Acid-Base Titration and Salt Hydrolysis in Chemistry
Acid-Base Titration
In both industrial processes and in the laboratory, it is very common to determine the amount of acid or base in a sample. Titration, specifically acid-base titration, is a method of chemical analysis used to determine the unknown concentration of an acidic or basic solution. This is achieved by using a standard solution, which is a basic or acidic solution of known concentration.
The experimental procedure involves placing a known volume of the solution to be analyzed in a conical flask. Then, from a burette, the standard solution is added drop by drop until the equivalence point is reached. The equivalence point is where there is no free acid or base in the sample. At this point, the number of moles of H3O+ ions produced by the acid equals the number of moles of OH– produced by the base.
For an acid concentration Ma, whose valence is va, and a base of concentration Mb, whose valence is vb:
- Number of moles of H3O+ = va * Ma * Va
- Number of moles of OH– = vb * Mb * Vb
The equivalence point is reached when va * Ma * Va = vb * Mb * Vb
Experimental Calculation of the Equivalence Point
To experimentally determine when the equivalence point is reached, a few drops of an indicator are added to the initial solution. The moment a color change is observed in the indicator is called the endpoint of the assessment.
The equivalence point and the endpoint can never match exactly because the former is a theoretical point, while the latter is an experimental point. A good evaluation is one in which, by appropriate choice of the indicator, the endpoint is very close to the equivalence point. The closer the endpoint is to the equivalence point, the smaller the error in determining the concentration of the solution.
Hydrolysis of Salts
Hydrolysis is the reaction that occurs between water molecules and ions from certain salts. When a salt is dissolved in water, it dissociates into cations and anions, which interact with the water molecules. Depending on the characteristics of these ions, two different behaviors can occur:
- Ions from strong acids (their conjugate bases, which are very weak) or strong bases (their conjugate acids, which are very weak) do not react with water and do not produce hydrolysis. These ions simply become hydrated.
- Ions from weak acids or bases (their conjugate acids or bases) react with water, producing hydrolysis. This results in the formation of H3O+ or OH–, changing the pH of the solution.
Salts of Strong Acid and Strong Base (e.g., NaCl)
When dissolved, NaCl dissociates into Na+ (conjugate acid of NaOH, a strong base) and Cl– (conjugate base of HCl, a strong acid). Neither of these ions reacts with water; therefore, no hydrolysis occurs. No hydronium or hydroxide ions are released into the solution, so the pH of the solution will be neutral (pH = 7).
Salts of Strong Acid and Weak Base (e.g., NH4Cl)
When dissolved, ammonium chloride dissociates into NH4+ (conjugate acid of NH3, a weak base) and Cl– (conjugate base of HCl, a strong acid). The Cl– anion does not undergo hydrolysis, but NH4+ does react with water:
NH4+ + H2O ⇌ NH3 + H3O+
This reaction releases H3O+ ions. Therefore, the solution will become slightly acidic (pH < 7).
Salts of Weak Acid and Strong Base (e.g., NaCH3COO)
Upon dissolution, sodium acetate dissociates into Na+ (conjugate acid of NaOH, a strong base) and CH3COO– (conjugate base of CH3COOH, a weak acid). The Na+ cation does not produce hydrolysis, but CH3COO– reacts with water:
CH3COO– + H2O ⇌ CH3COOH + OH–
This reaction releases OH– ions. Therefore, the solution will become slightly basic (pH > 7).
Salts of Weak Acid and Weak Base (e.g., CH3COONH4, NH4CN)
When a salt of this type is dissolved in water, both ions formed produce hydrolysis. To determine whether the resulting pH will be acidic or basic, compare the constants of both hydrolysis reactions. The one with the higher value of K will determine the acidity or alkalinity of the solution.
Concept of pH
The measure of the acidity or alkalinity of a solution is given by the hydronium ion concentration [H3O+] present in it. Since the most common solutions are very dilute, we often work with very small numbers and powers of 10 with negative exponents.
To avoid this, we define the concept of pH using the following equation:
pH = -log[H3O+]
Similarly, we define pOH as:
pOH = -log[OH–]
The same mathematical approach can be applied to the acidity or basicity constant.
From the ionic product of water, it is easy to prove, using logarithms, that at a temperature of 25°C, pH + pOH = 14. In general, at any temperature, it follows that pH + pOH = pKw.