Titration

An acid–base titration is the determination of the concentration of an acid or base by exactly neutralizing the acid or base with an acid or base of known concentration. The analyte (titrand) is the solution with an unknown molarity. The reagent (titrant) is the solution with a known molarity that will react with the analyte. The equivalence point is the point in the titration at which the amount of titrant added is just enough to completely neutralize the analyte solution. At the equivalence point in an acid-base titration, moles of base = moles of acid and the solution only contains salt and water. The term end point is where the indicator changes colour. As you will see on the page about indicators, that isn’t necessarily exactly the same as the equivalence point. The analyte is prepared by dissolving the substance being studied into a solution. The solution is usually placed in a flask for titration. A small amount of indicator is then added into the flask along with the analyte. The reagent is usually placed in a burette and slowly added to the analyte and indicator mixture. The amount of reagent used is recorded when the indicator causes a change in the color of the solution. pH indicators are frequently weak acids or weak bases. The indicator is present in such a small amount that it doesn’t affect the solution’s pH. The general reaction scheme of a pH indicator can be formulated as:

H-Ind  +  H2  H3O+  +  Ind

 

 

A pH value is determined from the negative logarithm of this concentration and is used to indicate the acidic, basic, or neutral character of the substance you are testing.

 

  

It is important here to note that the equation expressed above is in equilibrium, meaning Le Chatelier’s principle applies to it. Thus, as the concentration of H3O+ increases or decreases, the equilibrium shifts to the left or right accordingly.

Neutralization is a chemical reaction in which an acid and a base react quantitatively with each other. The neutralization of a strong acid and strong base has a pH equal to 7. The neutralization of a strong acid and weak base will have a pH of less than 7, and conversely, the resulting pH when a strong base neutralizes a weak acid will be greater than 7.

A titration curve is the plot of the pH of the analyte solution versus the volume of the titrant added as the titration progresses.

 

The following titration curves is when the acid is being run into the base. Titration curve for strong acid with a strong base is shown below.

You can see that the pH only falls a very small amount until quite near the equivalence point. Then there is a really steep plunge.

The titration curve for a strong acid with a weak base is shown below:

Because you have got a weak base, the beginning of the curve is going to be different. However, once you have got an excess of acid, the curve is essentially the same as before. At the very beginning of the curve, the pH starts by falling quite quickly as the acid is added, but the curve very soon gets less steep. This is because a buffer solution is being set up – composed of the excess weak base and the product salt being formed. Notice that the equivalence point is now a bit acidic because the salt produced isn’t neutral. However, the equivalence point still falls on the steepest bit of the curve. That will turn out to be important in choosing a suitable indicator for the titration.

The titration curve for weak acid with a strong base is shown below:

For the first part of the graph, you have an excess of sodium hydroxide. The curve will be exactly the same as when you add hydrochloric acid to sodium hydroxide. Once the acid is in excess, there will be a difference.

The titration curve for weak acid with a weak base is shown below:

This is really just a combination of graphs you have already seen. Up to the equivalence point it is similar to the strong acid- weak base titration curve. After the equivalence point it is like the end of the weak acid – strong base curve. Notice that there isn’t any steep bit on this graph. Instead, there is just what is known as a point of inflexion. That lack of a steep bit means that it is difficult to do a titration of a weak acid against a weak base.

There is a particular titration known as a redox titration. While Brønsted acid-base titrations involve proton transfers, redox titrations involve electron transfers. In this type of titration, One species gains electrons while the other species loses electrons. Redox is a combination of the words reduction and oxidation. Reduction reactions refer to the reduction in charge of a chemical species. It can also refer to the decrease oxidation number of that species or if it has gained electrons. Oxidation reactions refer to the increase in charge of a chemical species. It can also refer to the increase oxidation number of that species or if it has lost electrons.

5H2O2  +  2MnO4  +  6H+ → 2Mn2+  +  5O2  +  8H2O

Normally oxygen has an oxidation state of -2, but in peroxides, it is -1. The reactants here include a peroxide. Oxygen, and anything else in its elemental state has an oxidation number of 0. The product O2 is one such case. Hydrogen is always +1 unless it is a hydride, in which case it’s negative 1. For this reaction, all hydrogens are +1. The reactant Mn has an oxidation number of +7.

The half reactions (reactions that depict electron transfer only) are as follows:

Reduction: Mn7+ + 5e- → Mn2+

Oxidation: O- → O0 + e-

 

References

1) Katherine Dunn, H. W. (2017, February 12). Retrieved from Chemistry LibreTexts: https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Acids_and_Bases/Acids_and_Bases_in_Aqueous_Solutions/Water_Autoionization

2) Hayek, D. (2014, April). Conjugate Acids and Conjugate Bases. Retrieved from Socratic: https://socratic.org/chemistry/acids-and-bases/conjugate-acids-and-conjugate-bases

3) Chieh, C. (2016, March). Retrieved from University of Waterloo : http://www.science.uwaterloo.ca/~cchieh/cact/c123/salts.html

4)

California, U. o. (2016, December 14). The Formation of Complex Ions. Retrieved from https://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Chemistry_(Averill_and_Eldredge)/17%3A_Solubility_and_Complexation_Equilibria/17.3%3A_The_Formation_of_Complex_Ions

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