Preparations of Standard Solutions

Standard solution is the solution whose concentration is accurately known.

Stand solutions are of two types:

• Primary standard solution
• Secondary standard solution

1. Primary Standard Solution

• Made from a primary standard (a highly pure, stable compound).
• Directly prepared by dissolving an exact weighed amount in a volumetric flask.
• Does not require standardization.

Characteristics of a Primary Standard:

• They should have high purity to enable preparation of exact concentrations of solutions.
• They should be stable, that is, should not decompose with time or when exposed to air.
• They should have no water of hydration, that is, should not be hygroscopic or efflorescent.
• They should be less volatile or non-volatile so that cannot easily lose any component due to evaporation.
• They should be highly soluble in water.
• They should have high molecular mass to enable measurement of a significant amount of a substance for a specific concentration.

Examples are:

• sodium carbonate (Na₂CO₃) for acid-base titrations
• potassium dichromate (K₂Cr₂O₇) and Oxalic acid (H₂C₂O₄) for redox titrations

2. Secondary Standard Solution

• Made from a less pure or unstable substance.
• Must be standardized against a primary standard solution before use.

Characteristics of a Secondary Standard:

• Less pure or stable than primary standards
• Concentration may vary (must be standardized before use)

Key Differences Between Primary and Secondary Standards

Preparation of secondary standard solution:

• Approximate concentration is prepared first.
• Standardized by titrating against a primary standard solution.

Examples of Secondary Standards:

Preparation of primary standard solution from solid substances

1. Identify the volume and concentration of the standard solution required
2. Calculate the number of moles of the substance needed to make up the standard solution.
3. Work out the molar mass of the substance.
4. Calculate the mass of the substance needed to prepare the standard solution.
5. Weigh the required mass of the substance using analytical balance.
6. Transfer the primary standard into beaker. Rinse the watch glass by using distilled water from a wash bottle ensuring that the rinsing water goes into the beaker containing the substance.
7. Stir the mixture in the beaker with a glass rod until all the solute is dissolved and then transfer the solution into a volumetric flask.
8. Rinse the beaker and the glass rod by using distilled water from a wash bottle twice times and pour the rinsing water into the volumetric flask each time.
9. Add water to the solution to just below the graduation mark on the volumetric flask.
10. Top up the solution with water up to the graduation mark by using a clean dropper. Ensure that the lowest part of the meniscus is exactly at the graduation mark.
11. Close the mouth of the volumetric flask with a stopper and invert it several times to ensure that the solution is homogeneously mixed.
12. Work out the molarity of the solution.
13. Label the solution correctly by indicating its name, concentration and preparation date if the solution is to be used for a long time.

Preparation of standard solution from Stock solution

Solutions of mineral acids are prepared by dilution of concentrated acids.

• Concentrated acids are bought as stock solutions from chemical suppliers.
• The molar concentrations of the stock solutions are usually not indicated on the reagent bottles.
• The reagent/Winchester bottles may however contain the following information:
  • Density or specific gravity (₰)
  • Percentage purity
  • Molar mass = molecular weight =M.W

Preparation of standard solution from Stock solution

1. Determine the molarity of the concentrated solution by using the formula:

   Molarity (Mc) = (Percentage Purity × density × 1000) / (100 × molar mass)

2. Calculate the volume needed to prepare a required molarity by using dilution law:

   Number of moles of solute before dilution = number of moles of solute after dilution

   Number of moles = molarity (mol/dm³) × volume (dm³)

   McVc = MdVd

   Where: Mc = Molarity of concentrated solution

   Vc = Volume of concentrated solution

   Md = Molarity of dilute solution

   Vd = Volume of dilute solution

   Therefore:

   Volume Vc = (MdVd)/Mc

3. Using measuring cylinder, measure exactly volume (Vc) of concentrated solution from the Winchester bottle.
4. Put a small amount of distilled water in a beaker.
5. Add the volume of concentrated solution measured in beaker. Stir the solution with glass rod to mix it thoroughly.
6. Pour the solution into a distilled water which is in a volumetric flask.
7. Wash both glass rod and beaker and pour the washings into the volumetric flask. Repeat twice times.
8. Carefully add distilled water to the solution up to the calibration mark of the volumetric flask.
9. Invert the flask several times to obtain a uniform solution.
10. Label the name of solution and its concentration.

WORKED EXAMPLES

1. What volume of 2M hydrochloric acid solution has to be diluted to obtain 250cm³ of 0.5M solution?

   Solution

   Data given

   Molarity of concentrated acid (Mc) = 2M

   Molarity of dilute acid solution (Md) = 0.5M

   Volume of dilute acid solution (Vd) = 250cm³

   Volume of concentrated acid solution =?

   From dilution law:

   McVc = MdVd

   Vc = (MdVd)/Mc

   (Ans = 62.5cm³)

2. Calculate the volume of concentrated nitric acid 18mol/dm³ which is required to prepare 5dm³ of 2mol/dm³ nitric acid (Ans = 0.5556dm³ or 555.6cm³)
3. What volume of 2mol/dm³ sulphuric acid is needed to prepare 100cm³ of 0.5mol/dm³ sulphuric acid? (Ans = 25cm³)
4. What mass is required to prepare 250cm³ of 0.1mol/dm³ sodium carbonate solution? (Ans = 2.65g).
5. What volume of hydrochloric acid with 37.1% purity and a density of 1.2g/cm³ should be used to prepare 1 litre of 0.1000M of hydrochloric acid solution? (The molar mass of hydrochloric acid is 36.5g/mol) (Ans = 8.20cm³) TIE
6. Analytical nitric acid assay indicates that Winchester contains 68% of the acid with relative density of 1.42g/cm³. Calculate:
   1. The concentration of the acid in mol/dm³
   2. The volume of the acid required to prepare 250mL of 0.2M acid solution.

   (Ans. (a) 3mol/dm³

   (b) 3.27cm³

Titration

Titration is a laboratory technique which is used to determine unknown concentration of a solution using a standard solution.

Conditions necessary for titration

• The reaction must be fast so that the titration can be performed by using convenient time.
• The reaction should go to completion, that is it should not be reversible reaction.
• The reaction should be free from side reactions; meaning that it can be represented by a single chemical equation.
• The reaction should have a definite end-point that can be determined accurately.

Types of titrations

• Acid-base titration reaction
• Redox titration reactions
• Precipitation titration reactions
• Complexometric titration reactions

Acid-base titrations

• Titrant is a solution of known concentration.
• Titrand (analyte) is a solution of unknown concentration.
• Indicator is a chemical substance that changes color at the end-point.
• End-point is the point in the titration at which the indicator changes color.
• Equivalence point is the point during titration where the number of moles of the titrant added is exactly equal to the number of moles of the analyte present in the solution.

Common acid-base indicators

Choice of indicators

Procedure for carrying out acid-base titrations