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 NaO_{2}C—CH(‗OH‗)—CH(‗OH‗)—CO_{2}Na. 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}(CO_{2}^{−})_{2} + 2 Na^{+} ⇄ (CHO^{−})_{2}(CO_{2}^{−})_{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}(CO_{2}^{−})_{2} + Cu^{2+} ⥂ [(CHO)_{2}Cu](CO_{2}^{−})_{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.