WHAT IS THE EQUIVALENCE POINT AND WHAT IS IT FOR? - CHEMISTRY - 2025

The equivalence point is that at which two chemicals have completely reacted. In acid-base reactions, this point indicates when an entire acid or base has been neutralized. This concept is the daily bread and butter of volumetric titrations or valuations, and is determined by simple mathematical calculations.

But what is the degree? It is a process in which a volume of a solution of known concentration, called titrant, is carefully added to a solution of unknown concentration, in order to obtain its concentration.

Source: Pixabay

The use of a pH indicator allows to know the moment at which the titration ends. The indicator is added to the solution that is being titrated to quantify the concentration that it is desired to know. An acid-base indicator is a chemical compound whose color depends on the hydrogen concentration or pH of the medium.

However, the color shift of the indicator indicates the end point of the titration, but not the equivalence point. Ideally, both points should coincide; but in reality, the color change may occur within a few drops after the acid or base neutralization is complete.

What is the equivalence point?

A solution of an acid of unknown concentration placed in a flask can be titrated by slowly adding a sodium hydroxide solution of known concentration using a burette.

The selection of an indicator should be done in such a way that it changes color at the point that the same amount of chemical equivalents of the titrant solution and the solution of unknown concentration have reacted.

This is the equivalence point, while the point at which an indicator changes color is called the end point, where the titration ends.

Final point

The ionization or dissociation of the indicator is represented as follows:

HIn + H ₂ O <=> In ^- + H ₃ O ⁺

And therefore has a constant Ka

Ka = /

The relationship between the undissociated indicator (HIn) and the dissociated indicator (In ^-) determines the color of the Indicator.

The addition of an acid increases the HIn concentration and produces indicator color 1. Meanwhile, the addition of a base favors an increase in the concentration of the dissociated indicator (In ^-) (color 2).

It is desirable that the equivalence point matches the end point. To do this, an indicator with a color change interval that includes the equivalence point is selected. Also, it tries to reduce any errors that produce a difference between the equivalence point and the end point.

The standardization or titration of a solution is a process where the exact concentration of a solution is determined. It is methodologically a degree, but the strategy followed is different.

A solution of the primary standard is placed in the flask and the titrant solution that is being standardized is added with a burette.

Equivalence point of the titration of a strong acid with a strong base

100 mL of 0.1 M HCl is placed in the flask and a 0.1 M NaOH solution is gradually added through a burette, determining the pH changes of the solution that originates the hydrochloric acid.

Initially before adding NaOH the pH of the HCl solution is 1.

The strong base (NaOH) is added and the pH gradually increases, but it is still an acidic pH, since excess acid determines this pH.

If NaOH continues to be added, there comes a time when the equivalence point is reached, where the pH is neutral (pH = 7). The acid has reacted with the base being consumed, but there is no excess base yet.

The concentration of sodium chloride predominates, which is a neutral salt (neither Na ⁺ nor Cl ^- are hydrolyzed).

If NaOH continues to be added, the pH continues to increase, becoming more basic than the equivalence point, due to the predominant NaOH concentration.

Ideally the color change of the indicator should occur at pH = 7; but due to the sharp shape of the titration curve, phenolphthalein can be used which changes to a pale pink color at a pH around 8.

Weak acid-strong base titration curve

Source: Quantumkinetics, from Wikimedia Commons The titration curve for a strong acid with a strong base is similar to the image above. The only difference between the two curves is that the first one has a much more abrupt pH change; while in the titration curve of a weak acid with a strong base, it can be observed that the change is more gradual.

In this case, a weak acid, such as acetic acid (CH ₃ COOH), is titrated with a strong base, sodium hydroxide (NaOH). The reaction that occurs in the titration can be outlined as follows:

NaOH + CH ₃ COOH => CH ₃ COO ^- Na ⁺ + H ₂ O

In this case the acetate buffer with a pKa = 4.74 is formed. The buffered region can be seen in the slight and almost imperceptible change before pH 6.

The equivalence point, as the image indicates, is around 8.72 and not 7. Why? Because CH ₃ COO ^- is an anion that after hydrolyzing generates OH ^-, which basifies the pH:

CH ₃ COO ^- + H ₂ O <=> CH ₃ COOH + OH ^-

Stages

The titration of acetic acid by sodium hydroxide can be divided into 4 stages for analysis.

-Before starting to add the base (NaOH), the pH of the solution is acidic and depends on the dissociation of acetic acid.

-As sodium hydroxide is added, the acetate buffer solution is formed, but in the same way the formation of the acetate conjugate base increases, which brings with it an increase in the pH of the solution.

-The pH of the equivalence point occurs at a pH of 8.72, which is frankly alkaline.

The equivalence point does not have a constant value and varies depending on the compounds involved in the titration.

-When you continue adding NaOH, after reaching the equivalence point, the pH increases due to an excess of sodium hydroxide.

Selection of the indicator according to the equivalence point

Phenolphthalein is useful in determining the equivalence point in this titration, because it has its color turning point at a pH around 8, which falls in the pH zone of the acetic acid titration that includes the point equivalence (pH = 8.72).

On the other hand, methyl red is not useful in determining the equivalence point, since it changes color in the pH range 4.5 to 6.4.

References

1. Wikipedia. (2018). Equivalence point. Recovered from: es.wikipedia.org
2. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Equivalence Point Definition. Recovered from: thoughtco.com
3. Lusi Madisha. (April 16, 2018). Difference Between Endpoint and Equivalence Point. » DifferenceBetween.net. Recovered from: differencebetween.net
4. Look J. (2018). Equivalence Point: Definition & Calculation. Recovered from: study.com
5. Whitten, KW, Davis, RE, Peck, LP & Stanley, GG Chemistry. (2008) Eighth Edition. Cengage Learning Editors.