Theory Involved in Titrations of Strong, Weak, and Very Weak Acids and Bases and Neutralization Curves : Pharmaceutical Guidelines -->

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Theory Involved in Titrations of Strong, Weak, and Very Weak Acids and Bases and Neutralization Curves

Strong acid and strong base, Strong acid and weak base, Weak acid and strong base, Weak acid and weak base and Neutralization curve.
Most titrations are conducted to calculate concentrations of unknowns with knowns (titrants or standards). Using the stoichiometry of the reaction and knowing how much of a first species there is, it is easy to find out how much of a second species there will be; the trick is to determine when the two species are stoichiometrically equivalent. Indicators, which are substances that change color when the reaction is complete, can be used as a way to determine the outcome. Titration is composed of adding one reactant incrementally until stoichiometric proportions are reached, and then measuring how much of the analyte is added to reach a given level of concentration. In addition to measuring the mass of analytes, the molar mass of analytes can also be determined that way.

Potentiometric and indicator acid-base titrations are the two most common types. The titration method involves adding another chemical that changes color when the acid and base reach an equivalence point, assuming the pHs of both acids and bases are the same. Potentiometric titration involves recording the pH while adding the titrant, and determining the KaKa or KbKb. value of an analyte that is weakly acidic or basic.

Titrant (the compound added to the analyte in a burette) always has a high concentration, whereas analyte can have low levels. Therefore, the titrant must react completely during the titration. Therefore, the molarity times the volume of the titrant will equal the molarity times the volume of the titrant (n=MV). The titrant can be consumed up to the triple point and therefore the salt produced by the consumption can be calculated. The "initial" concentration can be calculated based on how much titrant you added when the salt reacted to form the acid in its conjugate relationship.

Strong acid and strong base
The phenolphthalein indicator is used for measuring the strength of acid and base. Its color changes when the pH ranges between 8.3 and 10. Phenolphthalein is selected for its pH range of 8.3 - 10. If it is placed in a basic solution, it appears pink; when it is placed in an acidic solution, it appears clear. It is possible to cause pH transitions within fractions of a drop of actual neutralization for acids and bases with high acidity. By using a burette, reactants are added. The analyte is deposed in an Erlenmeyer flask containing the reaction product of unknown concentration. Other reactants of known concentration are stored in a burette to be released during the reaction. Titrants are used for this purpose. As the analyte is added to the Erlenmeyer flask, the indicator has been added --phenolphthalein.

When two acids react with one another, a salt is formed and a neutralized base is produced. An example is the formation of sodium chloride and water by hydrochloric acid and sodium hydroxide:

HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)H

Titration is based on neutralization. Equivalence is determined by the mole ratio of bases and acids as indicated by a pH indicator. In the case of a strong acid and strong base titration, it is often incorrectly assumed that neutralization should lead to a pH 7.0 solution.

Strong acid and weak base
In a titration, the amount of one compound is measured by adding measured amounts of another compound (normally using a burette) until an indicator becomes visible, indicating the reaction is complete. Acidic solutions are buffers that serve to maintain a liquid's pH level. When acid and base have been mixed in equal amounts, the equivalent point in the chemical reaction has been reached.

Strong acid-weak base titrations are produced by reacting ammonia (a weak base) with hydrochloric acid (a strong acid) in the aqueous phase:

NH3 (aq) + HCl (aq) → NH+4(aq) + Cl(aq)

Titration usually occurs between the acid and the base. Upon taking the burette, the titrant is added to a small amount of acid solution of known concentration. Using a measured volume of the base with an unknown concentration, an Erlenmeyer flask is filled with the analyte, and if the pH can be measured using an electrode, the titration curve can be calculated based on the pH and volume measurements.

In a titration with strong acid into a weak base, the pH of the base is normally high and falls rapidly when the acid is added. When the equivalence point is reached, the pH will change more slowly, but eventually, one drop will change it rapidly. Chemical indicators, such as methyl orange, will change their color from basic to acidic when exposed to acidic environments.

It is not pH 7 that is the equivalence point in strong acid-weak base titration, but pH 6 or lower. Due to the hydrogen evolution reaction, while titrating, conjugate acids will be formed. These acids will react with water to produce hydronium ions (H3O+).

The conjugate acid formed from HCl titration into ammonia solution (NH4+) behaves as follows:

NH+4 + H2O → H3O+ + NH3

Weak acid and strong base
A titration takes place when specific reactants are combined -- in this case a strong base and a weak acid. Titration curves show the strength of corresponding acids and bases over time, as well as their pH change during titration. The pH changes slowly and gradually when the strong base is titrated with a weak acid, as shown by the titration curve. Titration approaches the point of equivalence, thus forming a buffer system.

A titration of NaOH and HCl, for example, will follow the same curve at equivalence and beyond. Any system dominated by NaOH changes pH when its concentration is overexposed to NaOH.

Titrations of weak acids in water consistently end up with pH values about the same as those of the solution before the experiment. Equivalence is achieved by neutralizing all weak acids and converting them into their conjugate bases. The pH at the equilibrium point is not 7, however. A conjugate base is produced during titration, which is why this occurs. This leads to slightly basic solutions being formed. A functional equivalence point and the endpoint are fundamentally different because a functional equivalence point is determined by the reaction stoichiometry and the endpoint by the color change related to the indicator.

Example
Acetic acid (HC2H3O2) titration with NaOH

HC2H3O2 + OH→ H2O + C2H3O−2

This reaction causes the OH– from the acetic acid to react with the H+ from acetate to form the acetate ion (C2H3O2–). A slightly basic solution is formed when this conjugate base reacts with water.

C2H3O−2 + H2O → HC2H3O2 + OH

Weak acid and weak base
Equally strong products will have a neutral pH if they are both of the same strength. A pH of neutral will result if both substances are of equal strength. Observers will have difficulty identifying color changes with the indicator because the change of color is often rapid, and therefore hard to see.

Neutralization curve
pH is plotted against the volume of alkali (mL) when measuring neutralization or titration. The curve is generated by alkali (or acid) additions, which result from 'potentiometric titration'. Near the equivalence point of the titration, there is a sudden and very sharp change in pH. This is the most distinguishing characteristic of the curve. This will occur around pH 7 for acid with high alkalinity. To locate the approximate equivalence point, preliminary titrations are necessary to find either the acid or base concentration. An accurate titration is then conducted. It is optimal to use indicators between 4.5 and 9.5.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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