4.2 Standards

Standard Solution

A reagent solution of accurately known concentration

Stock Solution

A concentrated solution that will be diluted to some lower concentrated for actual use

Working Solution

A name given to a chemical solution made for actual use in the lab, usually made from diluting or combining stock or standard solution

Definition

A reagent that is extremely pure and serve as a reference material

Properties

High purity

Atmospheric stable

Has no waters of hydration (non hydroscobic)

Has relatively high molecular weight

Easily available and at modest cost

Examples

For titration of acids

i) Sodium carbonate, Na2CO3,
mol wt= 105.99 g/mol

ii) tris-(hydroxymethyl)aminimethane,
(CH2OH)3CNH2 mol wt= 121.12 g/mol

For titration of bases

i) potassium hydrogen phthalate (KHP): KHC8H4O4, mol wt= 204.23 g/mol

ii) potassium hydrogen iodate : KH(IO3)2,
mol wt= 289.92 g/mol

For redox titration

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

Definition

A reagent (usually prepared in the laboratory) whose purity has been established by standardizing it against a primary standard

Properties

Secondary standard are influenced atmosphere / environment

Concentration change over time

Usually powerful reactants

Usually cheap and easy to use

Examples

Sodium hydroxide (NaOH)

Definition

A process to determine the concentration of a solution by titrating with a primary standard or with a solution of known concentration

Process

1. Weight out 1.325g of Na2CO3
2. Dissolve it in water
3. Add enough water to fill 250mL volumetric flask
4. Use this solution to standardized the HCl acid

Definition

A procedure for preparing a less concentrated solution from a more concentrated one by adding a solvent

Formula

M1V1 = M2V2

How to do?

How many cm3 of a 10.00M HCl stock solution would you use to prepare 5dm3 of 0.5M HCl

M1 = 10.00 M M2 = 0.5 M
V1 = ? V2 = 5 dm3

M1V1 = M2V2
(10)V1 = (0.5)(5)
V1 = 0.25 dm3