In inorganic chemistry, when is the prefix meta used? (as in metaborate and metasulphite) What about the terms pyro and ortho (as in orthophosphorous acid)?
For example, I recently came to know about the prefixes per and hypo which are used to refer to the highest and lowest oxidation states of an element respectively (like perchlorate and hypochlorite).
Are there any terms and conventions I should be aware of?
Answer
In the older literature, the prefixes “hypo-”, “per-”, “ortho-”, “meta-”, and “pyro-” were used in some cases for the distinction between different acids with the same characteristic element.
The prefix “hypo-” was used to denote a lower oxidation state, and the prefix “per-” was used to designate a higher oxidation state. (The prefix “per-” is not be confused with the prefix “peroxo-”.)
$\ce{HClO}$ hypochlorous acid
$\ce{HClO2}$ chlorous acid
$\ce{HClO3}$ chloric acid
$\ce{HClO4}$ perchloric acid
The prefixes “ortho-” and “meta-” were used to distinguish acids differing in the formal content of water.
$\ce{H3BO3}$ orthoboric acid
$\ce{(HBO2)$_n$}$ metaboric acid$\ce{H4SiO4}$ orthosilicic acid
$\ce{(H2SiO3)$_n$}$ metasilicic acid
The prefix “pyro-” was used to designate an acid that is formally formed by removing one molecule of water from two molecules of an ortho-acid.
$\ce{H4P2O7}$ pyrophosphoric acid $(\ce{2 H3PO4 -> H4P2O7 + H2O})$.
Various traditional names are retained for use in IUPAC nomenclature, though the number of retained names has been reduced with each succeeding edition of the IUPAC recommendations. The Second Edition (1970 Rules) of the IUPAC Nomenclature of Inorganic Chemistry (published in 1971) still included all of the above-mentioned prefixes; however, the approved use was limited to only some selected retained names. The further development and the current situation of the nomenclature are well explained in @Jan’s thorough answer.
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