Chapter 3: Volumetric Analysis - missballinger



Chapter 3: Volumetric Analysis

In volumetric analysis, the amount (in mol) of the substance being analysed is determined by measuring the volume of the test solution required to react completely with a volume of solution of known concentration.

Concentration

Concentration is a measure of the relative amounts of solute and solvent. It is measured in molarity: mol L-1 or M.

To convert a concentration measured in mol L-1 to (g L-1), simply multiply the molarity by the molar mass. UNITS ARE THE KEY!

Eg. 2.00 mol L-1 NaCl

2.00mol L-1 x (23.0 + 35.5) g mol-1

= 117 g L-1

Other ways to express concentration are:

• Parts per million (ppm) ie grams per million grams

• Parts per billion (ppb) ie grams per billion grams

• Percentage mass/volume (%m/v) ie grams per 100mL

• Percentage by volume (%v/v) ie mL per 100 mL

• Percentage by mass (%m/m) ie grams per 100g

Standard Solutions

Solutions with accurately known concentrations are called standard solutions. Preparing a standard solution is not a simple matter of dissolving a measured mass as many chemicals are impure as they decompose or react with chemicals in the atmosphere.

Substances that are so pure that the amount of substance, in mole, can be calculated accurately from their mass are called Primary Standards.

They should:

•

•

•

• Be readily obtainable in pure form

• Have a known formula

• Be easy to store without decomposing or reacting with the atmosphere

• Have a high molar mass to minimise the effect of errors of weighing

• Be inexpensive

E.g.’s Basic primary standard = Na2CO3 (anhydrous sodium carbonate), Na2B4O7.10H2O (sodium borate)

Acidic primary standard = H2C2O4.2H2O (hydrated oxalic acid),

To make a standard solution:

1. Place weighed mass of solute in volumetric flask

2. Half fill with water and shake to dissolve

3. Add water to the calibration line.

Volumetric Analysis

A common way of finding an unknown concentration is by using the technique of Volumetric Analysis. This technique involves reacting a measured volume of a standard solution with a measured volume of the solution of unknown concentration.

Steps involved in Volumetric Analysis:

1. pipette a known volume (aliquot) of solution into a conical flask

2. dispense other solution slowly through a burette

3. stop when indicator just changes colour and measure volume (titre) dispensed

4. repeat until you have three concordant titre results (no more than 0.1mL difference between all three) then average the concordant results to minimise errors.

Define:

• Equivalence point:

• End point:

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Accuracy of Instruments used

• Burette - within 0.02mL

• 20 mL Pipette - within 0.05mL

• 250mL volumetric flask – within 0.3mL

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Concentration = amount of solute, mol

Volume of solution, L

Mol x g = g

L mol L

Percentage means ‘out of one hundred’

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