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In chemistry, the equivalence point is a concept that is applied to titrations or volumetric titrations. In turn, these are analytical techniques to determine the content or concentration of a substance, which is called analyte, within a sample of unknown composition. The equivalence point of a titration corresponds to the precise moment in which the number of equivalents of the titrant added is exactly equal to the number of equivalents of the analyte or titrant present in the aliquot analyzed .
In other words, it is the exact moment during a titration in which it is true that:
Seen from another point of view, it is the exact point during a titration in which the titrant and the titrant are in stoichiometric proportions according to the chemical reaction involved in the titration.
Use of the equivalence point
The purpose of any titration or volumetric titration, no matter what type, is always to find the equivalence point or, more precisely, the volume of titrant required to reach the equivalence point. This is so because said volume is what makes it possible to determine the concentration or the number of equivalents of the analyte in the sample from the known concentration of the titrating agent and, possibly, the volume of the aliquot.
Determination of the number of analyte equivalents in the aliquot
Since the number of equivalents is related to the normal concentration and volume by the equation
where V is the volume and N is the normal concentration, then the equivalence point condition can be rewritten as
Using this equation, the total number of equivalents present in the aliquot being titrated can be determined. The number of equivalents can then be converted to mass via the equivalent weight of the analyte, or to moles via the number of equivalents per mole depending on the particular titration reaction.
Determination of the normal concentration of the analyte
The equation for the equivalence condition can also be rewritten as
Where do you get that
Using this equation, the normal concentration of the sample being titrated can be obtained. Said concentration can be transformed into a molar concentration by dividing it by the number of equivalents per mole according to the particular titration reaction.
Regardless of the use that is given to said volume, the experimental procedure of the titration consists of finding the volume of the titrant at the equivalence point. However, this represents a problem, as we will see below.
The equivalence point is a theoretical point.
The equivalence point is a theoretical point that can never be known with absolute certainty during a titration. This is due, first of all, to the inevitable existence of experimental errors. These errors include both random and judgment errors related to the measurement of masses and volumes, as well as errors related to the skills of the analytical chemist when preparing solutions and carrying out the titration.
But there is a more important underlying reason why we cannot know the equivalence point in a titration: there is no way to know exactly when it has been reached, as explained in the next section.
The equivalence point is estimated by means of the end point
When during a titration we observe a change in color or the appearance of a precipitate, this indicates that we should stop the titration and note the volume of titrant added. This volume is the one that we then use as if it were the volume of the equivalence point in the previous equations.
However, it turns out that this is not actually the equivalence point. That point at which we stop the titration is actually called, conveniently, the end point of the titration . The difference between the end point and the equivalence point is that the end point is what we actually see or detect by using an indicator that undergoes an observable change, presumably at or very close to the equivalence point. . For this reason, the end point is nothing more than an experimental estimate of the equivalence point, which is a merely theoretical point.
Due to the way the different types of indicators work, they rarely undergo an observable change exactly at the equivalence point. Some change a bit earlier, leading us to underestimate the equivalence point, while others change a bit later, leading us to overestimate it. But even if we had an ideal indicator that changed exactly at the equivalence point, it would be very difficult to notice this change until we had added even a very slight excess of titrant.
For these reasons and more, the end point will never be more than an estimate, sometimes better, sometimes worse, of the true equivalence point we are looking for.
Importance of the normal concentration and the number of equivalents at the equivalence point
Many chemistry students find it difficult to understand, at first, why the concept of normal concentration and the number of equivalents exists. In addition, they are confused by the fact that the same solution can have different normal concentrations, depending on its use.
However, everything makes sense when we face titrations or volumetric titrations and the equivalence point.
Suppose a titration reaction has the following form, where A is the analyte, T the titrant, P represents the reaction products and a, b and c are the stoichiometric coefficients:
For this reaction, the point at which A and T are in stoichiometric proportions would be when it holds that
which corresponds to the equivalence point.
This equation can be perfectly used to carry out the calculations of a titration. However, in order to use it, it is essential to know the adjusted chemical equation, otherwise the stoichiometric coefficients a and b would not be available.
On the other hand, due to the way in which the numbers of equivalents are defined, both members of the previous equation end up representing the number of equivalents of A and T, which reduces this equation to the first one that we showed at the beginning of this article. for any titration reaction that is involved, as long as it is of the same type.
For example, if the number of equivalents of an acid are known, they will react with the same number of equivalents of a base, no matter what the acid is or what the base is (as long as it is an acid-base reaction). .
Likewise, the number of equivalents of an oxidizing agent in a redox titration will always be equal to the number of equivalents of the reducing agent, no matter what they are, as long as they are involved in a redox reaction.
In this way, the procedure to make the calculations related to the equivalence point is simplified, since it is not necessary to adjust the chemical equations of the titration if we work with equivalents and normality, something that would be necessary to work with moles and molarity.
References
Volumetric analysis – Titration – Acid / base indicators . (nd). National University of Rosario. https://www.fbioyf.unr.edu.ar/evirtual/pluginfile.php/131892/course/section/4402/titulacion%202021.pdf
Byjus. (2021, March 22). General Data Protection Regulation(GDPR) Guidelines BYJU’S . https://byjus.com/chemistry/difference-between-endpoint-and-equivalence-point/
Chang, R. (2012). Chemistry (11th ed .). McGraw-Hill Education.
glossaries. (2017, June 12). Normality (Chemistry) . specialized glossaries. https://glosarios.servidor-alicante.com/quimica/normalidad
Skoog, D.A., West, D., Holler, J., & Crouch, S. (2014). Fundamentals of Analytical Chemistry (9th ed .). Cengage Learning.
Teixidó, CM (2020, June 19). The old chemical normality and the new normality of deconfinement . Research and Science. https://www.investigacionyciencia.es/blogs/fisica-y-quimica/24/posts/la-vieja-normalidad-qumica-y-la-nueva-normalidad-del-desconfinamiento-18735
