= Solution
\b[An organic substance may also contain <nitrogen>, <sulfur>, <phosphorus>, <halogens> and, in some cases, <metals> or other elements.] “Organic” does not mean “made of only carbon, hydrogen and oxygen”. Detection generally requires converting a covalently bound element into a species that an appropriate inorganic test can recognize.
A historically available general method is the <sodium fusion test>. Fusion converts suitable nitrogen-containing material into <cyanide>, sulfur into <sulfide>, and chlorine-, bromine- or iodine-containing material into the corresponding soluble halides. The cooled reaction material is worked up to an aqueous extract, with separate portions reserved for separate tests. The transformations are analytical conversion principles, not claims that every nitrogen or sulfur compound must give quantitative recovery under any fusion conditions. Blanks and a known positive material are essential when a negative result is to be interpreted.<Basic Tables for Chemical Analysis by National Bureau of Standards>[Organic qualitative analysis, sodium-fusion screening for nitrogen, sulfur and halogens, printed p. 203 and associated tables]{ref}
For <nitrogen>, a suitable cyanide-containing extract is treated with a fresh <iron(II)> reagent and then brought through the appropriate oxidation and acidification sequence to obtain <Prussian blue>. Formation of <ferrocyanide> is represented by
$$
\mathrm{Fe}^{2+}+6\mathrm{CN}^-\longrightarrow[\mathrm{Fe(CN)}_6]^{4-}.
$$
The eventual ferric ferrocyanide material provides the blue observation. It is not sufficient simply to add any iron salt and assume that a positive blue must appear. Another classical principle converts nitrogen in suitable organic materials into <Ammonia> by heating with alkaline reagents, then recognizes the ammonia in a receiver. That test has substrate-dependent limitations and is not a universal replacement for complete nitrogen analysis. A later specialized nitrogen-digestion protocol must not be silently attributed to 1865.
For <sulfur>, a sulfide-containing alkaline extract can give black <lead sulfide> with a suitably introduced lead reagent:
$$
\mathrm{Pb}^{2+}+\mathrm S^{2-}\longrightarrow\mathrm{PbS}\downarrow.
$$
The strongly acidic form of the reagent conditions changes the available sulfide, so conditions must be controlled. Oxidizing the material to <sulfate> and identifying a persistent <barium sulfate> precipitate supplies another principle, using sulfur-free reagents and excluding pre-existing sulfate contamination. If nitrogen and sulfur occur together, thiocyanate formation can alter a fusion test; failure of the expected cyanide reaction is not conclusive evidence that nitrogen is absent.
For <chlorine>, <bromine> and <iodine>, remove interfering cyanide and sulfide by an appropriate controlled treatment of a separate extract portion, then use <silver nitrate> under suitable nitric-acid conditions. The precipitates are ordinarily white <silver chloride>, cream <silver bromide> and yellow <silver iodide>. Their differing behaviour with aqueous <Ammonia> provides corroboration: chloride dissolves relatively readily, bromide requires stronger conditions, and iodide is resistant. Do not use <hydrochloric acid> to prepare the portion in which original chloride is being sought. <Fluorine> requires a different recognition method: soluble <silver fluoride> means this precipitation scheme does not provide an equivalent fluoride test.
For <phosphorus>, oxidize a suitable portion to <phosphate>, then apply a phosphate test such as yellow <ammonium phosphomolybdate> formation under the proper acidic conditions. <Arsenic> species and other interferences must be excluded when relevant. Mineral ash can reveal <metal> constituents through their specific precipitation or flame reactions, but oxidation during ashing can change their original chemical form. Absence of a visible ash does not prove absence of every metallic or otherwise volatile element.
A positive result identifies an element or an operationally generated ion; it does not establish which <functional group> contained it in the original molecule. Conversely, an incomplete conversion can give a false negative. This is why the three-element premise used in the preceding combustion calculation should be checked rather than accepted merely because a substance is described as organic.
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