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?

up vote 18 down vote accepted

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.

  • 3
    Hey sorry but I still Did not get the ortho and meta part! What do you mean by formal content of water? – Karan Singh Nov 11 '15 at 20:23
  • @KaranSingh For example, metaboric acid can be formed by condensation (dehydration) of orthoboric acid. Heating of orthoboric acid releases one equivalent of water to give metaboric acid: $\ce{$n\,$H3BO3 -> (HBO2)$_n$ + $n\,$H2O}$. Therefore, the formal difference is just the content of one equivalent of water. – Loong Nov 11 '15 at 20:43
  • @Loong But meta phosphoric acid is not formed by loosing one water molecule from orthophosphoric acid. So what is general equation? – JM97 Mar 27 '16 at 14:37
  • @Loong What's the difference between -ous and -ic? – Abcd Sep 13 at 13:41

Are there any terms and conventions I should be aware of?

Yes. IUPAC publishes the Red Book also called Nomenclature of Inorganic Chemistry which is the authoritative source of nomenclature terms. Its current version (2005; the only one I will be referencing above the horizontal rule) does not contain the word pyro meaning that that is unacceptable for inorganic nomenclature purposes.

Ortho and meta are sometimes allowed as traditionalised common names. Ortho only in the cases of ortho-periodic acid and ortho-telluric acid and their derivatives ($\ce{IO(OH)5}$ and $\ce{Te(OH)6}$, respectively) to distinguish them from periodic acid ($\ce{HIO4}$) and telluric acid ($\ce{H2TeO4}$). Meta is only allowed for metasilicate and metaborate, both describing long-chain anions whose systematic name would be catena-poly[(oxidoborate-μ-oxido)($1-$)] (or with a dioxidosilicate instead of oxidoborate) to distinguish them from silicate ($\ce{SiO4^4-}$) and borate ($\ce{BO3^3-}$), respectively.

The usage ‘*ortho-phosphoric acid’ and the like has been deemed unnecessary and obsolete because there is no ‘other’ form of phosphoric acid that one could want to differ between. The former meta-forms are better described as what they actually are: cyclo-triphosphoric acid, for example.

Note that the prefixes hypo and per are also non-systematic as per IUPAC every instance where they occur can also be named systematically. However, they remain as acceptable alternatives in many instances due to their simplicity and commonness, e.g. peroxide, periodic acid, hypochlorite. Note that not always does per refer to the highest oxidation state: persulphate is still a sulphur(VI). However, there is a peroxido ligand attached to it.

Historically, the terms were also used in an often non-systematic way. Ortho is Greek and means true, implying that an ortho-something be the true form of something: Hence ortho-phosphoric acid for ‘the true’ monophosphoric $\ce{H3PO4}$.

Meta means after in Greek, so Aristotle’s Metaphysics was a book that was conventionally placed after Aristotle’s Physics in the library of Alexandria. Thus, meta compounds are the ones which arrived after ortho compounds by some mechanism. meta-Phosphoric acid was seen as a monohydrate of $\ce{P2O5}$ while ortho-phosphoric acid was the ‘true’ trihydrate of $\ce{P2O5}$. ($\ce{P2O5} + \ce{H2O} = 2 \ce{HPO3}$; while the formula for phosphorous pentaoxide is actually $\ce{P4O10}$ and that of cyclo-triphosphoric acid would be $\ce{H3P3O9}$.) The logic is that the ‘true’ hydrate needs to be hydrated, to afterwards give the meta-hydrate.

Para is not used in inorganic chemistry to the best of my knowledge.

Pyro, meaning fire in Greek, is a prefix for something that has to do with being burnt in some way. Hence, pyrophosphates were initially prepared by heating hydrogen phosphates to red glow. A few decades later, researchers realised that the ‘pyro’-phosphate was, in fact, a diphosphate. Likewise, pyruvic acid was originally prepared by heating of tartaric acid. According to a quick search across Wikipedia, pyro is only applicable to phosphate and sulphate and their respective acids meaning the dehydrated dimer. Personally, I have never encountered pyro-anything outside biochemical lectures (and my school’s biology classes) when connected to phosphate. You can consider all pyros entirely obsolete and replaced by di- (disulphuric acid, disulphate, diphosphate, etc.).

  • Also, a question:) Does it all also mean that orthovanadate and metavanadate are obsolete and shouldn't be used? – andselisk Sep 20 '17 at 8:10
  • 2
    @andselisk To the best of my knowledge yes. – Jan Sep 20 '17 at 9:55

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