Cerium complexes fluorides

This method involves very simple and inexpensive equipment that could be set up in any laboratory [9,10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed in the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluorides and has been applied to unbuffered solutions from Willard-Winter distillation, to ion-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10-4 M This method is based on complexing by fluoride ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(III) ions... 

The direct method for determining traces of fluoride is based on the coloured ternary complex formed by fluoride with Alizarin Complexone and lanthanum or cerium(III). Fluoride ions form stable complexes with some multivalent metals, namely Zr, Th, Ti, Fe(III), and Al. The colour changes resulting from the reactions of fluoride with coloured complexes of these metals provide indirect methods for the determination of fluoride. Examples of these methods are the Eriochrome Cyanine R-zirconium method, and the (less sensitive) Fe(III)-sulphosalicylate method. 

Tetravalent plutonium. Solutions of tetravalent plutonium salts are generally similar to tetravalent cerium and uranium. The fluoride PUF4, potassium complex fluoride K2PUF6, iodate Pu(I03)4, and phosphate Pu3(P04)4 are insoluble. Excess soluble hydroxides precipitate Pu(OH)4. The... 

Because stable tetravalent rare earth halides are formed only for the fluorides of Ce, Pr and Tb, the number of MX-MX4 complexes is much lower than for the MX-MX3 systems. Early reports of stable complex fluorides of Nd(IV) and Dy(IV) have not been confirmed. The complex tetravalent fluorides are predicted by radius ratio correlations (Thoma, 1962) and are reviewed by Brown (1968). An extensive investigation by Delaigue and Cousseins (1972) has greatly expanded the data for the cerium systems. Seven complex fluorides and the corresponding metal systems are shown in table 32.11. As expected, these phases are similar to the corresponding complexes of the actinides (Brown, 1968). 

Fluoride ions forms very stable complexes with cerium(IV) and lowers the redox potential considerably. A cerium(IV) sulfate solution is not able to oxidize iodide to iodine when appreciable amounts of fluoride ions are present. The most important fluoro complex is hexafluo-rocerate(IV), [CeFe] ", which can be formed by addition of ammonium fluoride to a solution of cerium(IV). The behavior of cerium(IV) in presence of fluoride ions is markedly different from that of cerium(III). Addition of an ammonium fluoride solution to a solution containing cerium(III) results in the precipitation of gelatinous cerium(III) fluoride, CeF3, which becomes powdery upon standing (Svehla, 1979). The use of cerium(IV) fluoride as a fluori-nating agent is discussed in sections 5.16 and 6.7. 

As a result, the electromotive force (EMF) of the cell is zero In the presence of fluoride ions, cerium(IV) forms a complex with fluoride ions that lowers the cerium(IV)-cerium(IIl) redox potential The inner half-cell is smaller, and so only 5 mL of cerium(IV)-cenum (III) solution is added To the external half-cell, 50 mL of the solution is added, but the EMF of the cell is still zero When 10 mL of the unknown fluonde solution is added to the inner half-cell, 100 mL of distilled water IS added to the external half-cell The solution in the external half-cell is mixed thoroughly by turning on the stirrer, and 0 5 M sodium fluonde solution is added from the microburet until the null point is reached The quantity of known fluonde m the titrant will be 10 times the quantity of the unknown fluoride sample, and so the microburet readings must be corrected prior to actual calculations... 

This reaction takes place quite rapidly on boiling, and hence hydrochloric add cannot be used in oxidations which necessitate boiling with excess of cerium(lV) sulphate in add solution sulphuric add must be used in such oxidations. However, direct titration with cerium(IV) sulphate in a dilute hydrochloric add medium, e.g. for iron(II) may be accurately performed at room temperature, and in this respect cerium(IV) sulphate is superior to potassium permanganate [cf. (2) above]. The presence of hydrofluoric add is harmful, since fluoride ion forms a stable complex with Ce(lV) and decolorises the yellow solution. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

Fletsch and Richards [51] determined fluoride in seawater spectrophotometri-cally as the cerium alizarin complex. The cerium alizarin complex and chelate was formed in 20% aqueous acetone at pH 4.35 (sodium acetate buffer) and, after 20-60 min, the extinction measured at 625 nm (2.5 cm cell) against water. The calibration graph was rectilinear for 8-200 ig/l fluoride the mean standard deviation was 10 xg/l at a concentration of 1100 ig/l fluoride. 

A number of fluoride complexes have been made, both by crystallization for the ammonium salts like (NH4)4CeFg, which has square-antiprismatic coordination of cerium, also found in the (CeF6 )oo chains in (NH4)2CeF6, its thermal decomposition product. (NH4)3CeF7-H20 acquires dodecahedral eight-coordination by dimerization. (NH4)4[CeFg] can... 

Recent investigations have been devoted to the measurement of fluorides [91]. A ternary complex colored by fluoride ion is formed from a cerium(III)-alizarin complexone binary complex (alizarin fluorine blue). The reagent is immobilized on an Amberlite polymer matrix. The reflectance is measured in a flow-cell assembly with a bifurcated optical flber. The response is linear for 0.16-0.9S mM fluoride at pH 4.1. The response time is about 12 min. The major interferents are aluminium, iron, and phosphate. 

Many of the fluoride complexes are anhydrous, being prepared from melts, and even hydrates are not numerous among those prepared from aqueous solution. In the case of hydrates, the refractivity of water must be allowed for. An example of a hydrate, (NH4)3CeF7 H2O obtained in the NH4F-CeF4-H20 system (81, 86), is shown in Fig. 12. When the refractivity of H2O is subtracted, the point for the yet unknown anhydrous (NH4)sCeF7 compound falls on the line established by the known anhydrous ammonium fluoride-cerium tetrafluoride complexes. 

The pH of the test solution offers that a yellow solution is expected but in the presence of cerium(III) a red complex of the structure (Figure 6.5.3) is formed. Most likely it has a red color because the electronic structure of tiie chelate is similar to the red form of tiie pure substance in pH up to about 10. When the fluoride is added a new structure (6.5.4) is formed. This is blue. 

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. 

A method for the determination of fluorine in fluorinated polymers such as polytetrafluoroethylene (PTFE) is based on decomposition of the sample by oxygen flask combustion followed by spectrophotometric determination of the fluoride produced by a procedure involving the reaction of the cerium(III) complex of alizarin complexan (1,2-dihydroxy-anthraquinone-3-ylmethylamine N,N-diacetic acid). The blue colour of the fluoride-containing complex (maximum absorption, 565 nm) is completely distinguishable from either the yellow of the free dye (maximum absorption, 423 nm) or the red of its cerium(III) chelate (maximum absorption, 495 nm). 

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