Back to Library

(AI-Powered Suggestions)

Related Topics

A topic from the subject of Standardization in Chemistry.

  • 11 months ago
  • 3 min read
Use of Primary and Secondary Standards in Chemistry
Introduction

In chemistry, a standard is a substance used to calibrate instruments or standardize solutions. Primary standards are highly pure substances with known compositions that can be used to standardize solutions. Secondary standards are substances that have been standardized against a primary standard and can be used to standardize other solutions. They provide a reliable reference point for quantitative analysis.

Basic Concepts
  • Equivalence point: The point in a titration at which the moles of acid and base are chemically equivalent (not necessarily equal volumes).
  • Stoichiometry: The study of the quantitative relationships between reactants and products in a chemical reaction, crucial for calculating amounts in titrations.
  • Normality: A measure of the concentration of a solution in terms of the number of equivalents of reactive species per liter. Less commonly used now than molarity.
  • Molarity: A measure of the concentration of a solution in terms of the number of moles of solute per liter of solution. The most common unit of concentration.
Equipment and Techniques
  • Analytical balance: Used to precisely weigh the primary and secondary standards to ensure accurate molarity calculations.
  • Burette: Used to deliver precise volumes of titrant solution during titrations.
  • Pipette: Used to accurately measure specific volumes of solutions, especially the analyte.
  • pH meter: Used to measure the pH of a solution, especially important in acid-base titrations to monitor the equivalence point.
  • Indicator: A substance that changes color near the equivalence point of a titration, providing a visual signal of the endpoint.
Types of Experiments
  • Acid-base titrations: Used to determine the concentration of an acid or base using a standardized solution of the opposite type.
  • Redox titrations: Used to determine the concentration of an oxidizing or reducing agent based on electron transfer reactions.
  • Complexometric titrations: Used to determine the concentration of a metal ion by forming a stable complex with a chelating agent.
Data Analysis
  • Moles of acid or base: Calculated from the precisely weighed mass of the standard and its molar mass (grams/mole).
  • Concentration of the solution: Calculated from the moles of the acid or base and the accurately measured volume of the solution (Molarity = moles/liter).
  • Equivalence point: Determined from the graph of the titration data (e.g., pH vs. volume of titrant) – often identified by a sharp change in pH or indicator color change.
Applications
  • Calibration of instruments: Primary standards are used to calibrate pH meters, spectrophotometers, and other analytical instruments to ensure accuracy.
  • Standardization of solutions: Primary and secondary standards are crucial for preparing solutions of known and precise concentrations (e.g., standardizing NaOH solution with KHP).
  • Quantitative analysis: Standard solutions are used in various analytical techniques to accurately determine the concentration of unknown substances in samples.
Conclusion

The use of primary and secondary standards is fundamental in analytical chemistry for achieving accurate and reliable quantitative results. Their careful use ensures the precision and validity of experimental findings.

Use of Primary and Secondary Standards in Chemistry
Primary Standards
  • Substances that have high purity and well-defined compositions.
  • Used to standardize solutions of other substances (secondary standards).
  • Examples: Potassium hydrogen phthalate (KHP), sodium carbonate (Na2CO3), sodium chloride (NaCl).
Secondary Standards
  • Substances calibrated against primary standards.
  • Used for routine titrations and other analytical procedures.
  • Examples: HCl solution, NaOH solution, AgNO3 solution.
Key Points
  • Primary standards are used to calibrate secondary standards, ensuring accuracy in analytical measurements.
  • The purity and composition of primary standards must be known with high precision.
  • Secondary standards are widely used in titrations, redox reactions, and other analytical procedures.
  • By using standardized solutions, accurate and reliable results can be obtained in chemical analyses.
  • Primary standards should ideally exhibit the following characteristics: high purity, stability (resistant to atmospheric changes), high molar mass (to minimize weighing errors), readily available, inexpensive, and non-hygroscopic (does not absorb moisture from the air).
  • The process of standardizing a secondary standard involves carefully reacting a known mass of primary standard with a solution of the secondary standard. This allows for the precise determination of the concentration of the secondary standard.
Experiment: Use of Primary and Secondary Standards in Chemistry
Objective:
  • To understand the concept of primary and secondary standards
  • To calibrate a burette using a known primary standard
  • To determine the concentration of an unknown solution using a secondary standard

Materials:
  • Primary standard: Potassium hydrogen phthalate (KHP)
  • Secondary standard: Sodium carbonate (Na2CO3)
  • Burette
  • Titrating flask (Erlenmeyer flask)
  • Pipette
  • Phenolphthalein indicator
  • Analytical balance
  • Deionized water

Procedure:
Calibration of Burette:
  1. Accurately weigh approximately 0.2 g of KHP using an analytical balance and record the mass. Dissolve it in a known volume (e.g., 100 mL) of deionized water in a titrating flask.
  2. Add 2-3 drops of phenolphthalein indicator to the KHP solution.
  3. Fill the burette with the titrant (e.g., standardized NaOH solution – the experiment needs a known titrant). Record the initial burette reading.
  4. Slowly add the titrant from the burette to the KHP solution, swirling constantly until a persistent faint pink color appears.
  5. Continue titrating until the solution turns a faint pink color that persists for at least 30 seconds.
  6. Record the final burette reading. Calculate the volume of titrant used.

Determination of Concentration of Unknown Solution:
  1. Accurately weigh approximately 0.1 g of Na2CO3 using an analytical balance and record the mass. Dissolve it in a known volume (e.g., 100 mL) of deionized water in a titrating flask.
  2. Add 2-3 drops of phenolphthalein indicator to the Na2CO3 solution.
  3. Fill the burette with the unknown solution (e.g., HCl solution of unknown concentration).
  4. Slowly add the unknown solution from the burette to the Na2CO3 solution, swirling constantly.
  5. Continue titrating until the solution turns a faint pink color that persists for at least 30 seconds.
  6. Record the final burette reading. Calculate the volume of unknown solution used.

Calculations:
Calibration of Burette (Determining the concentration of the titrant):
The concentration of the titrant (e.g., NaOH) (MNaOH) can be calculated using the following formula:

MNaOH = (Weight of KHP (g) / Molecular weight of KHP (g/mol)) / (Volume of NaOH used (L))

The molecular weight of KHP is 204.22 g/mol.


Determination of Concentration of Unknown Solution:
The concentration of the unknown solution (Munknown) can be calculated using the following formula:

Munknown = (MNaOH * Volume of NaOH used (L) * 2) / (Volume of unknown solution used (L))

The factor of 2 accounts for the stoichiometry of the reaction between Na2CO3 and HCl (or the appropriate titrant used). Na2CO3 reacts with 2 moles of HCl for every mole of Na2CO3. Adjust this factor for other titrations accordingly.


Significance:
Primary and secondary standards are essential in chemistry for accurate and precise determination of the concentration of solutions. Primary standards, such as KHP, have a known high purity and can be used to standardize solutions. Secondary standards, such as Na2CO3, can then be used to determine the concentration of unknown solutions. This experiment demonstrates the practical application of primary and secondary standards in analytical chemistry. Note that careful attention to detail, including proper weighing and clean glassware, is crucial for accurate results.

Share on: