Lanthanum EDTA complex

At what pH should 10% of the lanthanum-EDTA complex be dissociated in 0.020 M equivalent LaEDTA" solution ... 

Compared to later elements in their respective transition series, scandium, yttrium and lanthanum have rather poorly developed coordination chemistries and form weaker coordinate bonds, lanthanum generally being even less inclined to form strong coordinate bonds than scandium. This is reflected in the stability constants of a number of relevant 1 1 metal-edta complexes ... 

Fuger, J. and Cunningham, B. B. (1964). Microcalorimetric determination of the enthalpy of formation of the complex ions of trivalent plutonium, americium and lanthanum with EDTA, J. Inorg. Nucl. Chem. 27,1079. 

There are a number of interferences that can occur in atomic absorption and other flame spectroscopic methods. Anything that decreases the number of neutral atoms in the flame will decrease the absorption signal. Chemical interference is the most commonly encountered example of depression of the absorption signal. Here, the element of interest reacts with an anion in solution or with a gas in the flame to produce a stable compound in the flame. For example, calcium, in the presence of phosphate, will form the stable pyrophosphate molecule. Refractory elements will combine with 0 or OH radicals in the flame to produce stable monoxides and hydroxides. Fortunately, most of these chemical interferences can be avoided by adding an appropriate reagent or by using a hotter flame. The phosphate interferences, for example, can be eliminated by adding 1 % strontium chloride or lanthanum chloride to the solution. The strontium or lanthanum preferentially combines with the phosphate to prevent its reaction with the calcium. Or, EDTA can be added to complex the calcium and prevent its combination with the phosphate. 

The complex anions [M(edta)(H20)3] where M = Pr, Sm, Gd,410 Tb411 and Dy412 all have similar coordination to the lanthanum complex, but for Er and Yb, [M(edta)(H20)2] , which are eight-coordinated, are obtained.412,413 The change in coordination number occurs at some point in the Ho area. Of course, it is by no means certain that a change, if there is one, occurs at exactly the same position in the series in solution. 

Fig. 7.27. Dependence of the isotopic exchange rate constants (k) and the stability constants (K) of the complexes R(EDTA) on the atomic number of the rare earths. The vertical bars shown in the figure represent the probable error in the k values. The dotted line describing the behavior of the elements lanthanum, neodymium and europium is from the comparison of exchange reactivity attempted by Fomin.

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

An example of the first approach (matrix assimilation) would be to match the acid content in the standards with the acid content in the samples. Matrix assimilation is only effective provided that the interference is not severe and the sample matrix is relatively simple. For more marked interferences, a second cation can act as a release agent. As an example, lanthanum [as La(N03)3] can be added to solutions in which Ca is to be determined in the presence of P04 , silicate or aluminate in an air/C2H2 flame. An example of the third approach would be to add a strong complexing agent (such as EDTA) to both samples and standards. Many metals have an appreciable tendency to hydrolyse in aqueous media moreover the hydroxides can be sparingly soluble yet precipitates can be difficidt to detect visually in dilute solutions. To limit this process, samples are customarily prepared in acidic media. 

No appreciable polymerization occurred in concentrated aqueous solutions of La or Nd hydroxonitrilotriacetates but, with the heavier lanthanides, dimerization, via hydroxo bridges, occurred in dilute solution. A series of mixed ligand chelate complexes was obtained with Ho and Er and 5-sulpho-salicylate (ssa) and edta, iV-hydroxyethylethylenediamine-triacetic acid, 1,2-diaminocyclohexanediamine-tetra-acetic acid (YX i.e. [M(Y)(ssa)], and iminodiacetic acetate, hydroxyethyliminodiacetic acid and nitrilotriacetic acid, [M(Y)(ssa)2] with higher carboxylates, no mixed chelate complexes were produced. In the presence of an excess of hydroxyethylethylenediamine triacetic acid (H3A), [Eu(A)(HA)] was formed. Lanthanum(m) formed 1 1 complexes with nitrilotriacetic acid and 8-hydroxyquinoline, or 2-picolinic acid and 8-hydroxyquinoline-5-sulphonic acid. 

Tokmadzhyan MA, Dobrynina NA, Martynenko LI, Alchadzhyan AA (1974) Mixed complexes of lanthanum, praseodymium, neodymium, and samarium with EDTA, nitrilotriac-etic acid, and citric acid. Zhum Neoig Khim 19 2888-2889... 

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