Back to Library

(AI-Powered Suggestions)

Related Topics

A topic from the subject of Standardization in Chemistry.

  • 1 year ago
  • 6 min read
Standardization in Gravimetric Analysis
Introduction

Gravimetric analysis is a quantitative analytical technique used to determine the concentration of an analyte in a sample by measuring the mass of the precipitate formed by a chemical reaction between the analyte and a reagent. Standardization is a crucial process in gravimetric analysis. It involves determining the exact concentration of a reagent, typically a titrant, used in the analysis. This is achieved by reacting the reagent with a precisely known amount of a primary standard, a highly pure substance with a known chemical composition. The standardization ensures accurate and reliable results in determining the analyte's concentration.

Basic Concepts

The basic concepts underlying gravimetric analysis include:

  • Gravimetric factor: The gravimetric factor is a stoichiometric ratio that converts the mass of the precipitate to the mass of the analyte. It is calculated from the molecular weights of the analyte and the precipitate.
  • Equivalent weight: The equivalent weight (or equivalent mass) of a substance is the mass of the substance that reacts with or is equivalent to one mole of hydrogen ions (H+) or one mole of electrons in a chemical reaction. This concept is particularly useful in acid-base and redox titrations often associated with standardization.
  • Percentage composition: The percentage composition of an analyte is the mass of the analyte expressed as a percentage of the total mass of the sample. This is the ultimate goal of many gravimetric analyses.
Equipment and Techniques

Key equipment and techniques employed in gravimetric analysis are:

  • Analytical balance: An analytical balance is used to accurately measure the mass of the sample and the precipitate.
  • Crucible: A crucible, often made of porcelain or platinum, is a heat-resistant container used to hold the precipitate during heating and weighing.
  • Filter paper/Filter crucible: Filter paper or a filter crucible (e.g., Gooch crucible) are used to separate the precipitate from the solution.
  • Filtration: Filtration is the process of separating the precipitate from the supernatant liquid. Techniques include gravity filtration, vacuum filtration, and centrifugation.
  • Drying/Ignition: The precipitate is dried or ignited in a furnace to remove any volatile impurities and achieve a constant mass before weighing.
Types of Gravimetric Analysis

Common types of gravimetric experiments include:

  • Precipitation gravimetry: In this most common type, the analyte is converted into a sparingly soluble precipitate by adding a suitable reagent. The precipitate is then filtered, washed, dried, and weighed.
  • Volatilization gravimetry: This involves heating the sample to volatilize the analyte. The volatile analyte is collected (e.g., in a drying tube) and weighed, or alternatively, the mass loss of the sample is measured to determine the analyte's concentration.
  • Electrogravimetry: The analyte is deposited as a solid coating on an electrode using electrolysis. The increase in electrode mass is used to determine the analyte's concentration.
Data Analysis

Data analysis in gravimetric analysis involves these steps:

  1. Determine the mass of the precipitate after drying or ignition to constant weight.
  2. Calculate the mass of the analyte using the gravimetric factor.
  3. Calculate the percentage or concentration of the analyte in the original sample.
Applications

Gravimetric analysis finds applications in diverse fields including:

  • Determination of the purity of compounds: Assessing the percentage of a specific component in a substance.
  • Analysis of environmental samples: Measuring pollutants or other analytes in water, air, or soil.
  • Analysis of food and drug products: Determining the concentrations of specific components to ensure quality control.
  • Clinical analysis: Analyzing biological samples for specific substances.
Conclusion

Gravimetric analysis is a precise and accurate technique with wide-ranging applications. Proper standardization of reagents and meticulous attention to experimental details are essential for obtaining reliable and accurate analytical results.

Standardization in Gravimetric Analysis

In gravimetric analysis, precise and accurate measurements of mass are essential for obtaining reliable results. Standardization is a crucial process that establishes the relationship between the mass of a precipitate (containing the analyte) and the concentration of the analyte in the original sample. This allows for accurate determination of the analyte's amount.

Key Aspects of Standardization

  • Primary Standards: These are pure, highly characterized compounds with known stoichiometry and a precisely determined molar mass. Examples include potassium dichromate (K₂Cr₂O₇) and sodium carbonate (Na₂CO₃). Primary standards are used to calibrate analytical balances and standardize titrants (solutions of precisely known concentration) used in volumetric analysis, often a precursor step to gravimetric analysis. They are crucial for ensuring accuracy in mass measurements.
  • Method of Standard Addition: This technique involves adding known amounts of the analyte to a separate aliquot of the unknown sample. The increase in precipitate mass is then plotted against the added analyte mass. Extrapolating the resulting graph to the x-intercept (where the added analyte mass is zero) determines the original concentration of the analyte in the unknown sample. This method is particularly useful when the sample matrix interferes with the analysis.
  • Gravimetric Factor: This is the ratio of the molar mass of the analyte to the molar mass of the precipitate multiplied by the appropriate stoichiometric coefficients. It converts the measured mass of the precipitate to the mass of the analyte. For example, if you are analyzing for chloride (Cl⁻) and precipitating it as silver chloride (AgCl), the gravimetric factor would account for the molar mass ratio of Cl⁻ to AgCl.
  • Significance: Standardization ensures the accuracy and precision of gravimetric analysis results. By using primary standards and carefully controlling experimental conditions, systematic errors associated with instrument calibration and reagent purity are minimized, leading to reliable and reproducible data.
  • Applications: Gravimetric analysis, employing standardization techniques, has broad applications in various fields, including:
    • Determining the purity of metals and metal compounds
    • Analyzing environmental samples for pollutants (e.g., determining sulfate levels in water)
    • Analyzing food samples for nutrient content
    • Studying biochemical reactions and determining the composition of biological samples
    • Quality control in industrial processes
Standardization of Sodium Hydroxide Solution
Materials:
  • Sodium hydroxide (NaOH) pellets
  • Potassium hydrogen phthalate (KHP)
  • Analytical balance (0.0001 g precision)
  • Erlenmeyer flask (250 mL)
  • Buret (50 mL)
  • Phenolphthalein indicator
  • Distilled water
Procedure:
  1. Prepare the NaOH solution: Dissolve approximately 5 g of NaOH pellets in 1 L of distilled water. Allow the solution to cool to room temperature before standardization.
  2. Dry the KHP: Heat about 1 g of KHP in an oven at 110°C for at least 2 hours to remove any moisture. Allow to cool in a desiccator before weighing.
  3. Accurately weigh the KHP: Weigh approximately 0.2500 g of dried KHP into the Erlenmeyer flask. Record the exact mass.
  4. Dissolve the KHP: Add about 50 mL of distilled water to the flask and swirl gently to dissolve the KHP.
  5. Add 2-3 drops of phenolphthalein indicator: The solution should remain colorless.
  6. Fill the buret with NaOH solution: Record the initial buret reading to the nearest 0.01 mL.
  7. Titrate the KHP solution: Slowly add the NaOH solution from the buret to the KHP solution while swirling continuously. The endpoint is reached when a faint pink color persists for at least 30 seconds.
  8. Record the final buret reading: Record the final buret reading to the nearest 0.01 mL. Subtract the initial reading from the final reading to obtain the volume of NaOH solution used.
  9. Calculate the molarity of the NaOH solution: Use the following formula:

    Molarity of NaOH = (Mass of KHP (g) / Molecular weight of KHP (g/mol)) / Volume of NaOH solution (L)

    The molecular weight of KHP is 204.22 g/mol.

  10. Repeat the titration: Perform at least two more titrations to ensure reproducibility. Calculate the average molarity of the NaOH solution from the three (or more) trials.
Significance:

Standardizing the NaOH solution is crucial in gravimetric analysis because it determines the accurate concentration of the solution. This allows for precise measurements of analyte concentrations by relating the mass change (i.e., gravimetrically determined) to the moles of analyte present in the solution. A standardized NaOH solution ensures that the analysis results are accurate and reliable.

Share on: