Certain organic acids, which contain no alcohol groups,[489] are [p241] also capable of forming fairly stable complexes with metal ions: thus, acetates form a complex with lead-ion, that is sufficiently stable to render lead sulphate, which is difficultly soluble in water, readily soluble in ammonium acetate solution[490] (‹exp.›). Soluble oxalates readily combine with ferric, ferrous, cupric and other oxalates and interfere, more or less, with the detection of the metal ions, as the result of complex formations. All of these complexes are ‹decomposed› rather readily ‹by the addition of strong acids›, whose hydrogen-ion breaks up the complex ions, by suppressing the anions[491] of the much weaker organic acids and alcohols. Consequently, these organic compounds do not interfere with tests which may be carried out in strongly acid solution. For instance, the addition of potassium ferrocyanide to a solution of ammonium ferrioxalate (NH_{4})_{3}Fe(C_{2}O_{4})_{3}, to which an excess of ammonium oxalate has been added, gives only a slight indication of the presence of ferric-ion (a greenish blue solution is obtained); when hydrochloric acid, in excess, is added to the mixture, Prussian blue, ferric ferrocyanide, is immediately precipitated in quantity (‹exp.›). FOOTNOTES: [425] The naming of the complex ions, which ammonia forms with metal ions, has not yet been satisfactorily settled. English writers frequently speak of "ammonio-argentic" ion and "ammonio-argentic" nitrate. German writers speak of "Silber-ammoniak" ion (Abegg, ‹Handb. der anorg. Chem.›, II, 728), which would read "silver-ammon‹ia›" ion in English terms. The terminology "silver-ammoni‹um›" ion, used in this book, is based on the idea, that all these complex ions are essentially ‹of the same nature› as the well-known ‹ammonium-ion›, NH_{4}^{+}, the positive charge being, almost certainly, carried by ‹nitrogen› in these complex ions, as it is in ammonium-ion. The latter is a ‹complex ion of ammonia› with ‹hydrogen›-ion. The name "ammonio-argentic" ion does not bring out this close relationship and puts the emphasis on the silver, which is probably little concerned in the reactions of the complex as such. The names "silver-ammon‹ia›" ion and "silver-ammon‹ia›" nitrate sound badly and do not emphasize the relation to ammoni‹um›, potassi‹um›, sodi‹um› and similar positive ions and their salts. The term "‹ammonium›" is, for the reasons given, used here in a ‹generic› sense for all complex ions of ammonia with simple metal ions (such as H^{+}, Ag^{+}, Cu^{2+}, Zn^{2+} etc.), and the number of ammonia molecules, entering into the composition of a complex ion, is not indicated in the names. A similar nomenclature has long been in vogue, and has worked well, for the complex ions of metal ions with the cyanide-ion (see below). We speak of ferro‹cyanide›, Fe(CN)_{6}^{4−}, argenti‹cyanide›, Ag(CN)_{2}^{−} and Ag_{2}(CN)_{3}^{2−}, etc.,