic, basic, and neutral salts?
Yes, salts are classified based on the pH of their aqueous solutions. Here are some common examples:
- Neutral Salts (pH ≈ 7): Formed from a strong acid and strong base. Examples include Sodium Chloride (NaCl), Potassium Nitrate (KNO₃), and Sodium Sulphate (Na₂SO₄).
- Acidic Salts (pH < 7): Formed from a strong acid and weak base. Examples include Ammonium Chloride (NH₄Cl), Copper Sulphate (CuSO₄), and Aluminium Chloride (AlCl₃).
- Basic Salts (pH > 7): Formed from a weak acid and strong base. Examples include Sodium Acetate (CH₃COONa), Potassium Cyanide (KCN), and Sodium Carbonate (Na₂CO₃).
4. What is the pH of a sodium chloride (NaCl) solution and why?
The pH of an aqueous sodium chloride (NaCl) solution is approximately 7 (neutral) at 25°C. This is because NaCl is the salt of a strong acid, hydrochloric acid (HCl), and a strong base, sodium hydroxide (NaOH). When dissolved, it dissociates into Na⁺ and Cl⁻ ions. Neither ion is strong enough to react with or hydrolyze water, so the concentrations of H⁺ and OH⁻ ions in the water remain equal.
5. What is salt hydrolysis and how does it cause a change in pH?
Salt hydrolysis is a chemical reaction in which the cation, the anion, or both ions of a salt react with water. This interaction disrupts the natural equilibrium of water (H₂O ⇌ H⁺ + OH⁻).
- If the anion from a weak acid reacts with water, it removes H⁺ ions, leaving excess OH⁻ and making the solution basic.
- If the cation from a weak base reacts with water, it produces H⁺ ions, making the solution acidic.
6. What are the formulas used to calculate the pH of different types of salt solutions?
The pH of a salt solution depends on the type of salt. As per the CBSE syllabus, the key formulas are:
- Strong Acid - Weak Base Salt: pH = 7 - ½ (pKb + log C)
- Weak Acid - Strong Base Salt: pH = 7 + ½ (pKa + log C)
- Weak Acid - Weak Base Salt: pH = 7 + ½ (pKa - pKb)
Here, 'pKa' is the dissociation constant of the weak acid, 'pKb' is for the weak base, and 'C' is the molar concentration of the salt.
7. How is the pH of a salt of a weak acid and a weak base determined?
For a salt formed from a weak acid and a weak base, such as ammonium acetate (CH₃COONH₄), the pH of the solution depends on the relative strengths of the acid and base, indicated by their pKa and pKb values. The pH is independent of the salt's concentration. The formula is pH = 7 + ½(pKa - pKb). If pKa = pKb, the solution is neutral. If pKa < pKb, the solution is acidic. If pKa > pKb, the solution is basic.
8. What is the difference in hydrolysis between ammonium chloride (NH₄Cl) and sodium acetate (CH₃COONa)?
The key difference lies in which ion hydrolyzes. Ammonium chloride (NH₄Cl) is a salt of a strong acid (HCl) and a weak base (NH₄OH). Here, the cation (NH₄⁺) hydrolyzes water to produce H₃O⁺, making the solution acidic (cationic hydrolysis). In contrast, sodium acetate (CH₃COONa) is a salt of a weak acid (CH₃COOH) and a strong base (NaOH). Here, the anion (CH₃COO⁻) hydrolyzes water to produce OH⁻, making the solution basic (anionic hydrolysis).
