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A topic from the subject of Standardization in Chemistry.

  • 1 year ago
  • 6 min read

Primary Standards and Secondary Standards in Titration

Introduction

Titration is a laboratory technique used to determine the concentration of a solution by reacting it with another solution of known concentration. The sample solution is known as the "analyte," while the solution of known concentration is known as the "titrant." In order for titrations to be accurate, the concentration of the titrant must be known with high accuracy. Primary and secondary standards are used to ensure that the concentration of the titrant is as accurate as possible.

Basic Concepts

Primary Standard

A primary standard is a substance that is highly pure and has a known concentration. It can be weighed accurately and dissolved in a solvent to create a solution of known concentration. This standard is used to standardize solutions of other substances, such as the titrant.

Secondary Standard

A secondary standard is a substance that is not as pure as a primary standard, but it is still accurate enough to be used to standardize other solutions. They are less expensive and easier to acquire than primary standards.

Equipment and Techniques

Equipment

Several pieces of equipment are used in titration, including:

  • Volumetric flasks
  • Pipettes
  • Burettes
  • pH meter (for acid-base titrations)
  • Weighing paper
  • Analytical balance

Techniques

A technician needs to master techniques such as:

  • Weighing accurately
  • Preparing solutions
  • Using volumetric flasks and pipettes
  • Measuring the volume of a solution
  • Using a burette to deliver a precise volume of a solution
  • Recording data accurately
  • Calculating the concentration of an unknown solution

Types of Experiments

Titrations may be used to determine the concentration of a solution of acid, base, or oxidizing agent. Few examples are:

Acid-Base Titration

In an acid-base titration, the solution of known concentration is a strong acid (e.g., hydrochloric acid) or strong base (e.g., sodium hydroxide). The endpoint is the point at which the solution changes color, indicating that the reaction has reached completion. The change in color is caused by the indicator, which is a weak acid or base.

Redox Titration

Redox reactions involve the transfer of electrons between atoms or ions. In a redox titration, the solution of known concentration is an oxidizing agent (e.g., potassium permanganate) or reducing agent (e.g., sodium thiosulfate). The endpoint is detected using an indicator, or by the change in color of the solution itself.

Data Analysis

The data from a titration is plotted on a graph. The x-axis of the graph shows the volume of titrant added, while the y-axis of the graph shows the pH or the concentration of the titrant. The endpoint of the titration is the point at which the two lines intersect.

Applications

  • To determine the concentration of a new solution or reagent.
  • In environmental monitoring to determine the concentration of pollutants in air, water, and soil.
  • In the food industry to determine the acidity or alkalinity of food products.
  • In the pharmaceutical industry to ensure the quality and consistency of drug products.
  • In chemical research for various investigations and experiments.

Conclusion

Titrations are an important technique used to determine the concentration of solutions. Primary and secondary standards are essential for ensuring the accuracy of these titrations. By understanding how to use primary and secondary standards, you can ensure that your titrations are as accurate as possible.

Primary Standards and Secondary Standards in Titration

Titration is a quantitative analytical technique where a solution of known concentration (the titrant) is reacted with a solution of unknown concentration (the analyte) until the reaction is complete. The point at which the reaction is complete is called the equivalence point. At the equivalence point, the moles of titrant added are stoichiometrically equal to the moles of analyte present.

Primary Standards

Primary standards are highly pure substances used to prepare solutions of accurately known concentration. These solutions are then used to standardize other solutions. A substance qualifies as a primary standard if it meets the following criteria:

  • High purity (typically >99.9%): Minimal impurities to ensure accurate molar mass determination.
  • Stable composition: Does not decompose or change composition over time under normal storage conditions.
  • Well-defined chemical formula: The molar mass can be calculated precisely from its formula.
  • Readily available in pure form: Easily obtained and purified.
  • High molar mass: Minimizes weighing errors.
  • Reacts rapidly and completely with the analyte: Ensures a sharp endpoint in the titration.
  • Does not absorb moisture or carbon dioxide from the air (non-hygroscopic): Maintains a constant composition.

Examples of common primary standards include potassium hydrogen phthalate (KHP) for acid-base titrations and sodium chloride (NaCl) for precipitation titrations.

Secondary Standards

Secondary standards are solutions whose concentrations have been determined by standardization against a primary standard. They are less precise than primary standard solutions because their concentration is dependent on the accuracy of the primary standard used to standardize them. Secondary standards are convenient for routine analysis because they eliminate the need to weigh out a primary standard each time a titration is performed.

Key Differences Summarized:

  • Primary Standards: Pure substances used to standardize other solutions. Their concentration is determined directly from their mass and molar mass.
  • Secondary Standards: Solutions whose concentrations have been determined by titration against a primary standard. Their concentration is indirectly determined.
  • Equivalence Point: The point in a titration where the moles of titrant equal the moles of analyte, according to the stoichiometry of the reaction.
Primary Standards and Secondary Standards in Titration
Objective

To demonstrate the use of primary and secondary standards in titration and determine the concentration of a secondary standard solution.

Materials
  • Primary standard: Potassium hydrogen phthalate (KHP), pure, dried
  • Secondary standard: Sodium carbonate (Na2CO3), reagent grade
  • Analytical balance
  • Buret (e.g., 25 mL)
  • Volumetric flask (e.g., 100 mL)
  • Pipet (e.g., 25 mL)
  • Erlenmeyer flask (e.g., 250 mL)
  • Phenolphthalein indicator solution
  • Distilled water
Procedure
  1. Preparation of Primary Standard Solution: Accurately weigh approximately 0.8-1.2 g of dried KHP using an analytical balance. Record the exact mass. Quantitatively transfer the KHP to a 100 mL volumetric flask. Dissolve the KHP completely in distilled water and dilute to the mark. Calculate the exact molar concentration of the KHP solution. The molar mass of KHP is 204.22 g/mol.
  2. Preparation of Secondary Standard Solution (if not pre-made): Accurately weigh approximately 1-1.5 g of sodium carbonate. Record the exact mass. Dry the sodium carbonate thoroughly in an oven at 105-110°C before weighing. Dissolve in distilled water and make up to 100mL in a volumetric flask.
  3. Titration:
    1. Pipet 25.00 mL of the KHP solution into a clean Erlenmeyer flask.
    2. Add 2-3 drops of phenolphthalein indicator.
    3. Rinse a buret thoroughly with the Na2CO3 solution. Fill the buret with the Na2CO3 solution and record the initial buret reading.
    4. Slowly titrate the KHP solution with the Na2CO3 solution, swirling the flask constantly. The endpoint is reached when a persistent faint pink color appears and persists for at least 30 seconds.
    5. Record the final buret reading. Calculate the volume of Na2CO3 solution used.
    6. Repeat the titration at least two more times to ensure consistent results.
Results

Record the mass of KHP, the molar mass of KHP, the volume of KHP solution used, the initial and final buret readings for each titration, and the volume of Na2CO3 solution used in each titration. Calculate the average volume of Na2CO3 used.

Calculations:

  1. Calculate the moles of KHP used in each titration: Moles KHP = (mass KHP / molar mass KHP)
  2. Calculate the molarity of the KHP solution: Molarity KHP = (moles KHP / volume KHP solution in Liters)
  3. For each titration, calculate the moles of Na2CO3 used: Moles Na2CO3 = moles KHP (from the stoichiometry of the reaction. Assume a 1:1 molar ratio for simplicity, but this depends on the specific reaction. The balanced reaction needs to be included to account for the correct stoichiometry)
  4. Calculate the molarity of the Na2CO3 solution for each titration: Molarity Na2CO3 = (moles Na2CO3 / volume Na2CO3 solution in Liters)
  5. Calculate the average molarity of the Na2CO3 solution.
Significance

Primary standards are highly pure substances with known chemical composition used to standardize other solutions. Secondary standards are solutions whose concentrations are determined by titration against a primary standard. This experiment demonstrates how to accurately determine the concentration of a secondary standard solution using a primary standard and titration techniques.

Accurate standardization is crucial for reliable quantitative analysis in chemistry.

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