aristo chemistry

Topics: Ion, Molecule, Atom Pages: 41 (5164 words) Published: October 16, 2014


Contents

PART XV ANALYTICAL CHEMISTRY
Chapter 63Detecting the presence of chemical species
Action of heating solid sample strongly

1
Chapter 64Separation and purification methods
Centrifugation
Sublimation
Partition equilibrium of a solute between two immiscible solvents Two-dimensional thin-layer chromatography

2
2
3
6
Chapter 65Quantitative methods of analysis
Detection of end point in acid-alkali titration

8
Chapter 66Instrumental analytical methods
More about infrared spectroscopy
More about mass spectrometry

12
15
Chapter 67Contribution of analytical chemistry to our society Gas chromatography-mass spectrometry

32
PART XVANALYTICAL CHEMISTRY

Chapter 63Detecting the presence of chemical species

Action of heating solid sample strongly
Heating the solid sample strongly is one of the preliminary tests of substances. This can be done by heating the sample over a non-luminous Bunsen flame in the laboratory. In case there is any gas evolved, further tests can be done to identify the gas. See Table S63.1. Observation

Inference
Yellow sublimate
Sulphur
Violet vapour and dark grey shiny sublimate
Iodine
Dark red sublimate
Iron(III) chloride, FeCl3
White sublimate
Ammonium halides e.g. AlCl3
Water vapour evolved
Hydrated salts; hydrogencarbonates; some hydroxides
Colour change
Some hydrated salts
Rust red residue
Fe2O3 formed (from decomposition of iron compounds)
Black residue from blue or green sample
CuO formed (from decomposition of copper(II) compounds)
Yellow residue when hot, white when cold
ZnO formed (from decomposition of zinc compounds)
Orange residue when hot, yellow when cold
PbO formed (from decomposition of lead(II) compounds)
Ammonia evolved
Some ammonium salts
Carbon dioxide evolved
Some carbonates; hydrogencarbonates
Nitrogen dioxide evolved
Nitrates and nitrites (except those of potassium, sodium and ammonium) Oxygen evolved
Nitrates (except NH4NO3); some oxides of metals (e.g. PbO2, HgO, Ag2O); chlorates (e.g. KClO3) Sulphur dioxide evolved
Sulphites (except those of sodium and potassium); some sulphates Table S63.1 Observation and inference from the action of heating solid sample strongly. Chapter 64Separation and purification methods

Centrifugation
Centrifugation can be used instead of filtration to separate a solid from a liquid. It is particularly useful when there is only a small amount of material, or when very fast separation is required.

Firstly, the mixture containing an undissolved solid and a liquid is put into a centrifuge tube. Then the centrifuge tube, together with other tubes, are put into the tube holders in a centrifuge (Figure S64.1). The holders and tubes are spun around very rapidly, and are thrown outwards. The undissolved solid and the liquid in the mixture are spun at the same time (Figure S64.2). Finally, the denser solid is collected as a lump at the bottom of the tube. Then the liquid can be removed by decantation.

Figure S64.2 Centrifugation.

Sublimation
A few substances change directly from solid to vapour on heating, without going through the liquid state. This physical change is called sublimation. On cooling, the vapour changes back to solid directly.

Consider the following example. If a mixture of iodine and sand is heated in a beaker, the iodine changes from solid to vapour directly (Figure S64.3). The vapour changes back to solid directly on a cool surface. The sand is not affected by the heat and remains in the beaker.

Figure S64.3 Sublimation of iodine.

Other substances which sublime include solid carbon dioxide (‘dry ice’) (Figure S64.4), anhydrous iron(III) chloride, anhydrous aluminium chloride and some ammonium salts. Since only a few solids sublime, this method of separation is limited in use.

Partition equilibrium of a solute between two immiscible solvents Water and heptane are immiscible solvents. When iodine is extracted from an...
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