The relations may be illustrated by the discussion of the complex formed by the cupric-ion with tartrates. The structure of sodium tartrate is expressed by NaO2C─CH(OH)─CH(OH)─CO2Na. The underscored hydroxide groups OH are known in organic chemistry as alcohol groups.[487] Now, alcohols resemble water in very many properties and, among others, in the capacity to form metal derivatives or alcoholates, in which the hydrogen (ion) of the hydroxide group is replaced by metal ions. The alcoholates correspond, thus, to the metal hydroxides, which are the analogous [p240] derivatives of water. Exactly as there is a vast difference in the readiness with which the various metal hydroxides, or bases, ionize, many of them being only slightly ionizable (the weakest bases), so certain alcoholates are much less readily ionizable than others. The alkali alcoholates are most readily ionized.
When sodium tartrate is mixed with an excess of sodium hydroxide, some of the readily ionizable sodium salt of the alcohol groups of the sodium tartrate is formed and we have:
(CHONa)2(CO2−)2 + 2 Na+ ⇄ (CHO−)2(CO2−)2 + 4 Na+.
When cupric sulphate is added to this mixture, a slightly ionizable complex cupri-tartrate-ion is formed by the union of the cupric-ion with the "alcoholate-tartrate-ion":
(CHO−)2(CO2−)2 + Cu2+ ⥂ [(CHO)2Cu](CO2−)2.
The complex ion is not perfectly stable and so the action is a reversible one, as indicated in the equation. The greater portion of the copper, however, is present as part of the complex negative ion of cupric-tartaric acid and its salts. This may be demonstrated by subjecting the solution to electrolysis in a U-tube (p. [45]). It is readily seen (exp.) that a deep blue ion, obviously containing copper, migrates to the positive pole.[488] The concentration of cupric-ion is so small that its hydroxide and its phosphate are not precipitated from the solution by the addition, respectively, of alkali or of a soluble phosphate (exp.). Cupric sulphide, however, is so insoluble that it may be precipitated completely from the solution by the addition of a sulphide (exp.), the concentration of cupric-ion being much smaller in the saturated solution of the sulphide than in the solution of sodium cupri-tartrate.
Citrates, sugars, glycerine, contain alcoholic groups of the same nature as found in the tartrates, and they are capable of forming similar complexes, or little ionizable alcoholates, with metal ions.
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 (NH4)3Fe(C2O4)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.).