๐ Introduction to Titration
Titration is a quantitative chemical analysis technique used to determine the concentration of an identified analyte. In the context of acid-base chemistry, it involves gradually adding a known concentration of an acid or base (the titrant) to a solution containing the unknown concentration of a base or acid (the analyte) until the reaction is complete. The point at which the reaction is complete is known as the equivalence point. When titrating a weak acid with a strong base, the equivalence point will be at a pH greater than 7 due to the formation of the conjugate base of the weak acid.
๐ Historical Context
The concept of titration dates back to the late 18th century, with early applications in the analysis of acids and bases. The method became more refined in the 19th century with the development of improved glassware and indicators. Karl Friedrich Mohr is often credited with popularizing the method in its modern form.
๐ Key Principles
- โ๏ธ Stoichiometry: Understanding the balanced chemical equation for the reaction between the weak acid and the strong base is critical for calculating the concentration of the analyte.
- ๐งช Equivalence Point: The point at which the moles of titrant added are stoichiometrically equal to the moles of analyte in the solution. For a weak acid-strong base titration, this occurs at a pH > 7.
- ๐ Indicator Selection: Choosing an appropriate indicator that changes color near the equivalence point is essential for accurate endpoint determination. Phenolphthalein is commonly used because it changes color around pH 8.3-10.0.
- ๐ Titration Curve: A plot of pH versus volume of titrant added. It shows the gradual change in pH as the strong base is added to the weak acid. The curve helps visualize the buffering region and equivalence point.
๐งช Lab Procedure: Titration of a Weak Acid with a Strong Base
- โ๏ธ Materials:
- ๐ฌ Weak acid solution of unknown concentration (e.g., acetic acid, $CH_3COOH$)
- NaOH solution of known concentration (the titrant)
- ๐ง Distilled water
- ๐งช Indicator solution (e.g., phenolphthalein)
- ๐ถ Beakers
- ๐บ Erlenmeyer flask
- ๐ Burette
- ๐๏ธ Burette clamp
- ๐ฅข Stirring rod or magnetic stirrer
- ๐ Volumetric pipette
- pH meter (optional, for more precise determination)
- ๐ Procedure:
- ๐ข Preparation: Clean and rinse all glassware with distilled water. Fill the burette with the standardized NaOH solution, ensuring there are no air bubbles. Record the initial burette reading.
- ๐ Analyte: Pipette a known volume (e.g., 25.00 mL) of the weak acid solution into the Erlenmeyer flask.
- ๐ Indicator: Add 2-3 drops of phenolphthalein indicator to the flask. The solution should remain colorless.
- ๐ง Titration: Place the flask under the burette. Slowly add the NaOH solution to the weak acid solution while constantly stirring.
- โฑ๏ธ Endpoint: As you approach the expected endpoint, add the NaOH dropwise. The endpoint is reached when a faint, permanent pink color appears and persists for at least 30 seconds.
- ๐ Reading: Record the final burette reading. Calculate the volume of NaOH used by subtracting the initial reading from the final reading.
- ๐ Repeat: Repeat the titration at least three times to ensure accuracy and precision.
- ๐งฎ Calculations:
- ๐ข Calculate the moles of NaOH used: $moles_{NaOH} = Molarity_{NaOH} \times Volume_{NaOH}$ (in Liters)
- โ๏ธ Use the stoichiometry of the reaction to determine the moles of weak acid in the Erlenmeyer flask. For a monoprotic weak acid (HA) reacting with NaOH: $HA + NaOH \rightarrow NaA + H_2O$, the mole ratio is 1:1.
- โ Calculate the concentration of the weak acid: $Molarity_{HA} = \frac{moles_{HA}}{Volume_{HA}}$ (in Liters)
โ๏ธ Real-World Examples
- ๐ Vinegar Analysis: Determining the acetic acid concentration in vinegar.
- ๐งช Pharmaceutical Analysis: Determining the purity of weak acid drugs.
- ๐ท Wine Analysis: Measuring the titratable acidity of wine.
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Conclusion
Titration of a weak acid with a strong base is a fundamental analytical technique in chemistry. By understanding the principles and following the lab procedure carefully, accurate determination of unknown concentrations can be achieved. Proper technique, careful observation, and precise calculations are key to successful titration.
โ Practice Quiz
- ๐งช What is the purpose of titration?
- ๐งฎ At the equivalence point in a weak acid/strong base titration, is the pH equal to 7? Why or why not?
- ๐ What is the purpose of an indicator in titration?
- ๐ง Why is it important to rinse the glassware with distilled water before performing a titration?
- ๐ What is the role of stoichiometry in calculating the concentration of the unknown acid?
- โฑ๏ธ Why should the NaOH be added dropwise near the expected endpoint?
- ๐ Sketch a typical titration curve for the titration of a weak acid with a strong base. Label the axes and important points (e.g., equivalence point, buffering region).