= Solution
TODO: Solutions e and f and their test observations are absent. Hydrocyanic acid, iron and magnesia are testing targets, not supplied positive findings; general test methods can still be described.
\b[The presence or absence of <hydrogen cyanide> in e, and of <iron> or <magnesium> in f, remains unestablished.] The named substances are targets of the examination. They are not positive findings.
The period name “hydrocyanic acid” refers to aqueous <hydrogen cyanide>. “Magnesia” is the historical name for <magnesium oxide>; examination of a <chemical solution> for magnesia normally concerns dissolved <magnesium> compounds, rather than a claim that intact <magnesium oxide> particles are dissolved unchanged. The modern <ionic equations> below express test principles and are not observations made on f.
For e, the analytical task is to obtain a response attributable to <cyanide>, distinguish it from responses of other constituents, and check the entire procedure against a <reagent blank> and a known reference. The distinction between free <cyanide> and <cyanide> held in a <coordination complex> matters: a test for one fraction need not measure the other. As a modern comparison, the <Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency> provides <titrimetric analysis> and <spectrophotometric analysis>, with specified sample preparation, interferences and quality controls. This 2014 procedure illustrates how a result is validated; it is not being attributed to the 1865 candidate.<Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency>[§§1–2: analytical scope and measurement principles; §§4 and 9: interferences and controls]{ref} No numerical <concentration>, positive colour or negative result can be extracted from the label e.
An odour is neither an acceptable identifying experiment nor a reliable negative test. <Hydrogen cyanide> is volatile and highly toxic, and many people cannot detect its supposed characteristic smell. Any actual analysis belongs in a suitably contained laboratory method; uncontrolled acidification of a possible <cyanide> solution can release <hydrogen cyanide>.<Hydrogen cyanide emergency response information by NIOSH>[Agent Characteristics: volatility, toxicity and unreliable odour warning]{ref}<Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency>[§5.2: hydrogen cyanide release on acidification]{ref}
For f, distinguish <iron(II)> from <iron(III)> when describing a test. On suitable separate portions, <hydroxide> can produce a green <iron(II) hydroxide> <precipitate> or a brown hydrated <iron(III) hydroxide> <precipitate>. <Iron(II)> products can oxidise during handling, so the time at which a colour is observed matters. The simplified precipitation balances are:
$$
\mathrm{Fe^{2+}+2OH^-\longrightarrow Fe(OH)_2(s)},\qquad
\mathrm{Fe^{3+}+3OH^-\longrightarrow Fe(OH)_3(s)}.
$$
These equations describe possible chemical responses, not a choice between them for the missing f. Tests intended for a particular <oxidation state> cannot be treated as universal tests of every form of <iron>.
Dissolved <magnesium> can give a white <magnesium hydroxide> <precipitate>:
$$
\mathrm{Mg^{2+}+2OH^-\longrightarrow Mg(OH)_2(s)}.
$$
The white solid does not characteristically dissolve in excess <sodium hydroxide> under the usual qualitative-test conditions. This differentiates it from some amphoteric <hydroxides>, but is not uniquely specific for <magnesium>.<Testing Salts for Anions and Cations by the Royal Society of Chemistry>[Metal hydroxides: iron(II), iron(III), magnesium and the distinction from aluminium in excess sodium hydroxide]{ref} If <iron> and <magnesium> were together, a single mixed <precipitate> could obscure their separate behaviour. An analytical separation with controlled <pH>, followed by confirmation on the separated fractions, would be needed; indiscriminately adding excess alkali is not proof of separation.<Qualitative Analysis of Metal Ions in Solution by Williams College>[printed II & III-15–17: hydroxide concentration, magnesium interference and iron confirmation]{ref} The actual <oxidation state> of any <iron>, the presence of <magnesium>, and the outcome of such a separation all remain unknown.
Back to article page
