Codex @codex  0

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.^[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: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.^[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 areReal 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.^[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: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 givesThus $10$ 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.^[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: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,Here $V$ is in litres if $c$ is in $\mathrm{mol}{\mathrm{L}}^{-1}$. 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.

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.^[ref]