Question

In a titration with a strong acid and weak base, where is the equivalence point?

Answer 1

Titration is a popular quantitative chemical analysis procedure used in laboratories to quantify the concentration of a specified analyte (a substance to be analyzed). The titrant or titrator is a reagent that is produced as a standard solution with a known concentration and volume. The titrant interacts with an analyte solution (also known as the titrant) to determine the concentration of the analyte. The titration volume is the amount of titrant that interacted with the analyte.

Complete answer:
A chemical reaction's equivalence point, also known as the stoichiometric point, is the point at which chemically equivalent quantities of reactants have been combined. The equivalence point for an acid-base reaction is the point at which the moles of acid and base would neutralise each other according to the chemical reaction. This does not necessarily mean that the acid:base molar ratio is 1:1, but rather that it is the same as in the chemical reaction. An indicator, such as phenolphthalein or methyl orange, can be used to detect it. In a colorimetric titration, the endpoint (which is related to but not the same as the equivalence point) refers to the point at which the indicator changes colour.
Consider $\text{N}{{\text{H}}_{3}}(\text{aq})+\text{HCl}(\text{aq})\to \text{NH}_{4}^{+}(\text{aq})+\text{C}{{\text{l}}^{-}}(\text{aq})$
In most cases, the acid is titrated into the base. In the burette, a tiny quantity of an acid solution of known concentration is put (this solution is called the titrant ). In an Erlenmeyer flask (the analyte), a known volume of base with unknown concentration is put, and if pH measurements can be acquired by an electrode, a graph of pH vs. volume of titrant may be created (titration curve). When titrating acid into a weak base for a strong acid-weak base titration, the pH of the base will often start high and decline fast as acid is added.
The pH will gradually alter as the equivalent point is approached, until one drop causes a fast pH shift through the equivalence point. When a chemical indicator is used—methyl orange is a suitable choice here—the colour shifts from basic to acidic. The pH at the equivalence point in strong acid-weak base titrations is not 7, but lower. This is owing to the titration's creation of a conjugate acid, which will react with water to form hydronium (${{H}_{3}}{{O}^{+}}$) ions.
The conjugate acid produced ($N{{H}_{4}}^{+}$) interacts as follows in the case of HCl titration into ammonia solution:
$\text{NH}_{4}^{+}+{{\text{H}}_{2}}\text{O}\to {{\text{H}}_{3}}{{\text{O}}^{+}}+\text{N}{{\text{H}}_{3}}$
Hence the pH is below 7.

Note:
A pH indicator is a material that changes colour when a chemical reaction occurs. The hue of an acid-base indicator (such as phenolphthalein) varies with the pH. The use of redox indicators is also common. At the commencement of the titration, a drop of indicator solution is introduced; when the colour changes, the endpoint has been achieved; this is an estimate of the equivalence point.