Critieria of Purity | Chemistry for EmSAT Achieve PDF Download

Paper Chromatography

• This method is employed to segregate substances with distinct solubilities in a given solvent, such as different colored inks amalgamated to form black ink.
• To perform chromatography, a pencil line is drawn on chromatography paper, and sample spots are applied onto it. Pencil is chosen for this task to prevent ink from merging into the chromatogram alongside the samples.
• Subsequently, the paper is immersed into a solvent container, ensuring the pencil line remains above the solvent level to prevent sample washout.
• The solvent ascends the paper through capillary action, carrying some of the colored substances along.
• Because different substances possess varying solubilities, they migrate at different rates, causing them to disperse. Those with higher solubilities travel farther than others.
• This process reveals the distinct components of the ink or dye.

Interpret Simple Chromatograms

• If two or more substances are the same, they will produce identical chromatograms
• If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
• An impure substance will show up with more than one spot, a pure substance should only show up with one spot

Assessing Purity

• Pure substances melt and boil at specific and sharp temperatures. Eg water has a boiling point of 100°C and a melting point of 0°C
• Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
• Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
• An unknown pure substance can be identified by experimentally determining its m.p and b.p and comparing to data tables
• Mixtures melt over a range of temperatures as they contain two or more substances

Importance of Purity

• A pure substance consists of only one substance and contains nothing else.
• To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts
• Melting and boiling point analysis is routinely used to assess the purity of food and drugs
• For example, if a sample of water melts at exactly 0°C and boils at exactly 100°C then the water is pure
• If the melting and boiling points of the water aren’t these exact values then the water must be impure and contain other substances i.e. it must be a mixture

Retention Factor (Rf) Values

• Extended
  • These values are used to identify the components of mixtures
  • The Rf value of a particular compound is always the same
  • Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions
• Calculation
  • Retention factor = distance moved by compound  ÷ distance moved by solvent
  • The Rf value is a ratio and therefore has no units

Locating Agents

• For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for invisible samples such as proteins
• Locating agents are substances which react with the sample and produce a coloured product which is then visible
• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye