Cerium color change

Ce4+ is yellow and Ce3+ is colorless, but the color change is not distinct enough for cerium to be its own indicator. Ferroin and other substituted phenanthroline redox indicators (Table 16-2) are well suited to titrations with Ce4+. 

Anatol M. Zhabotinsky (b. 1938) is best known as the Zhabotinsky. of the Belousov-Zhabotinsky, or BZ, oscillating reactions. The BZ reactions involve the oxidation of various organic acids and ketones by bromate in the presence of cerium or ferroin ions. Waves of oxidation are easily observed as a color change from red (ferroin) to blue (ferriin). Originally from Moscow, Dr. Zhabotinsky currently does research at the Department of Chemistry of Brandeis University in Waltham, Massachusetts, and that is where we recorded our conversation on July 24,... 

Br2 then brominates the organic acid. This reaction depletes Br- and lets a second pathway take over that starts with a self-accelerating two-step sequence that also oxidizes the cerium catalyst and so produces the color change ... 

The temporal oscillating patterns of certain chemical intermediates have been observed only in a stirred BZ reaction system. Similar to cerium-catalyzed BZ reaction, the oscillation is occurred between colorless and yellow color at an assured time interval. There are some other important indicators where oscillations can be monitored due to the gradual color change, either direcdy in batch reactor [37, 38] or via spectrophotometric measurements [39, 40]. The most excellent illustration of oscillations manifested in batch reactors in the form of color variation of ferroin-catalyzed BZ reaction system. On the other hand, in some other reaction systems such as the manganese-catalyzed system and the cerium-catalyzed reaction, oscillations can be observed with the help of UV-visible spectrophotometer [41] where change in color might be monitored less distinctly. 

The BZ oscillating reaction has been discovered by B.P. Belousov, in 1951. In his early attempt, he wanted to make a chemical model for Krebs cycles (an energy pathway). For experimentation, he used KBr03, acidified solution of citric acid and a metal catalyst (Cerium (TV) metal ions). A redox indicator has also been used for observing the end of the reaction-phase. They found that the citric acid is oxidized into CO2, and B1O3 is reduced to Br ion. He observed a periodic color changes for duration of 10 min. He wanted to publish this report but editor refused his proposal and explanation because, it seems to be contrary with 2nd laws of thermodynamics. 

Belousov-Zhabotinskii reaction (B-Z reaction) An oscillating chemical reaction in which there are periodic oscillations in the color of a mixture of sulfuric acid, potassium bromate, cerium (or iron) sulfate, and propanedioic acid. The period of oscillation is about one minute. The color changes are caused by repeated oxidations and reductions of cerium (or iron) ions. The reaction was first observed by the Russian chemist B. P. Belousov in the case of cerium and modified to iron by A. M. Zhabotinskii in 1963. The mechanism of the B-Z reaction is highly complicated and involves a large number of individual steps. 

There is still considerable interest in the mode of oxidation of organic substrates by this reagent. Using a scanning spectrometer, the series of rapid color changes observed in the reaction with phenothiazine have been monitored.With cerium(IV) in excess, a transient at 419 nm was observed which disappeared slowly, whereas with substrate in excess, several absorptions are identified with the formation of a dark green solution considered tentatively as a binuclear cation. The intermediates have been identified as the radical cation and the phenothiazinium cation [equation (35)]. The first reaction is too fast for... 

In step R6, cerium oxidizes from oxidation state (III) to oxidation state (IV), giving the color change from red to blue. The growth of HBr02 is limited by quadratic termination... 

B. Self-indicating reagents. This is well illustrated by potassium permanganate, one drop of which will impart a visible pink coloration to several hundred millilitres of solution, even in the presence of slightly coloured ions, such as iron(III). The colours of cerium(IV) sulphate and of iodine solutions have also been employed in the detection of end points, but the colour change is not so marked as for potassium permanganate here, however, sensitive internal... 

Natural teeth exhibit blue-white fluorescence in the long-wavelength UV [5.435]-[5.437]. Luminescent pigments are used to imitate this phenomena in artificial teeth. They are added to the ceramic paste at a concentration of 0.3-0.5 wt%. Yttrium silicates doped with cerium, terbium, and manganese give the best results [5.438]. The excitation maximum of these phosphors is in the range 325-370 nm. The fluorescence color of the teeth can be varied by changing the concentration of activators. 

The most interesting result is the formation of a transparent colloidal solution of ceria with 2 nm particles. Cerium metal tips with the superficial layers of oxide are allowed to react in 2-methoxyethanol at 250 to 300°C, and removal of coarse ceria particles originating from the superficial layers yields the colloidal solution. Addition of water to the solution does not cause any change except dilution of the color of the solution, but addition of a drop of a solution of any kind of salt immediately causes precipitation of ceria particles. - The reaction mechanism is as follows The solvent slowly dissolves the superficial layers, and when the solvent reaches the metal, rapid reaction occurs, yielding an alkoxide solution. The concentration of the ceria precursor becomes so high that a burst of nucleation occurs, yielding the colloidal solution. The reaction of cerium acetylacetonate in the same solvent yields ceria particles but does not give a colloidal solution. 

Irradiations were within a 60Co source (— 2000 curies) with a dose rate of about 1018 e.v. ml.-1 min. 1 in the ferrous sulfate dosimeter. Changes in cerium (IV) concentration were followed with a Cary recording spectrophotometer at 340 mfx. No detectable effect on measurements at 340 m[a could be attributed to coloration of the S 18-260 silica windows during irradiation. Therefore, irradiations and spectrophotometric measurements were made in the same cell. 

Belousov did not succeed in publishing his results as they were extremely unusual at that time (1950s), displaying changes in color (oscillations) in a system containing a mixture of potassium bromate, cerium(IV) sulfate, and citric acid in dilute sulfuric acid, due to changes in the ratio of concentration of the cerium(IV) and cerium(III) ions (Figure 8.23). 

As a certain concentration of CHBr (C02H)2 is needed for reaction 9 to occur long induction period for oscillations is expected, a phenomenon, which is also observed experimentally. During this induction period, the concentration of Br" is small and mechanism II dominates due to the slow conversion of Ce4+ into Ce3+ and the accumulation of brommalonic acid (reaction 8). Step 9 (8.71) results in the change of the blue color of solution to red resetting the chemical clock for the next oscillation. In fact, the oxidized form of the catalyst can also react directly with malonic acid, so there may be less than one bromide ion per cerium (III) ion produced. 

Several other cations, such as calcium, barium, cadnium, nickel, magnesium, and mangane(II), are capable of forming a complex analogue to (Figure 6.5.3) thereby changing the color from yellow to red. But only the complex of cerium(lll) will form by fhe addition of fluoride. 

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