= Solution
\b[Confirm <hydrogen cyanide> by chemical tests on a suitable separated fraction, then estimate its amount with a defined quantitative assay.] Here “hydrocyanic acid” means $\mathrm{HCN}$ in <water>, also historically called prussic acid. Smelling or tasting is not an analytical test: <hydrogen cyanide> is acutely poisonous, and its characteristic odour is not reliably perceived.<Hydrogen cyanide emergency response information by NIOSH>[Agent characteristics; odor warning and exposure hazards]{ref}
A coloured or complex <liquid> can mask the tests or contain interfering ions. Historically, controlled <distillation> separated volatile <hydrogen cyanide> from many nonvolatile constituents; a receiving solution captures the analyte. In a modern analytical apparatus, the closed transfer system includes a condenser and an alkaline <gas> absorber. The receiver converts volatile <hydrogen cyanide> into less volatile <cyanide>:
$$
\mathrm{HCN}+\mathrm{OH}^-\longrightarrow\mathrm{CN}^-+\mathrm H_2\mathrm O.
$$
The measured fraction must be defined: free <hydrogen cyanide> in the original <liquid> is not automatically identical to “total <cyanide>” released from complexes by a more aggressive preparation. A blank, a known positive control and recovery checks help distinguish a negative sample from failed separation or reagent failure.<Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency>[§§1–4, scope and interferences; §§8–9, sample handling and quality control]{ref}
For the historical <iron> test, make a separate test portion alkaline, introduce fresh <iron(II)> salt together with an <iron(III)> source, and finally acidify as specified by the historical procedure. <Cyanide> first forms <ferrocyanide> with <iron(II)>; <iron(III)> then gives the characteristic <Prussian blue> precipitate. Idealized balances explaining the recognition chemistry are
$$
\mathrm{Fe}^{2+}+6\mathrm{CN}^-\longrightarrow[\mathrm{Fe(CN)}_6]^{4-},
$$
$$
4\mathrm{Fe}^{3+}+3[\mathrm{Fe(CN)}_6]^{4-}\longrightarrow\mathrm{Fe}_4[\mathrm{Fe(CN)}_6]_3\downarrow.
$$
Real <Prussian blue> can be hydrated and nonstoichiometric; the second <chemical equation> is a useful idealization. The acid stage also removes obscuring <iron hydroxide> precipitates. These steps explain the old mixed <iron>-salt test, rather than assuming that <cyanide> simply reacts directly with any <iron> salt to give blue material.<British Pharmacopoeia (1867)>[Acidum Hydrocyanicum Dilutum, printed p. 14, mixed ferrous/ferric salt, potash and acid sequence]{ref}
A second aliquot can be examined with <silver nitrate>. <Silver cyanide> is white, but whiteness alone is not specific because <silver chloride> and other precipitates can interfere:
$$
\mathrm{Ag}^++\mathrm{CN}^-\longrightarrow\mathrm{AgCN}\downarrow.
$$
For a purified aliquot, precipitate the <cyanide> quantitatively with <silver nitrate>, separate the <silver cyanide>, wash it free of soluble salts, dry it to constant <mass> and weigh it. The precipitation conditions must avoid losses as volatile <hydrogen cyanide> and dissolution as <dicyanoargentate>; interfering precipitates must be excluded. For complete collection of pure, dry <silver cyanide>, <gravimetric analysis> gives
$$
\boxed{m(\mathrm{HCN})=\frac{27.03}{133.89}\,m(\mathrm{AgCN})\simeq0.2019\,m(\mathrm{AgCN}).}
$$
Thus $10$ <mass> units of <silver cyanide> correspond to about $2.02$ <mass> units of <hydrogen cyanide>. The 1867 pharmacopoeial check was $10$ grains of dry <silver cyanide> from $100$ grains of the diluted acid, corresponding approximately to its stated $2\%$ strength by <mass>.<British Pharmacopoeia (1867)>[Acidum Hydrocyanicum Dilutum, printed pp. 13–14, identity, strength and silver-precipitation assay]{ref} A fraction or dilution of a sample requires the corresponding scaling back to the original quantity.
Alternatively, an alkaline <silver nitrate> <titration> depends on soluble <dicyanoargentate> formation:
$$
\mathrm{Ag}^++2\mathrm{CN}^-\longrightarrow[\mathrm{Ag(CN)}_2]^-.
$$
Before the endpoint, added <silver cyanide> redissolves in excess <cyanide>. In the classical version, the first persistent turbidity marks the end of this clear-solution stage. For a clean sample, a standardized <silver nitrate> <molar concentration> $c$, a blank-corrected endpoint <volume> $V$, and an aliquot fraction $f$ of the total recovered sample,
$$
\boxed{n(\mathrm{HCN})_{\rm original}=\frac{2cV}{f},\qquad m(\mathrm{HCN})_{\rm original}=\frac{2cV}{f}(27.03\ \mathrm{g\,mol^{-1}}).}
$$
Here $V$ is in litres if $c$ is in $\mathrm{mol\,L^{-1}}$. Divide by the original sample <mass> for a <mass fraction>, or by its original <volume> for a <mass concentration>. No numerical sample amount is supplied, so no particular measured <concentration> can be inferred.
\b[Keep the two assay ratios separate: precipitation gravimetry gives one mole of <silver cyanide> per mole of <hydrogen cyanide>; alkaline complex-formation <titration> uses one mole of <silver> per two moles of <cyanide>.] A modern version of the latter uses a rhodanine indicator and a blank, as in EPA Method 9014; that indicator procedure is a modern comparison, not a detail assigned to 1867.<Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency>[§11.3, silver-to-cyanide reaction, rhodanine endpoint and blank; §12.2, calculation]{ref}
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