Chemical analysis

Purity and formulations

  • Chemical analysis is the different instruments and methods used to separate, identify and quantify different substances

  • A pure substance contains only one type of compound or element

    • For example, pure water with only H and O atoms

    • They melt and boil at specific temperatures, like 100 degrees for water

    • Their specific temperatures can be used to find what solutions they are - a physical test

  • Tests that react substances with other chemicals are known as chemical tests

  • Impure substances, like salt water, are a mixture of two or more different compounds

    • They melt and boil over a range of temperatures, dependent on the ratio between the two substances

  • Formulations are mixtures that have been prepared by using a specific formula 

    • They have precise amounts of different substances, and have a particular function

      • For example, fuels, paints, medicines or alloys

    • Different components are always present in the same proportions

    • Each component contributes a different property 

Tests for gases

  • Chlorine is tested for by placing damp blue litmus paper in a test tube with the sample of gas 

  • If chlorine is present, the paper turns white

    • Chlorine is poisonous, so a mask should be worn, and the experiment done in a fume cupboard

  • Oxygen is tested for by placing a glowing splint in a test tube of the gas

    • If it relights, oxygen is present

  • Hydrogen is tested for by placing a burning splint above a test tube of the sample

    • If present, there will be a squeaky pop sound!

  • Carbon dioxide is tested for by bubbling the gas in a limewater solution

    • If present, the limewater goes cloudy 

    • This happens as the carbon dioxide and calcium carbonate react to produce water and CaCO3, a solid with particles that are visible

Paper chromatography

  • Chromatography is used to separate substances in a mixture

  • Paper chromatography can be used to separate dyes in an ink

  • The solvent doesn’t touch the line or the ink, but comes up the paper, and the lid is added to prevent evaporation

  • The solvent is seeped up by the paper, and it separates it’s colour

    • They move at different rates

  • Leave the paper to dry to create a chromatogram

  • The mobile phase is where the molecules move in, as a liquid or gas, and is the solvent in the chromatography

  • The stationary phase is where molecules can’t move, the paper in the chromatography

  • The different molecules in the ink constantly change phases

    • They dissolve, move a bit, bind to the paper, etc.

    • The more soluble they are, the faster they move

    • If they are less soluble, they are more attracted to the paper, so move slower

  • How far each chemical travels is dependent on the properties of that sample        

    • A sample will have the same results every time

  • We can compare the distance moves per chemical with a data log to find and identify the chemical

    • However, the distanced travelled is dependent on the time, so we use an Rf value

  • Rf value = distance travelled by substance (individual)/distance travelled by solvent (overall)

  • If a pure substance is used, there will be a single dot only shown on the chromatogram

    • Mixtures have multiple 

  • If the paper or solvent changed, results would differ and therefore the Rf value as mobile/stationary stages have changed

    • This has to be taken into account when comparing results

Testing for anions

  • Anions are negatively charged ions

  • Carbonates

    • A carbonate and an acid react to form a salt, carbon dioxide and water

    • We can test for carbon dioxide, which proves a carbonate was a reactant

  • Dilute HCl and the carbonate sample are reacted together, and the products are bubbled into limewater

    • If the limewater turns cloudy (CO2 present), a carbonate was reacted

  • Sulphates

    • Sulphate ions and barium ions react to form barium sulphate

    • Barium sulphate is a white precipitate

  • Hydrochloric acid and the sulphate ions are mixed to remove impurities that may effect the result

    • CO3 or SO3 may also create a white precipitate if present

  • Barium sulphate is then added

    • If a white precipitate is formed, sulphate was present as a reactant

  • Halides

    • Halides are ions with a negative 1 charge - for example, Cl-, Br- + I-

  • Dilute nitric acid is mixed with the halide to remove impurities - CO32- or SO32-

  • Silver nitrate is then added, to produce a precipitate if present

    • Chloride and silver nitrate - white precipitate

    • Bromide and silver nitrate - cream precipitate

    • Iodide and silver nitrate - yellow precipitate

Tests for cations

  • Cations are positively charged ions and they are mainly metals (not NH4+ - ammonium)

  • Flame tests

    • Take a platinum or nichrome wire loop and clean by dipping in dilute HCl, rinsing in distilled water and heating over a bunsen burner

    • Dip the loop into the substance that is being tested

    • Hold the loop in the clear blue section of the flame

    • See what colour the compound turns

  • Lithium turns crimson, sodium turns yellow, potassium turns lilac, copper turns orange-red and copper turns green

    • If there are more than two cations present in the sample, the colours are likely to mix

  • Metal hydroxide test for cations

    • Used for metal ions, by reacting them with sodium hydroxide (NaOH)

    • Possible as some metals form coloured precipitates when reacting with OH ions

  • Copper forms a BLUE precipitate when reacting with NaOH

    • Cu2+ + 2OH- → Cu(OH)2  (half equation)

    • Copper + sodium hydroxide → sodium chloride + copper hydroxide

  • Calcium forms a WHITE precipitate

    • Ca2+ + 2OH- → Ca(OH)2

    • Calcium + sodium hydroxide → sodium chloride + calcium hydroxide

  • Iron(II) forms a GREEN precipitate

    • Fe3+ + 2OH- → Fe(OH)3

    • Iron + sodium hydroxide → sodium chloride + iron hydroxide

  • Iron(III) forms a brown precipitate

    • Fe2+ + 2OH- → Fe(OH)2

    • Iron + sodium hydroxide → sodium chloride + iron hydroxide

  • Magnesium forms a WHITE precipitate

    • Mg2+ + 2OH- → Mg(OH)2

    • Magnesium + sodium hydroxide → sodium chloride + magnesium hydroxide

  • Aluminium forms a white precipitate, which turns colourless if NaOH is in excess

    • Al3+ + 3OH- → Al(OH)3

    • Aluminium + sodium hydroxide → sodium chloride + aluminium hydroxide

Flame emission spectroscopy 

  • When metal ions are heated, they emit light 

    • Wave lengths of light that are emitted are specific to particular metal ions

  • Spectroscopes detect the individual wavelengths of light that are emitted

    • Every metal has a different pattern of wave lengths (when heated), which can be used for identification

  • The intensity of the line shows the concentration of the ion, and therefore the concentration of the iron

  • If a sample contains multiple ions, the spectrum will show the lines for all of them     

    • From this, we can identify both ions and their concentrations

  • Flame tests (manual test) VS flame emission spectroscopy (instrumental)

    • Flame tests are basic and cheap, although less accurate

    • FEM is expensive (the machinery), but also more sensitive to change, more accurate and very fast (can be automated entirely)

DONE!!!