The following is an excerpt from GFS Monograph - Cerate Oxidimetry. If you would like to receive a complete copy, contact service@gfschemicals.com or call 800-858-9682.
TABLE OF CONTENTS
SECTION 1. INTRODUCTION
SECTION 2. COMMERCIAL CERATE CHEMICAL MANUFACTURER
SECTION 3. ELECTROCHEMICAL OXIDATION OF CERIUM (III) IN HCI,
H2SO., HNO AND HClO. SOLUTION DETERMINATION OF
ELECTRODE POTENTIALS
SECTION 4. PREPARATION OF CERIUM (IV) VOLUMETRIC SOLUTIONS
AND THEIR STANDARDIZATION
SECTION 5. THE METHINE CHROMOPHORE GROUP FERROIN OXI
DATION - REDUCTION INDICATORS AND RELATED COMPLEX
CHELATION TYPES
SECTION .6. MICRO-VOLUMETRIC DETERMINATION OF ARSENIC, IRON
AND THE OXALATE ANION
SECTION 7. CLINICAL TESTING IN DETERMINATION OF PROTEIN BOUND
IODINE
SECTION 8. CERATE OXIDIMETRY BIBLIOGRAPHY
SECTION 1
INTRODUCTION
The general application of the methods of Cerate Oxidimetry in volumetric oxidimetry, through the pioneer studies of H. H. Willard* and N. H. Furman** opening in 1928, marks one of modern analytical chemistry's most important innovations. Originally designated Ceric Oxidimetry, cerium(IV) quantitative oxidation-reduction applications were thought to be unique, as distinct from permanganate oxidimetry, as being cationic in principle. Ceric sulfate, the originally concept (Ce(S04H) is now recognized to be sulfatoceric acid H2Ce(S04H in sulfuric acid solution. The most important Ce(IV) analytical reagent is ammonium nitratocerate (NH4)Ce(NOg)6 derived from nitratoceric acid H2Ce(NOg)6 by original concept Ce (NOg)4 in nitric acid solution. Reactions employing cerium as electron donor thus involve anionic rather than cationic electron transfer distinctly contrary to the original concept.
"New developments" in chemical research are often found to involve prior documented chemical disclosures. Those responsible for their major promotion find their "original ideas" previously published. This is true for the case at hand. Probably the first published citation leading to the proposal that four valent cerium could be utilized to enrich analytical chemistry's field of oxidation-reduction reactions was made by 1. Th. Lang.
The separation of cerium from lanthanum and didymium was described by Walcott Gibbs. (2) in 1864 employing lead dioxide in nitric acid as oxidant to oxidize cerium and. separate it· by a precipitation process as an insoluble basic nitrate. G. von Knorre also described (3) the determination of cerium as oxalate or oxide.
Andre Jos employed hydrogen peroxide in the determination of cerium in nitric acid solutions of ammonium nitratocerate (NH4 HCe (NOs)6 employing a visual end point to define complete reduction. Oxygen was the reduction product of H202 and two moles of Ce(IV) were reduced by each mole of peroxide. Andre Jos also pointed out that thorium if present does not interfere and that "one can employ cerium(N) sulfate or nitrate in a goodly number of cases in which the use of permanganate is inapplicable," for example in the determination of oxalic acid in oxalo-rare earth chlorides, (4) and that "one can· employ the solution of Ce(IV) sulfate or nitrate in a great number of cases in which permanganate reactions are applicable."
Meyer and Aufrecht described the preparation of Ce(IV) sulfate from CeO2 or Ce(OH)4 which process was original practiced by the procedures developed by Professors Willard and Furman.
Barbieri developed a method for the determination of tlitrous acid employing Ce(IV) as oxidant which oxidation is not duplicated employing permanganate oxidimetry. The method employed excess Ce(IV) with iodimetric determination of excess employing thiosulfate. It was stated that hydroxylamine and hydrazine are also thus applicable in the use of -the same type process.
Sommer and Pincas described the oxidation and determination of hydrazoic acid employing Ce(IV} oxidimetry.
Benrath and Rulaf employing solutions of Ce(IV) - prepared as described by Meyer and Aufrecht and standardized by the method of vonKnorres developed Ce(IV) methods in the determination of a number of organic compositions. Their work followed the experimental observation that Ce(IV) sulfate in concentrated sulfuric acid oxidized ( qualitatively) toluol, naphthalene, and anthracene to benzaldehyde, naphthaquinone and anthraquinone. The determination. of tartaric, oxalic, malonic and citric acids, hydroxylamine, sulfite, thiosulfate and hypophosphite were described. The oxidation of formic acid was stated to be very· slowly effective by Ce(IV) when exposed for a number of days to sunlight. More complete studies of the determinations involved in the Benruth and Ruland work were carried out with extensions by Willard and Young also employing sulfuric acid solutions to give empirical stoichiometry and by Smithand Duke employing perchloratoceric acid in perchloric acid solution· to give stoichiometric reaction equivalents.
Jerome Martin described the determination of hydrazoic acid using excess Ce(IV) and iodimetric evaluation of the excess. The systematic introductory research in cerium(IV) volumetric oxidation-reduction reactions consisted in twenty-one papers, thirteen by H. H. Willard and Philena Young and eight papers by N. H. Furman, three of which were the joint authorships with associated authors. With one exception all of· these pioneer developments were burdened by the requisite use of potentiometric equivalence point determination. With few exceptions sulfuric acid solutions were employed. All these procedures were erroneously -designated "ceric oxidimetry." All but one (15) required potentiometric equivalence point determinations.
Three new innovations have served to enhance establishment of Ce(IV) in volumetric analysis as superior to permanganimetry in competitive adoption of the cerium procedures as preference techniques. These are the (1) the development of internal redox indicators of far greater sensitivity in brilliance of color production when compared to the permanganate self indicating equilibrium point determination and with far greater versatility in selective potential transition points. The general application of Ce(IV) reactions in perchloric acid solution represents the number two advance together with the establishment of the status of these reactions as "cerate oxidimetry" in substitution for the early concept of "eerie oxidimetry." The new concept involved the realization that conditions could be established whereby electrode potentials became applicable, by use of hydrochloric, sulfuric, nitric and perchloric acid solutions of Ce(IV), covering the potential range 1.21, 1.44, 1.61, and 1.71 volts at standard state. By employing perchloric acid solutions in 1 to 8 Formal concentrations the potential range for practical reaction conditions mounts to 1.85 volts. Improvement number three involves the need for commercial availability of Ce(IV) analytical reagents to free the early handicap of dependency upon ceric oxide as the predominant raw material for use in preparation of standard solutions. Soon the use of ammonium nitratocerate (NH4) 2Ce(NOs) 6 for the purification of ceriuin(IV) rare earth oxides solved this problem. With this advancement in preparational techniques pure Ce(IV) reagents presented no problem and the establishment of ammonium nitratocerate as a primary standard was a prime development differentiating cerate oxidimetry from permanganimetry as a procedural redox volumetric preference. The G. Frederick Smith Chemical Company were the original commercial distributors of a substantial number of required reagents including perchloric acid.
The need for suitable oxidation-reduction indicators applicable to Ce(IV) oxidimetry was clearly indicated by the work of Willard and by Furman and associates. It is an anachronism of historical reality that a suitable system had been developed as early as 1898 through the pioneer studies by F. Blau in the studies of the chemistry of bi -pyridine and later that of 1,10-phenanthroline. All the required chemistry of these methine chromophore group chemicals were disclosed by F. Blau. There only remained the re-discovery by Walden, Hammett and Chapman of the Blau innovations and to apply them to the field of cerate oxidimetry. The use of the complex 1,10-phenanthroline-ferrous ion as redox indicator was followed by the synthesis of the 5-nitro analogue. Their visual, deep red to faint blue color transitions, at 1.06 and 1.25 volts played a major role in placing cerate oxidimetry in a position of preferential acceptance by comparison with permanganimetry.
It was soon made clear that the commercial preparation and sale of new Ce(IV) reagent chemicals together with the marketing of the accompanying redox indicators was uniquely requisite leading to the general analytical acceptance of cerate oxidimetry. The publishers of the present book revision on the subject were first to supply the indicators and reagents involved.
The first applications of the newly designated "cerate oxidimetry" were developed by the research promotions having their origin at the University of Illinois. The preparation of ammonium nitratocerate as applied to the synthesis of sulfatocerate, and perchloratocerate reagents free from other rare earth associates following metathetic reactions as well as by the procedures of electrochemical oxidation, were developed. The determination of electrode potentials in mineral acid solutions of HC1, H2S04, HNO3 and HCl04 were evaluated. The preparation of ammonium nitratocerate as a primary standard effectively influenced standardization techniques. The innovations provided by employing Ce(IV) in perchloric acid which extended reaction kinetics and provided practical procedures in oxidations at potentials higher than 1.71 volts were described. This eliminated empiricism in the oxidation of a wide range of organic materials as a substitute for permanganimetry's Stamm reactions in strong alkali media.
The determination of iron, arsenic and oxalic acid on a micro-volumetric scale was provided for by employing perchloratoceric acid. oxidations in perchloric acid solution. The disclosure that electrochemical oxidations of Ce(III) to Ce(IV) without the use of a partition cell was made which was an innovation not previously thought to be an applicable technique.
By an additional 25 years of intense research activity (1928-1963), the great measure of which involved research at the University of Illinois, cerate oxidimetry has largely eliminated dependence upon the more complicatedprocedures of permanganimetry which it most closely duplicates.
The determination of glycerol in the manufacture of explosives and in the control laboratories of the soap industry as well as the many routine applications of cerate oxidimetry in the manufacture of iron and steel and ferrous alloys are but a few of the triumphs of Ce(IV) in solving the problems of wet chemical operations in a wide field of important analytical operations.
With the wide range of available redox potentials (1.21 to 1.85 volts) provided by the cerate oxidations, the titrations involve many variations in the applicable internal redox indicators to be preferentially employed. Pioneer studies by Doctor F. P. Richter resulted in the synthesis of a number of substituted 1,10-phenanthrolines whose ferrous sulfate complexes proved to have predictable redox potentials. These disclosures gave the first demonstration that the methine chromophore group organic chelation reagents could provide predictable property modulations depending upon specific substitutions in selective structural positions. The first clue indicating such possibility was provided by the synthesis of 5-nitro 1,10-phenanthroline by Walden Hammett and Edmonds. Following the many years of masterful synthetic studies contributed by Professor Francis Case* of Temple University in Philadelphia there have been created approximately 150 of the various substituted methine chromophore group organic reagents. One phase of the study as redox indicators in the form of their ferrous complexes resulted from structural property prognosis. Thus redox indicators have resulted a series of increasing magnitude of color transitions over the range, step by step of 0.85 volts to better than 1.3 volts in progressive increments of 0.01 to 0.02 volt magnitude. Their utility not only provides
a wide range of redox magnitudes but their color intensity is often from 5 to 10 fold greater in value than that of the molecular extinction coefficient of the permanganate anion at approximately 2500. The Francis Case developed reagents were put to practical analytical utility through the studies conducted at the University of Illinois by the author and at Iowa State University under the direction of Professor Harvey Diehl and their graduate students.
Cerate oxidimetry thus was freed from dependence upon potentiometric equivalence point evaluation of Ce(IV) volumetric methods devised in 1928 through 1930 by professors Willard and Furman.
Many of the procedures reviewed in the first edition of "Cerate Oxidimetry" by the present author have now been displaced or beneficially modified as disclosed in this second edition. Support in the project resulting in over 36 years research has been' augmented by many analytical research experts not herein individually cited. This help is gratefully acknowledged.
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