Inorganics - Copper Reagents: Cuproine, Neocuproine, Bathocuproine - Excerpt from Monograph
SECTION I
INTRODUCTION
The reaction of
1,10-phenanthroline and of 2,2'-bipyridine and their derivatives with the ferrous ion, yielding intensely red, soluble compounds, has proved of great value in analytical chemistry. Although the greatest interest in this reaction has been in the colorimetric determination of iron, other properties of the colored ions formed have been of great service too. The color change from blue to red of the oxidation-reduction couple Fe(l,10-Phenanthroline)
3***-Fe(l,lO-phenanthroline)
3*** furnishes a high potential oxidation-reduction indicator of great utility. The insolubility of tris (1,10phenanthroline) ferrous perchlorate has provided a satisfactory procedure for the gravimetric determination of perchlorate.
The trivial name ferroin was coined for the ferrous 1,lO-phenanthroline ion the oxidized and reduced forms of the couple being ferriin and ferroin, respectively. The terms ferroin reaction and ferroin group are used more generally for the color reaction of the ferrous ion with any 1,lO-phenanthroline or 2,2'-bipyridine or for any compound having the atomic grouping
=C-C=
/ \
=N N=
which is responsible for the reaction.
Quite early it was observed that the ferroin reaction failed with those compounds bearing substituent groups on the carbon atoms adjacent to the ring nitrogen atoms. It was only considerably later, however, that it was observed that the compounds which failed to give the ferroin reaction did produce colors with cuprous copper. The reaction was first discovered with 2,2'-biquinoline and was subsequently found for the 2,9-dialkyl-l,10-phenanthrolines. These substances are specific for copper and by analogy the terms cuproine reaction and cuproine group have been employed for the color reaction and the responsible atomic grouping.
Less happily the term cuproine has come to designate 2,2'-biquinoline rather than its colored cuprous derivative in contrast to ferroin which is used exclusively for the metal derivative.
The names neocuproine and bathocuproine were coined later and refer to the reagents 2,9-dimethyl-l,1O-phenanthroline and 2,9-dimethyl-4,7diphenyl-l,lO-phenanthroline, respectively. Sulfonated bathocuproine has been used as a common name for 2,9-dimethyl-4,7-diphenyl-l,10-phenanthrolinedisulfonic acid disodium salt.
These reagents are highly specific chromogens for copper for they react with no other metals to give colored products. The copper is present in the colored derivatives of these reagents in the univalent state. Two molecules of the reagent are associated with each copper atom, both nitrogen atoms of each molecule being bound to the copper so that two five-membered rings are formed. The four covalent bonds thus formed to the copper atom are directed toward the apexes of a regular tetrahedron about the copper atom so that the planes of the two five-membered rings lie at right angles to each other.
Fortunately, the copper compounds of three of these four cuproine reagents are not only intensely colored but are soluble in certain solvents immiscible with water. Thus, the copper may be concentrated by extraction into an immiscible solvent and a considerable increase in sensitivity achieved. Even more important the extraction process may be used to remove any copper in the distilled water and the solutions of the reagents used in the analysis, reducing the blank to zero and improving the certainty with which the determination of copper can be made. The sulfonated compound is water soluble and is used only in aqueous solution.
Hydroxylammonium chloride is usually chosen as the reducing agent and ammonium acetate as the agent for suitably buffering the solution. Other reagents can be used, however. Similarly, though isoamyl alcohol has usually been chosen as the extracting liquid, other higher alcohols can be used. Because the molar absorptivity of bathocuproine is the largest of the three, bathocuproine can be used for the determination of smaller amounts of copper than can be handled with cuproine or neocuproine. Curiously, though, as a visual qualitative test cuproine is the best of the four by a slight margin, its purple color being more easily detected by eye on a white background than the yellow or orange-yellow of the other three. The molar absorptivities and working ranges of the reagents are summarized in Table I.
Although copper is not an abundant element, not even among the first twenty in the earth's crust, its availability in large deposits and its excellent metallurgical properties have brought it into widespread use: in coinage, plumbing, electrical wire, and structural alloys. Its determination is thus a matter of considerable importance and excellent methods for its determination in macro amounts have been devised, notably the iodometric and electrodeposition methods (see for example, Quantitative Analysis, by Harvey Diehl, Oakland Street Science Press, Ames, Iowa, 1970). The,determination of small amounts of copper is almost equally important for traces of copper are essential to the well-being of the animals, and the small amounts which find their way into commercial products from natural sources or from piping and containers often play significant roles, as in the deterioration of food products on storage. Of the numerous methods which have been proposed for the determination of small amounts of copper, the colorimetric methods based on cuproine, neocuproine, and bathocuproine are the best in being both sensitive and specific. Small amounts of copper are almost always associated with iron, and not the least of the merits of the cuproine reagents is their immunity to disturbances by even large quantities of iron. The 3-(2-pyridyl)5,6-diphenyl-1,2,4-triazine reagent (PDT) can playa dual role, that of determining both iron and copper in a single sample with good sensitivity for each. Details are presented in Section VII.
These compounds are not easy to synthesize and their prices are high although dropping as their use increases and experience is gained in their manufacture. Colorimetric reagents, however, go a long way and the cost per determination is negligible, particularly in comparison with the value of time saved over older methods. Most of the reagents used in the procedures contained in this monograph are available from the company and are so noted. A list of these may be found in the Appendix, Section VIII.
