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Identifying unknown compounds requires a variety of methods such as IR, MS, EMS, XRD,
etc.
- First lets start with a flame test. Generally if it burns without leaving a residue, its
probably organic (containing carbon). If it doesn't burn and remains as a residue, its
probably an inorganic compound. Compounds which burn and leave a residue may be
organometallic.
- Determine the solubility of the compound in water, alcohol, and methylene chloride. The
pH of the water solution is also important; this can tell you if the salt is acidic or
basic.
- For inorganic compounds, X-Ray Fluorescence (XRF)
for metals composition and X-ray
diffraction (XRD) for identifying crystalline compounds give the most information.
XRF is
convenient since the material does not need to be dissolved. . Note that XRD is useful for crystalline compounds and not for amorphous
materials (glasses and other materials lacking a crystal lattice). Infrared spectroscopy
(IR) may also be helpful.
- For organic compounds, infrared spectroscopy
(IR), mass spectrometry (MS), nuclear
magnetic resonance (NMR), and elemental analysis (CHNS) will usually identify the
compound.
- Needless to say mixtures can be difficult, requiring several methods as well as
separation techniques.
- Frequently we are asked to identify salts in solution. Since salts ionize it is
difficult to determine which starting compounds were used. For example a solution of Na+,
K+, Cl-, and Br- may have started out as KCl and NaBr or visa versa. It may be possible to
determine the starting materials from stoicheometry, i.e. the molar concentrations of the
ions, but having more than one source for the ions in solution can make this impossible
and perhaps even irrelevant.
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