Methods of purification along with why it is important, how to tell whether a substance is pure or not, experimental techniques, Rf values, etc...

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  • Created by: hattie
  • Created on: 08-04-10 18:17

Criteria of Purity...

Why is it important?

It's important because...

  • impurities can be toxic, causing side effects which could be fatal.
  • Food and drug manufacturers need to ensure that all products are as pure as possible.

How can you tell if a substance is pure?

Check the melting and boiling point...

  • A pure substance has a definate melting and boiling point (plateau on graph).
  • When a substance is impure it's melting point falls, and boiling point rises. (mpt and bpt are not definate, but are a range of temps)
  • The more impurity, the larger change/wider range in temp.

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Conical flask containing filtrate, holding a funnel containing filter paper with the residue.

  • The residue of the insoluble material is left on the filter paper.
  • Filtrate is made of a solvent and a solute.
  • The solute is being dissolved.
  • Solvent is a liquid, Solute is the solid.
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Fractional distillation...

Bulb flask holding mixture, fractioning column coming out the top which is attatched to a condenser which drips into a beaker.

1.The mixture is heated. At about 78degrees the ethanol begins to boil. Somewater evaporates too. A mixture of ethanol and water vapour rise into the column.

2. The vapours condense on the glass beads in the column making them hot.

3. When the beads reach about 78degrees ethanol vapour no longer condense on them, only water does, so water drips back into the flaskwhile the ethanol goes into the condenser where it condenses to liquid and drips in the beaker.

4. The temp will rise when all ethanol has been distilled.

  • It is a way to separate a mix of two or more imiscible liquids, whoes boiling points are known.
  • It's used to refine oil, separtate gases in the air and to separate ethanolfrom a fermentationmixture of sugar cane.
  • The liquid with the lowest boiling point will distill off first.
  • Water goes in at the bottom and out at the top.
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Bunsen burner under a tripod holding a bowl of solution.

  • The removal of a solvent from a solution.

NB Solution should not be evaporated to dryness, (5cm3 of liquid should be left) as water of crystallisation will be removed.

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Simple distillation...

A bulb flask connected to a condenser leading to a beaker.

This is a way to obtain a solvent from a solution. e.g. to obtain water from salt water.

1. The solution is heated in a flask. It boils and steam rises into the condenser. The salt (solute) is left behind.

2. The condenser is cold so the steam condenses to water and then drips into the beaker. It is called distilled water and is almost pure.

  • A solvent is removed and captured.
  • The mixture could be a solution or a liquid with a small amount of impurity.
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It is used for the detection of drugs in urine - athletes, isolation pigments and dyes, forensic science testing blood found at crime scene, sugars and amino acids an also be separated.

The sample is applied to the paper in small amounts.

As the solvent rises up the paper it carries the dissolved solutes with it. The more soluble is in the solvent, the further the sulute will travel and be separted from the others.

Some spots may be colourless so they will be located by spraying with ninhydrin or treatment with iodine vapures (location agents). These substances form a new compound with the material on the paper and then produce a coloured compound which can be seen clearly.

The Rf value of a spot = distance moved by solute / distance moved by solvent.

NB values will always be less than 1.0

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