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A topic from the subject of Quantification in Chemistry.

  • 11 months ago
  • 9 min read
Introduction

Acid-base titrations are common techniques in chemistry used to determine the concentration of a given acid or base. By neutralizing the acid or base sample with a known quantity of base or acid of known concentration, one can deduce the concentration of the substances in question.

Basic Concepts
Understanding Acid-Base Reactions

In an acid-base reaction, an acid reacts with a base to form a salt and water. This neutralization process is the basis of acid-base titrations.

The pH Scale

The pH scale measures acidity or alkalinity. It ranges from 0 to 14, with 0 being the most acidic, 14 being the most alkaline, and 7 being neutral. Acid-base titrations usually involve monitoring the change in pH.

End Point and Equivalence Point

The equivalence point is when the amount of added titrant is just enough to fully neutralize the analyte solution. The end point is reached when the indicator changes color. The end point is an approximation of the equivalence point.

Equipment and Techniques
Burettes

A burette is a long, graduated tube with a tap at one end, used to add the titrant to the analyte.

pH Meter

A pH meter measures the solution's pH throughout the titration process, providing a more precise determination of the equivalence point than indicators alone.

Indicators

Indicators are chemicals that change color at or near the equivalence point. Phenolphthalein and bromothymol blue are commonly used indicators in acid-base titrations. The choice of indicator depends on the pH at the equivalence point.

Types of Titrations
Strong Acid-Strong Base Titrations

This involves titrating a strong acid with a strong base (or vice versa). The equivalence point occurs at pH 7.

Weak Acid-Strong Base Titrations

This involves titrating a weak acid with a strong base. The pH at the equivalence point is greater than 7.

Weak Base-Strong Acid Titrations

This involves titrating a weak base with a strong acid. The pH at the equivalence point is less than 7.

Data Analysis

Data from acid-base titrations typically includes the volume of titrant added and the pH of the solution. This data is used to create a titration curve (a graph of pH versus volume of titrant). The equivalence point is determined from the steepest point of the curve.

Applications

Acid-base titrations are used in various fields, including food and beverage quality control, pharmaceutical manufacturing, clinical chemistry, environmental testing, and academic research.

Conclusion

Acid-base titrations are fundamental tools in analytical chemistry. By understanding the basic concepts, mastering the equipment and techniques, and properly analyzing the data, one can accurately determine the concentration of an unknown acid or base solution.

Overview of Acid-Base Titrations

Acid-base titrations are crucial analytical procedures widely used to determine the concentration of an unknown acid or base solution. They are based on the neutralization reaction between an acid and a base, which produces a neutral salt and water.

Main Concepts in Acid-Base Titrations

The primary concepts in acid-base titrations include:

  • Titration: A method used in chemistry to determine the concentration of a substance in a solution by reacting it with a solution of known concentration.
  • Acid: A substance that donates hydrogen ions (H+), also known as a proton donor.
  • Base: A substance that accepts hydrogen ions (H+) or donates hydroxide ions (OH-), also known as a proton acceptor.
  • Neutralization: A chemical reaction where an acid and a base react quantitatively to form water and a salt. The pH of the resulting solution is approximately 7.
  • Equivalence Point: The point in a titration where the amount of acid and base are stoichiometrically equivalent; the moles of H+ equal the moles of OH-. This is a theoretical point determined by calculations.
  • Endpoint: The point in a titration where the indicator changes color, signaling the approximate completion of the reaction. The endpoint is experimentally determined and may differ slightly from the equivalence point.
  • Indicator: A substance that changes color at or near the equivalence point, visually indicating the endpoint of the titration. The choice of indicator depends on the pH range of the equivalence point.
Types of Acid-Base Titrations

Acid-base titrations can be classified into different types depending on the nature of the acid and base involved:

  • Strong Acid-Strong Base Titration: Involves a strong acid and a strong base. The equivalence point is at pH 7.
  • Weak Acid-Strong Base Titration: Involves a weak acid and a strong base. The equivalence point is above pH 7.
  • Strong Acid-Weak Base Titration: Involves a strong acid and a weak base. The equivalence point is below pH 7.
  • Weak Acid-Weak Base Titration: Involves a weak acid and a weak base. These titrations are less common due to the lack of a sharp change in pH at the equivalence point, making it difficult to accurately determine the endpoint.
Procedure for Acid-Base Titration
  1. A known volume of the solution with unknown concentration (analyte) is placed in a flask.
  2. A few drops of a suitable indicator are added to the flask.
  3. The titrant (solution with known concentration) is added gradually from a buret to the analyte solution.
  4. The solution is continuously stirred to ensure complete mixing.
  5. The addition of the titrant is stopped when the indicator changes color, indicating the endpoint of the titration.
  6. The volume of titrant used is recorded.
  7. The concentration of the unknown solution is calculated using the stoichiometry of the neutralization reaction and the volume and concentration of the titrant.
Calculations in Acid-Base Titrations

The concentration of the unknown solution is calculated using the following equation:

MaVa = MbVb

Where:

  • Ma = molarity of the acid
  • Va = volume of the acid
  • Mb = molarity of the base
  • Vb = volume of the base

This equation is based on the principle of stoichiometry, where the moles of acid and base are equal at the equivalence point.

Experiment: Standardization of a Sodium Hydroxide Solution using Oxalic Acid

In this experiment, we will titrate a NaOH (Sodium Hydroxide) solution with a standard solution of Oxalic Acid to determine the concentration of the NaOH. The reaction that occurs is:

(COOH)2 (aq) + 2NaOH (aq) → (COONa)2 (aq) + 2H2O (l)
Materials Needed:
  • Sodium Hydroxide Solution (approximately 0.1 M, exact concentration not known)
  • Oxalic Acid (H2C2O4) - we will prepare a 0.1 M solution
  • Phenolphthalein Indicator
  • Burette
  • Conical (Erlenmeyer) Flask
  • Pipette (25.00 mL)
  • Distilled Water
  • Weighing balance
Procedure:
  1. First, prepare a 0.1 M solution of Oxalic Acid by accurately weighing approximately 6.3g (calculate the precise mass needed based on the desired volume and molar mass of oxalic acid dihydrate if using the dihydrate form) and dissolving it in 1 liter of distilled water in a volumetric flask.
  2. Next, rinse the burette with the solution it will be dispensing – in this case, the Sodium Hydroxide. This will ensure our results are not skewed by any remaining water.
  3. Fill the burette with the NaOH solution.
  4. Pipette 25.00 mL of the Oxalic Acid solution into the Erlenmeyer flask.
  5. Add 2-3 drops of phenolphthalein indicator to the flask. The solution will be colorless.
  6. Slowly add the NaOH solution from the burette to the flask, continuously swirling the flask to mix. The solution will remain colorless until the endpoint is reached.
  7. Observe the color of the solution in the flask. Stop adding NaOH when one drop causes the solution to turn from colorless to a persistent light pink color that lasts for at least 30 seconds. This is called the "end point".
  8. Record the volume of NaOH dispensed from the burette.
  9. Repeat steps 4-7 at least two more times to obtain multiple measurements and improve accuracy.
Calculations and Significance

At the end point, the number of moles of Oxalic Acid is equal to half the number of moles of Sodium Hydroxide (due to the stoichiometry of the reaction). We can calculate the concentration (molarity) of the Sodium Hydroxide solution using the following formula: Molarity (NaOH) = (2 * Molarity (H2C2O4) * Volume (H2C2O4)) / Volume (NaOH) Knowing the exact concentration of the Sodium Hydroxide solution is essential in many laboratory scenarios. It can now be used to find concentrations of other unknown solutions through additional titrations. Remember to average the results from multiple titrations for a more accurate molarity.

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