Chapter 46 Rubidium

Chapter 46 Rubidium

Chapter 46 Rubidium Rubidium (Rb, atomic weight 85.47, melting point 39°C, d = 1.53 g cm -3 ) is a soft, silvery highly reactive alkali metal. It is...

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Chapter 46

Rubidium

Rubidium (Rb, atomic weight 85.47, melting point 39°C, d = 1.53 g cm -3 ) is a soft, silvery highly reactive alkali metal. It is widely abundant (0.003%) in the earth's crust and occurs together with Cs in certain potassium-bearing minerals. Rubidium is readily oxidized by air, forming a mixture of hydroxide and carbonate, and reacts vigorously with water and ethanol to evolve H2. In aqueous solutions Rb occurs as a monovalent cation, with no redox properties and only a weak tendency to form complexes with a restricted number of ligands, e.g. crown ethers. Rubidium determination is essential in geochronology, for the Rb-Sr isochrone dating. 46.1 ANALYTICAL TECHNIQUES Separationand preconcentration

The extraction of Rb with dicyclohexano-18-crown-6 and picrate as a counter-anion into CH 2 Cl2 is quantitative at pH 3-7. Rubidium may be stripped with 2M HNO 3. Most alkali and alkaline earth metal ions do not interfere, but potassium is co-extracted to a large degree [1]. Extraction of Rb from alkaline solution with tetraphenylboron (Ph4 B-) into DIBK proved to be reliable whereas in acidic media polyvalent cations may affect the recovery [2]. Cation-exchange separation of Rb has been reported but it generally lacks selectivity vs other alkali metal ions [3]. Atomic absorptionspectrometry

No HCL is available for Rb; vapour discharge or EDLs are used. A red cut-off filter (below 650 nm) should be used to reduce the background. Flame AAS offers a DL of 2 ng ml-l in the recommended 635

air-C 2 H 2 (oxidizing, lean, blue) flame at the 780.0 nm line [4]. The addition of a K salt as an ionization buffer is recommended [4-7]. Aluminium and strong mineral acids reduce the Rb signal; matrix matching is important. Graphite furnace AAS offers a DL of 0.05 ng ml-1 using platform atomization [4]. A DL of 2 ng ml-1 was reached with a tungsten coil atomizer [8]. Atomic emission spectrometry Flame AES offers a DL of ca 0.3 Ctg ml-' in a air--C2 H 2 flame at 780.02 nm [4,9]. Potassium interferes and matrix matching or standard additions calibration has to be used [2]. The fuel-rich H 2 -air flame shows virtual freedom from inter-alkali-metal interferences at the expense of sensitivity. Potassium or Cs should be added as ionization buffers [9,10]. Phosphorus interferes [10]. The DLs obtained in ICP AES are much poorer (0.5 gig ml-l at 780.02 nm) and this technique is restricted to the determination of Rb in a multielement array (cf: Part II). Mass spectrometry Rubidium has two natural stable isotopes: 8 5Rb (72.2%) and 87 Rb (27.8%). The determination by TI MS suffers from the inability to correct for fractionation and from difficulties in a clean separation of Rb from other alkali metals. In ICP MS the 85Rb peak is preferably counted as the 87Rb is interfered with by 87Sr [11-13]. Application of ID ICP MS has been reported [3]. Neutron activation analysis During irradiation a long-lived 86Rb (t1/ 2 = 18.6 d, E = 1.08 MeV) and short-lived s8Rb (tl/2 = 17.8 min, E = 1.85 and 0.91 Meit) are formed. As Rb is usually accompanied by large amounts of Na and K, radiochemical separation is necessary and the long-lived isotope must be used. Instrumental NAA was widely used in a multielement array for a variety of samples with a DL of ca 0.5 ppm [14-19]. Other determination techniques No reliable spectrophotometric method can be found in the literature. Laser-induced atomic ionization in flames has been used for direct determination of Rb in various samples [20,21]. Atomic fluorescence spectrometry with laser excitation allowed a DL of 0.2 ng ml-1 to be achieved [22]. Use of ED XRF with a 1 09Cd source and a Si(Li) detector has been reported [23]. 636

46.2 ANALYSIS OF REAL SAMPLES Environmental materials Rubidium can be determined in natural or mineral waters directly, after dilution with the ionization buffer by FAAS [6] or GF AAS [4]. The concentrations in fly ash can be measured by INAA [19] or ICP AES [24], usually as part of a multielement analysis. Geological materials Rubidium can be determined directly or on sample dissolution by GF AAS [7] and laser-induced atomic ionization in flames [21]. Lithium hydroxide is a suitable flux for basic and ultrabasic rocks as the bulk of the rock matrix (except some Al and Cr) is left behind whereas Rb and other alkali metals pass into solution [2]. Rubidium can be determined on extraction separation by FAES [1,2] and FAAS [1]. Dissolution of the barite samples by refluxing with an ammoniacal EDTA followed by disolution of the residue with HF-H 2SO 4 has been proposed for GF AAS [25]. Biological materials Rubidium in plasma or serum has roused only modest clinical interest. The technique most commonly used for analytical measurements is INAA [14,15], but FAES [9], GF AAS [26], ED XRF [23] and ICP MS [13] have been applied as well. Sensitivity of ICP AES and FAAS was found to be insufficient for Rb determination in blood [9]. Natural levels of Rb in insects can be determined by GF AAS [5]. Foodstuffs have been analyzed by INAA [17] and flame AES [10].

REFERENCES 1 2 3 4 5 6 7 8

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