How do acid-base titrations work?

Acid-base titrations determine the unknown concentration of an acid or base solution by neutralizing it with a solution of a known concentration.

Understanding Acid-Base Titrations

Acid-base titrations are a quantitative chemical analysis technique used to determine the concentration of an unknown acid or base solution. The process involves the gradual addition of a solution with a precisely known concentration (the titrant, or standard solution) to the solution being analyzed (the analyte or titrand) until the reaction between them is complete. The completion of the reaction is typically indicated by a color change in an indicator or by monitoring the pH.

The Process Explained:

1. Preparation:

   • A known volume of the solution with an unknown concentration (analyte) is placed in a flask.
   • A few drops of an appropriate acid-base indicator are added to the analyte solution. The indicator is a substance that changes color depending on the pH of the solution.
   • The titrant (standard solution) with a precisely known concentration is placed in a burette. A burette is a graduated glass tube with a tap at one end, used for delivering known volumes of a liquid, especially in titrations.

2. Titration:

   • The titrant is slowly added to the analyte while the solution is constantly mixed.
   • The pH of the solution in the flask changes as the titrant reacts with the analyte.

3. Endpoint and Equivalence Point:

   • The endpoint is the point in the titration when the indicator changes color, signaling that the reaction is complete. Ideally, the endpoint should coincide with the equivalence point.
   • The equivalence point is the point at which the acid and base have completely neutralized each other. At this point, the number of moles of acid is stoichiometrically equal to the number of moles of base.

4. Calculations:

   • The volume of the titrant used to reach the endpoint is recorded.
   • Using the known concentration of the titrant and the volume used, the number of moles of titrant that reacted can be calculated.
   • From the balanced chemical equation for the reaction between the acid and base, the number of moles of analyte that reacted can be determined.
   • Finally, the concentration of the unknown analyte can be calculated by dividing the number of moles of analyte by the initial volume of the analyte solution.

Example

Let's say you want to determine the concentration of a hydrochloric acid (HCl) solution using a standard solution of sodium hydroxide (NaOH).

1. You would add a known volume of the HCl solution to a flask.
2. Add a few drops of phenolphthalein indicator. (Phenolphthalein is colorless in acidic solutions and pink in basic solutions.)
3. Slowly add NaOH from the burette until the solution in the flask turns a faint pink color that persists for at least 30 seconds. This is the endpoint.
4. Record the volume of NaOH used.
5. Use the equation: M₁V₁ = M₂V₂, where M₁ is the molarity of the acid, V₁ is the volume of the acid, M₂ is the molarity of the base, and V₂ is the volume of the base used to calculate the molarity of the HCl.

Key Components and Their Roles:

Component	Role
Analyte (Titrand)	The solution with the unknown concentration being analyzed.
Titrant (Standard)	The solution with a precisely known concentration used to titrate the analyte.
Indicator	A substance that changes color to indicate the endpoint of the titration.
Burette	A graduated glass tube used to accurately deliver the titrant.

Common Indicators

• Phenolphthalein
• Methyl Orange
• Bromothymol Blue

Acid-base titrations involve the controlled neutralization of an acid or base with a solution of known concentration to determine the concentration of the unknown solution, utilizing indicators or pH monitoring to signal the endpoint of the reaction.