Preparation of various standard solutions –definition of primary & secondary standards

• Standard solution: A solution with a known solute concentration is called a standard solution. It is used to compare or determine the concentration of another solution in chemical analysis. Standard solutions are important in titration, calibration, and several chemical processes.

• Preparation of standard solution

  • · Materials and equipment: Analytical or digital balance, volumetric flask, beaker, stirring rod or magnetic stirrer, distilled water, and pure solute analytical grade.

• · Procedure:

  • o Solute weighing: Weigh pure solute accurately by using a high-precision analytical balance or digital balance. Record the exact weight of the solute.

  • o Transfer solute into the container: Carefully transfer the weighed solute into a clean and dry container.

  • o Solute dissolution: Add a small amount of distilled water or solvent into a container with weighed solute. Stir the mixture using a glass stirrer or magnetic stirrer. Ensure complete dissolution of solute in a solvent. Add more solvent gradually into a container with constant stirring. Make a homogeneous solution.

  • o Transfer of solution: Add solution into volumetric flask carefully by using a funnel or dropper.

• o Final volume adjustment: Add solvent into the volumetric flask and adjust the final volume up to the calibration mark..

Primary Standard: A primary standard is a stable and highly pure chemical to be used as a reference to prepare a primary standard solution. It is used for analytical purposes such as titration and other quantitative analysis. Common examples of primary standard substances are potassium permanganate (It is used in redox reactions), sodium hydroxide (It is used in acid-base titration), sodium thiosulfate (It is used in iodometric titration), etc.

Primary standard solution: It is prepared by dissolving accurately weighed primary standard chemicals in a known volume of solvent. It is used in chemistry for accurate and precise titration and other quantitative analyses. The primary standard solution has the following characteristics:

1. High purity: Primary standard chemical is used to prepare a primary standard solution. These chemicals are highly pure with purity above 99.9%. This minimizes the error in the quantitative analysis due to impurities.

2. Known concentration: The concentration of chemicals in the primary standard solution is accurately determined. It is a reference for use in other chemical analyses.

3. Stability: Primary standard solution and primary standard chemicals should not undergo chemical changes over time.

4. Ease of handling: Primary standard chemicals should be easy to weigh accurately

Secondary standard: Secondary standards are the chemicals or solutions that are used to calibrate or standardize laboratory instruments and techniques. It is not as pure as a primary standard. Secondary standards are required to ensure the accuracy and reliability of chemical measurements.

Commonly used terms in preparations of various standard solutions:

1. Normal solution (N): A solution containing one gram equivalent mass of solute per liter of solution is called a normal solution

2. Normality (N): The normality of a solution is the number of grams equivalent of solute per liter of solution.

         Normality = Number of gram equivalent of solute /Volume of solution (in liters)   

                            = Mass of solute in grams/equivalent mass/Volume of solution in ml/1000

Units: Gram equivalents/liter or Equivalents/liter

Normality of solution by mixing two solutions

      First solution: Volume = V1, Normality = N1

      Second solution: Volume = V2, Normality=N2

By mixing both solutions the normality of the resulting solution N3 is given by the formula

       N1 V1+ N2 V2 =N3(V1+ V2)

3. Molar Solution (M): Solution containing one molecular weight of chemical substance dissolved in one liter of solvent.

4. Molality (m): Molality of solution is the number of moles of solute present in 1000 gm of solution

Units: Gram moles/kg or Moles/kg.

• 5. Morality (M): Morality of solution is the number of gram moles of solute per liter of solution.

  • Morality: Gram mole of solute/Volume of solution (in liter)

  • Gram mole of solute = Morality x Volume of solution (in liters).

  • Units: Gram moles /liter or Moles /liter.

• Calculate molarity of solution:

  • The molarity of a solution containing 4.9 grams of H2SO4 dissolved in 250ml of the solution.

  • Molarity (M) = 4.9g/98mol-1/250mL/1000 = (0.05mol)/(0.25L)

  • =0.2mol L-1 or (0.2M)

  • Molarity of solution by mixing two solutions

    • · First solution: Volume = V1, Molarity = M1

    • · Second solution: Volume =V2, Molarity=M2

  • By mixing both solutions the molarity of the resulting solution M3 is given by the formula

    • M1V1+ M2V2=M3 (V1+ V2)

• Percent solution: Solid in liquid solution: If the solution is prepared by dissolving a solid solute in a liquid solvent, the concentration of the solution may be expressed as.

  • 1. Mass by mass percentage: Mass of solute in gram dissolved in 100gm of solution

  • For eg.10% solution of NaCl means 10gm of NaCl is present in 100gm of solution

  • ,Formula: NaCl.---10 gm, Solvent----90 gm

  • Mass by mass percentage of a solution= Mass of solute/Mass of solution X 100

  • OR                                                                = Mass of solute/Mass of solute+Mass of solvent X 100

  • 2. Mass by volume percentage: Mass of solute in gram dissolved in 100ml of solution.

  • For eg.10% mass by volume solution (10% M/V) of NaCl in water means 10 gm of NaCl present in 100 ml of solution.

  • Formula: NaCl----10gm, H2O q,s to produce 100ml

  • Mass by volume percentage solution=Mass of solute/Volume of solutionx100.

SI units and their uses.

• SI units are also called the International System of Units or Metric system. It has universal acceptability in science, industry, and everyday life. It provides a standardized way to express physical quantities. Some SI units that are in use:

  • 1. Meter (m): Meter is a SI unit to measure length and distance.

  • 2. Kilogram (kg): A Kilogram is a standard unit to measure weight and mass.

  • 3. Litre (L): A Litre is an SI unit to measure volume.

  • 4. Mole (mol): The mole is the SI unit for the amount of chemicals. It is used in chemistry to quantify a number of atoms, molecules, or ions in the sample.

  • 5. Kelvin (K): Kelvin is the SI unit for temperature. It is used to measure temperature on the absolute temperature scale.

  • 6. Ampere (A): Ampere is an SI unit to measure electric current.

Exercises

• Preparation of standard solutions/Stock solution

• Example: Prepare 1N Sulphuric acid.

• Preparation of 1N Sulfuric Acid

  • Understanding the Terms

  • Normality (N): A measure of concentration equal to the gram-equivalent weight of a solute per liter of solution.

  • Equivalent Weight: The mass of a substance that reacts with or displaces a fixed amount of another substance.

• Calculation

  • For sulfuric acid (H₂SO₄), the equivalent weight is its molecular weight (98.08 g/mol) divided by the number of replaceable hydrogen ions (2).

  • Equivalent weight of H₂SO₄ = 98.08 g/mol / 2 = 49.04 g/mol

• To prepare a 1N solution, we need 49.04 grams of H₂SO₄ in 1 liter of solution.

• Procedure

  • 1. Calculate the volume of concentrated sulfuric acid:

    • o Assuming you are using concentrated sulfuric acid (usually 98% w/w), the density is approximately 1.84 g/mL.

    • o Calculate the volume needed:

    • § Volume = (mass/density) = (49.04 g / 1.84 g/mL) ≈ 26.63 mL

  • 2. Dilute the concentrated acid:

    • o Safety first: Always add acid to water, never water to acid, as this can cause a violent reaction.

    • o In a 1-liter volumetric flask, add approximately 800 mL of distilled water.

    • o Slowly add the calculated volume of concentrated sulfuric acid, mixing gently.

    • o Add distilled water to the 1-liter mark, ensuring the solution is well mixed.

• Note: For precise measurements, especially in analytical applications, it's often recommended to standardize the prepared 1N sulfuric acid solution using a known standard, such as sodium carbonate or sodium hydroxide.

• Safety Precautions:

  • · Wear appropriate protective equipment, including gloves, eye protection, and a lab coat.

  • · Work in a well-ventilated area.

  • · Handle concentrated sulfuric acid with caution, as it is a strong corrosive acid.

• By following these steps and calculations, you can safely prepare a 1N sulfuric acid solution.

Dr Pramila Singh