Endpoint and equivalence point relationship

The difference between 'end point' and 'equivalence point' in a titration | Yeah Chemistry

endpoint and equivalence point relationship

The equivalence point, or stoichiometric point, of a chemical reaction is the point at which The endpoint (related to, but not the same as the equivalence point) refers to the point at which the indicator changes colour in a colourimetric titration . The endpoint of a titration is the point where the indicator just changes colour. The equivalence point is when the ratio of the reactants is in the. The end point of a titration indicates once the equivalence point has been . The graphs of titration curves effectively show the relationship.

In some reactions, there could be numerous equivalence points, especially in polyprotic acids and bases where multiple hydroxide ions exist. This article unfolds the key difference between the endpoint and equivalence point in chemical titrations.

endpoint and equivalence point relationship

Definition of Equivalence point Equivalence point in acid-base titration signals the completion of the reaction where the number of moles of the titrant and the analyte are equal as in the chemical equation. This point must be attained accurately by administering a number of drops of the standard solution to the unknown concentration.

Difference Between Endpoint and Equivalence Point

A pipette is usually used to pour drops of the titrant into the measuring flask where the analyte has been poured with a certain indicator. Indicators are important in acid-base titrations to clearly see the equivalence and endpoints. Acid and base titration can occur between a strong base and weak base; strong base and weak acid; strong base and strong base; or weak polyprotic acids. A different indicator may be used depending on the typical of solution being used for titration. If the pH of the titrant corresponds with the pH at equivalence point, the endpoint and equivalence point can occur simultaneously.

The picture below gives a clear understanding of titration set up in order to reach the equivalence point and then later reach the endpoint when the color changes.

  • Equivalence point
  • Equivalence Point
  • Difference Between Equivalence Point and Endpoint

At first, the solution with the unknown concentration is poured with the indicator. Then the titrant is added using the burette by controlling the drops with the valve. Definition of Endpoint Endpoint comes after the equivalence point in titrations. It indicates that the equivalence point has been achieved. This endpoint is indicated by the change in color of the solution. See the picture below: Chemistry LibreTexts To reach the endpoint, the amount of drops should be administered carefully because a single drop can change the pH of the solution.

pH Titration - Data Analysis in Excel

In situation where the endpoint has been passed, a Back Titration or reverse titration can be performed depending on the nature of the solution. If too much of the titrant has been poured, the endpoint might be passed.

The solution will be to add another solution of a different reactant in excess. Indicators are not always used in titrations.

endpoint and equivalence point relationship

The accuracy of titration will be high when the end point is quite close to the equivalence point. For example in the acid-base titration of a strong acid with a strong base, the equivalence point is at pH 7.

The indicator used for this titration is phenolphthalein which starts the color change at pH slightly above 8. Hence, in this reaction, the color change is noticed only when small excess amount added of titrant which introduces small positive error in the titration.

endpoint and equivalence point relationship

In other words, both equivalence point and the end point is different as equivalence point corresponds to the theoretical completion of the reaction while the endpoint is related to the actually measured physical change in the solution determined by an indicator or an instrument mentioned above.

The difference between the endpoint and the equivalence point of the titration is called as an indicator error. So equivalence point will comes at pH seven when both solutions have been mixed in exactly the right proportions. While at the end point indicator will show the color changethat will happen when the concentration of one the solution will more than other one. In any acid-base titration; titration curve based on the concentration of both reactants; acid as well as the base.

Titration Curve Equivalence Point There are total four possibilities with the different strength of acid and base. Strong acid and strong base Strong acid and weak base Weak acid and strong base Weak acid and weak base In each combination, there will be two titration curve depends on the titrant.

Strong acid and strong base: After equivalence point there is a steep plunge in titration curve.

Acid-base titration curves

In each combination, there will be two titration curve depends on the titrant. After equivalence point, there is a steep plunge in titration curve. On the other hand, if we add base solution to the acid solution, as expected the pH of solution increase.

endpoint and equivalence point relationship

As in the previous curve, here also the pH doesn't change very much until you get close to the equivalence point. Then it surges upwards very steeply. Methyl orange as well as phenolphthalein both acts as a good indicator for this titration. No doubt, neither indicator changes color at the equivalence point but the curve is so steep for a long pH which covers the range from methyl orange to phenolphthalein.

Strong acid and weak base: Titration of hydrochloric acid with ammonia gives ammonium chloride. For this combination, methyl orange is the perfect indicator as phenolphthalein does not show change in color.

Equivalence point - Wikipedia

Weak acid and strong base: If titration started with an excess of acid the equivalence point obtained at the formation of buffer solution containing sodium ethanoate and acetic acid results a large fall in pH value. If we started with excess of base the reaction as well as titration curve will be exactly same as in case of strong acid and base. Weak acid and weak base: There is no suitable indicator for this titration as phenolphthalein gets finish before equivalence point while methyl orange falls off the curve altogether.

Hence this titration will continue without indicator only.