Mendelevium separation

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

In 1961, enough einsteinium was produced to separate a macroscopic amount of 253Es. This sample weighted about O.OlMg and was measured using a special magnetic-type balance. 253Es so produced was used to produce mendelevium (Element 101). 

Exchange resins are also employed for the concentration of ions present in very dilute solutions instances are the recovery of silver from photographic residues, chromate from the waste liquor of chromium plating and magnesium from sea water. They have also been used for the separation of rare earths (p. 426), and of uranium, plutonium and radio-active fission products (p. 437), and for plutonium and uranium-233 purification. A striking application was the historic separation of single atoms of mendelevium on a sulphonated polystyrene resin and their elution therefrom, at 87 , with a-hydroxyisobutyrate (Seaborg, 1955). 

The isolation and characterization of these elements, particularly the heavier ones, has posed enormous problems. Individual elements are not produced cleanly in isolation, but must be separated from other actinides as well as from lanthanides produced simultaneously by fission. In addition, all the actinides are radioactive, their stability decreasing with increasing atomic number, and this has two serious consequences. Firstly, it is necessary to employ elaborate radiation shielding and so, in many cases, operations must be carried out by remote control. Secondly, the heavier elements are produced only in the minutest amounts. Thus mendelevium was first prepared in almost unbelievably small yields of the order of 1 to 3 atoms per experiment Paradoxically, however, the intense radioactivity also facilitated the detection of these minute amounts first by the development and utilization of radioactive decay systematics, which enabled the detailed properties of the expected radiation to be predicted, and secondly, by using the radioactive decay itself to detect and count the individual atoms synthesized. Accordingly, the separations were effected by ion-exchange techniques, and the elements... 

When the divalent state of Md was first discovered, extraction chromatography was used to prove that the behavior of Md + was dissimilar to that of Es + and Fm + (20). The extractant, bis(2-ethylhexyl)phosphoric acid (HDEHP), has a much lower affinity for divalent ions than it does for the tri- and tetravalent ones. Thus, the extraction of Md2+ is much poorer than the extraction of the neighboring tripositive actinides as indicated by the results shown in Table 2. This became the basis for a separation method in which tracer Md in 0.1 M HC1 is reduced by fresh Jones Reductor in the upper half of an extraction column containing HDEHP absorbed on a fluorocarbon powder in the lower half. Mendelevium, in the dipositive state, is rapidly eluted with 0.1 M HC1 whereas the other actinides are retained by the extractant. The separation is quickly performed, but the Md contains small amounts of Zn2+ from the Jones Reductor and also Eu2+, which was added prior to the elution to prevent reoxidation of Md2+ by the extractant. 

Ion-exchange chromatography is generally used for the separation of the transplutonium elements (americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawren-cium). Determinations are usually made directly by a-spectrometry with solid-state detectors. Some elements (americium, curium, berkelium, californium) also have long-lived isotopes and can be determined by chemical methods such ultraviolet-visible spectrophotometry. 

The discovery and identification of element 101 (mendelevium, Md) was a landmark experiment in many ways [ 1 ]. It was the first new transuranium element to be produced and identified on the basis of one-atom-at-a-time chemistry and it is also the heaviest element (to date) to be chemically identified by direct chemical separation of the element itself. All of the higher Z elements have been first identified by physical/nuclear techniques prior to study of their chemical properties. In fact, one of the criteria for chemical studies is that an isotope with known properties be used for positive identification of the element being studied. Due to relativistic effects [1] chemical properties cannot be reliably predicted and a meaningful study of chemical properties cannot be conducted with both unknown chemistry and unknown, non-specific nuclear decay properties ... 

The citrate ion was the eluting agent used in the first isolation of berkelium and californium [189,190]. Soon afterwards, however, the lactate and tartrate ions were found to give considerably improved separations between americium(iii) and curium (ill) [191]. Lactate elution also proved superior to the classic citrate elution for the first isolation of einsteinium and fermium [188]. Later on, an even more selective eluant for actinide(iii) ions has been found in the a-hydroxyisobutyrate ion [192], With this ligand, very clean separations of the heavy actinide(iii) ions can be achieved, as illustrated in Fig. 21.12. The first isolation of mendelevium was achieved by this procedure [193]. 

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