4.2 STANDARDS

1. SECONDARY STANDARD

1.1. Definition : A secondary standard is a reagent (usually prepared in the laboratory) whose purity has been established by standardizing it against a primary standard.

1.2. Properties : •Secondary standards are Influenced atmosphere/environment • Concentration change over time • Usually powerful reactants • Usually cheap & easy to use

1.3. Examples : NaOH

2. STANDARDIZATION

2.1. Definition : is a process to determine the concentration of a solution by titrating with a primary standard or with a solution of known concentration

2.2. Process : • Accurately weigh out 1.325g of Na2CO3 and dissolve it in a little water, then add enough water to fill a 250 mL volumetric flask. • This solution would then be used to standardized the HCl acid. This will give a very accurate determination of the concentration of the HCl solution.

3. DILUTION

3.1. Definition : Dilution is a procedure for preparing a less concentrated solution from a more concentrated one by adding a solvent.

3.2. Formula : M1V1 = M2V2 where, M1 = Molarity (M) of the concentrated solution M2= Molarity (M) of the diluted solution V1 = volume (cm3/dm3) of concentrated solution V2 = volume (cm3/dm3) of diluted solution

3.3. Process : 1) Concentrated solutions or ‘stock’ solutions are usually stored to the desired concentrations when required. 2) The concentrated solution with certain volume are diluted by simply mixing with solvent like adding distilled water until the mark of volumetric flask.

4. DEFINITION

4.1. Standard Solution : A reagent solution of ACCURATELY KNOWN CONCENTRATION.

4.2. Stock Solution : is a concentrated solution that will be diluted to some lower concentrated for actual use. Stock solutions are used to save preparation time, conserve materials, reduce storage space, and improve the accuracy with which working solutions are prepared.

4.3. Working Solution : is a name given to a chemical solution made for actual use in the lab, usually made from diluting or combining stock or standard solutions.

5. PRIMARY STANDARD

5.1. Definition : A primary standard is a reagent that is extremely pure and serve as a reference material.

5.2. Properties : • High purity. • Atmospheric stable. • Has no waters of hydration ( non hydroscobic). • Has relatively high molecular weight. • Easily available and at modest cost. • Readily soluble.

5.3. Examples:

5.3.1. Titration Acids

5.3.1.1. i) sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol ii) tris-(hydroxymethyl)aminomethane (TRIS or THAM) (CH2OH)3CNH2, mol wt. = 121.14 g/mol

5.3.2. Titration Bases

5.3.2.1. i) potassium hydrogen phthalate (KHP): KHC8H4O4,mol wt. = 204.23 g/mol ii) potassium hydrogen iodate: KH(IO3)2, mol wt. = 389.92 g/mol

5.3.3. Redox Titrations

5.3.3.1. potassium dichromate: K2Cr2O7, mol wt.= 294.19 g/mol