Strontium -EDTA complex

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. 

By use of releasing agents Considering the reaction M-X-i-R = R- Xh-M, it becomes evident that an excess of the releasing agent (R) will lead to an enhanced concentration of the required gaseous metal atoms (M) which will be of special significance if the product R-X is a stable compormd. Hence in the determination of calcium in presence of phosphate the addition of excess of strontium chloride to the test solution will lead to the formation of strontium phosphate and the calcium can then be determined in an acetylene-air flame without any interference due to phosphate. Also addition of EDTA to a calcium solution before analysis may increase the sensitivity of the subsequent flame spectrophotometric determination which may be due to the formation of an EDTA complex of calcium which is readily dissociated in the flame. 

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. 

Strontium, present as Sr(H), also showed increased solubility in the presence of EDTA and HEDTA due to the formation of stable soluble complex species (4). 

Strontium sorption behavior was consistent with the strontium solubility behavior described earlier. Both HEDTA and EDTA significantly decreased strontium sorption through formation of poorly sorbed anionic complexes (4). Close inspection of the test data suggested that increased sodium ion concentration might have led to decreased strontium sorption. The competition of strontium with sodium for sorption sites had been observed previously for Hanford sediments (13). 

Predicted rates of the radioelement transport through sediment were found to depend strongly on HLW compositions. Compositions of HLW that are high in HEDTA/EDTA concentrations resulted in 30 to 40 times faster migration rates for americium and strontium. Neptunium and plutonium migration rates increased by factors of 6 to 40 by changing HLW from dilute/noncomplexed to concentrated/complexed compositions. 

Strontium (Sr, at. mass 87.62) and barium (Ba, at. mass 137.33) occur in solution exclusively in the II oxidation state. The basicity and solubility in water increase from Ca(OH)2 to Ba(OH)2. Barium chromate and -sulphate are less soluble than the corresponding strontium compounds. The stability of the relatively weak complexes (e.g., with EDTA or tartrate) diminishes in the sequence Ca, Sr, Ba. 

Complexones such as EDTA (complexone III) [1-3] and DCTA (complexone IV) [4,5] are suitable eluents, but other complexing agents, such as citrate [3,6] and sulphate [7] are also applied. Barium has been separated from strontium and other metals by cation-exchange chromatography using mixed HCl-organic solvent eluents [8]. Strontium has been enriched and determined in sea water [5] and in milk [2]. 

N.m.r. studies of ligand exchange involving aminopolycarboxylate ligands include those of egta at calcium(ii) and of edta at magnesiura(ii), calcium(ii), and strontium(ii). The relative importance of dissociative and associative (i.e. via an L—M—L intermediate) paths has been assessed, and the reactivities of variously protonated and deprotonated forms of the ligands and the complexes have been considered. 

The usual way to avoid this interference is to prevent the formation of calcium phosphate during the desolvation of the sample solution in the burner. This can be done by adding another element in excess which binds the phosphate more strongly than calcium. Elements such as strontium, barium, and landianum have been used successfully for that purpose. The other possibility is to add a complexing agent such as EDTA, which forms a complex with calcium and hence prevents the analyte element from reacting with phosphate. 

Caldolysin is the trivial name of the serine proteinase from T. aquaticus strain T351 [284]. This enzyme did not have detectable esterase aetivity and hydrolysis of small peptides of less than four amino acids was not observed. The enzyme was highly stable in the presenee of ealcium ions. Caldolysin bound six calcium ions per molecule of enzyme, there being both high and low affinity binding sites [285]. Stability of apocaldolysin (i.e. caldolysin treated with EDTA to remove all calcium ions) was restored upon incubation with either calcium or lanthanide ions, the latter giving a lanthanide-caldolysin complex more stable than the native enzyme. Strontium ions were the only other divalent metal ions tested that could restore more than 50% activity. 

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