The discovery by Blau in the closing years of the last century of the red and blue colors that ferrous and ferric salts form with the organic bases phenanthroline and 2,2'-bipyridine was reinvestigated in 1931 by Walden, Hammett and Chapman who found that the ferric-ferrous (blue-red) system, being reversible in nature, constituted a fine, high-potential oxidation-reduction indicator. Somewhat later, H.H. Willard and G. Frederick Smith, practically simultaneously, utilized the formation of the red color of ferrous iron with 1,10-phenanthroline for the colorimetric determination of iron. This method has proved so satisfactory that it is now almost universally used.

Subsequently, G. Frederick Smith, having enlisted the aid of Professor Francis Case of Temple University in the synthesis phase of the research, investigated over one hundred derivatives of 1,10-phenanthroline. As a result of this work, there is now available a series of oxidation-reduction indicators covering the potential range 0.85 to 1.25 volts in small steps. A number of the organic bases and the ferrous derivatives in the form of the sulfate and perchlorate salts have now been made available commercially.

The investigation of the phenanthrolines by Smith has also led to a number of new colorimetric reagents. 4,7-Diphenyl-1,10-phenanthroline, given the common name Bathophenanthroline, is a sensitive reagent for iron, the ferrous derivative having a molar extinction coefficient almost twice that of the ferrous derivative of 1,10-phenanthroline. This ferrous complex may be extracted into immiscible solvents and this enables not only a concentration of the color which increases the sensitivity but provides a means of extracting the iron in the reagents and water used in the test and thus of reducing the blank to zero. Bathophenanthroline makes possible the quantitative determination of iron in amounts as low as 0.002 parts per million (2ppb).

Another outstanding development in the field of phenanthroline chemistry is the discovery that phenanthrolines substituted in the 2,9-positions do not react with iron but do react with cuprous copper. A similar effect is observed with 6,6'-substituted 2,2'-bipyridines. Of particular interest for the determination of copper are Cuproine (2,2'-biquinoline), Neocuproine (2,9-dimethyl-1,10-phenanthroline), and Bathocuproine (4,7-diphenyl-2,9-dimethyl-1,10-phenanthroline) all of which are excellent colorimetric reagents for copper.

Of special interest to clinical chemists are the sulfonated phenanthroline reagents. Sulfonated bathophenanthroline (No. 286) and sulfonated bathocuproine (No. 294) are readily soluble in aqueous solutions and retain the high sensitivity toward iron and copper of the parent reagents. Their use in clinical applications is free of solubility and turbidity complications when extraction procedures are to be avoided.

The six most sensitive iron reagents available are now included in our list, PPT,2,4-BDTP and PPDT and their sulfonated derivatives. These reagents are the latest in the continuing efforts of Case and Schilt to find the very best. The sulfonated reagents can be substituted in nearly any procedure using sulfonated bathophenanthroline or sulfonated PDT by merely changing the wavelengths used.

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