Optimale Bedingungen der Atomabsorption, erl?utert am periodischen System der Elemente

Zusammenfassung Die 42 wichtigsten, durch Atomabsorption erfaßbaren Elemente wurden nach einem einheitlichen Verfahren hinsichtlich der günstigsten Betriebsbedingungen untersucht. Die Elemente der 1. und 2. Neben gruppe und der 2. Hauptgruppe werden mit möglichst geringem Strom der Hohlkathodenlampe am empfind lichsten nachgewiesen. Alle übrigen Elemente liefern bei maximalem Lampenstrom günstigste Ergebnisse. In den einzelnen Gruppen kann man eine Abnahme der notwendigen Flammentemperatur zur Probenverdampfung mit Zunahme der Periode feststellen. Bei den Elementen der 4. Nebengruppe (Ti, Zr, Hf) kann Mußsäure und in geringem Maße Salzsäure die Verdampfungsblockierung durch Bildung von Oxiden vermindern.

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Optimum conditions of atomic absorption, illustrated by means of the periodic system

The 42 most important elements which can be determined by atomic absorption were examined according to a common method in order to establish the most favourable working conditions. The elements of groups IB and IIA and IIB were determined with the highest sensitivity at the lowest possible current of the hollow cathode lamp. All other elements yielded optimum results at maximum lamp current. In the individual groups the flame temperature required for sample evaporation decreased as the period increased. With the elements of group IVA (Ti, Zr, Hf) the evaporation blocking by formation of oxides could be reduced by hydrofluoric acid and, to a lesser extent, hydrochloric acid.

     

Polarographische Simultanbestimmung von Niob und Zinn in der bin?ren Legierung

Zusammenfassung Die Möglichkeit der gleichzeitigen Bestimmung von Niob und Zinn mit Hilfe der Polarographie wird untersucht. Es wird gefunden, daß die Bestimmung zwar nicht gleichspannungspolarographisch, wohl aber square-wave-polarographisch durchführbar ist. Zur Analyse der binären Nb-Sn-Legierungen (etwa Nb₃Sn) ist 8,0±0,1 m HCl+10 Vol-% Äthylenglykol als Grundelektrolyt geeignet. In diesem Elektrolyten erscheinen die erste Reduktionswelle des Niobs, Nb^VIV, und die zweite Reduktionswelle des Zinns, Sn^IISn, gut getrennt und vermeßbar. Die Differenz der Spitzenpotentiale (E _pNb^V/IV= -0.26V, E _pSn^II/O= -0.46V gegen Quecksilber-Bodenelektrode) beträgt 200 mV.Die beschriebene Methode hat im Vergleich zu den bekannten nichtpolarographischen Analysenvorschriften den Vorzug, eine bis zu 100 mal kleinere Einwaage (bei vergleichbarer Reproduzierbarkeit) und einen verringerten Zeitaufwand zu erfordern.Bei Einwaagen von 2–14 mg beträgt die Genauigkeit der Niobbestimmung 1% rel., die der Zinnbestimmung 2% rel. Eine einzelne Niob-Zinn-Simultanbestimmung dauert 4 Std.

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Summary The possibility of a simultaneous polarographic determination of niob and tin is investigated.It has been found that the determination cannot be carried out by direct current polarography, but square-wave polarography yields good results.8.0±0.1 M HCl+10 vol-% ethylene glycol is suitable as supporting electrolyte for the analysis of the binary Nb-Sn-alloys (Nb₃Sn).In this supporting electrolyte the first reduction-wave of niobium Nb^VNb^IV, and the second reduction wave of tin, Sn^IISn⁰, are well developed and separated. The difference of the peak potential (E _pNb^V/IV= -0.26V, E _pSn^II/O= -0.46V vs. mercury-pool electrode) is 200 mV. Compared with conventional methods the method described has the advantage of requiring smaller amounts of sample (up to 100 times smaller by weight at comparable reproducibility) with a saving of time.With sample amounts of 2 to 14 mg the accuracy of the niobium determination is 1% rel., that of the tin determination is 2% rel. One single simultaneous determination of niobium and tin takes about four hours.

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Herrn Prof. Dr. M. von Stackelberg zum 70. Geburtstag gewidmet.     

Determination of naturally occurring radionuclides in ultrapure organic liquids

For the determination of trace elements in organic liquids radiochemical neutron activation analysis has been combined with counting methods geared to various decay modes of indicator radionuclides leading to a high sensitivity required for ultrapure samples. The activation parameters such as irradiation time, sample mass and neutron flux have been enlarged to the maximum possible in the available irradiation facility. Separation yields and adsorption losses have been studied in detail for a set of elements in order to rule out losses during the separation process. The attainable limits of detection are 2 · 10^–16g/g for U and Lu, in the 5 · 10^–15g/g range for Th and Sm, in the 1 · 10^–14g/g region for La, 5 · 10^–13g/g for Rb, Cd and 2 · 10^–12g/g for K and In. Although the analysis focused on traces of naturally occurring radioisotopes, results for Cr, Fe, W and Zn are presented as well.     

Analytische Probleme der Darstellung radioaktiven Schwefelwasserstoffs mit hoher spezifischer Aktivit?t

Zusammenfassung Es wird ein Verfahren im Mikromaßstab zur Darstellung und Analyse von höchstreinem Schwefelwasserstoff von hoher bis sehr hoher spezifischer Aktivität beschrieben. Durch Anwendung der Adsorptions-Chromatographie und isothermer Destillation konnte bei hohen Ausbeuten der Gehalt an Verunreinigungen unter 10^–2 Molprozent gesenkt werden. Die massenspektrometrische H₂S-Analyse und ihre Problematik wurden ausführlich behandelt. Durch geeignete Versuche und ihre theoretische Interpretation konnte eindeutig nachgewiesen werden, daß im Massenspektrometer durch Nebenreaktionen des H₂S sowohl CS₂ als auch H₂O sekundär gebildet werden, wodurch eine Verfälschung des Analysenergebnisses eintritt.Als Kontrollmethode für die Reinheit des H₂S während des Ablaufes der Sulfidierungsreaktionen im Innern der geschlossenen Apparatur erwies sich die Messung des Tripelpunktdruckes als besonders geeignet. Sie wurde dafür zu einer Mikromethode für nur wenige Milliliter radioaktiven Gases modifiziert, die noch ca. 7 ppm Verunreinigungen zu erfassen erlaubt.

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Analytical problems of the preparation of radioactive hydrogen sulphide of high specific activity

A procedure in micro scale is described for the preparation and analysis of hydrogen sulphide of maximum purity with high to very high specific activity. High yield and impurity contents of less than 10^–2 Mol-% could be achieved by the use of adsorption chromatography and isothermal distillation.The mass-spectrometric H₂S analysis and its problems are discussed in detail. By suitable experiments and their theoretical interpretation it could clearly be shown that CS₂ and H₂O are formed secondarily by side-reactions of hydrogen sulphide in the mass spectrometer, thus causing erroneous results. Measurement of the triple-point pressure proved to be particularly suitable for checking the purity of H₂S during the course of the sulphurizing reactions within the closed apparatus. It has been modified to a micro method for only a few ml of radioactive gases, permitting the determination of about 7 ppm of impurities.

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Der Deutschen Forschungsgemeinschaft und dem Fonds der Chem. Industrie gebührt Dank für die Förderung dieser Untersuchungen.

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Herrn Prof. Dr. E. Asmus zum 60. Geburtstag gewidmet.     

Electrical resistivity and superconductivity of amorphous La−Ag alloys

We report on measurements of the electrical resistivity and the superconducting transition temperature of amorphous La–Ag alloys (60–74 at % La) obtained by liquid quenching. The temperature coefficient of the resistivity is always slightly negativ, its value cannot be described well by existing theories.T _c depends linearly on the La content and seems to be not very sensitive to the second alloy constituent.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday     

Mimicking primary processes in photosynthesis—covalently linked porphyrin quinones

Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed.     

Low-temperature specific heat of the cluster compound Au55 (P(C6H5)3)12Cl6

Specific-heat measurements on the cluster compound Au₅₅(P(C₆H₅)₃)₁₂Cl₆ at temperatures 0.06 K ≤T≤3 K and in magnetic fields 0≤B≤6 T are reported. While above 0.6 K the specific heatC is dominated by the inter-cluster vibrational contribution observed previously, an anomalous increase ofC towards lowT is observed below 0.3 K, withC ～T ^?2. This contribution develops into a Schottky-like anomaly forB≥0.4 T, indicating that it might be attributed to local moments which are also observed in ESR measurements. From the height of the anomaly one can infer that approximately one tenth of the Au₅₅ clusters carry a magnetic moment. For 0.6 K≤T≤1 K andB=0 our data indicate the absence of a linear electronic specific-heat contribution expected for bulk Au. This possibly constitutes the first direct observation of the quantum-size effect on electronic energy levels in the specific heat.     

Determination of oxygen in alkali halide salts by proton activation and radiochemical separation

A lanthanum fluoride precipitation method for the separation of¹⁸F produced from the¹⁸O(p,n)¹⁸F reaction in alkali halide salts is described. This radiochemical separation method minimizes interferences from other positron emitters produced by proton bombardment and makes the accurate determination of ppm-level¹⁸O in complex alkali halide systems feasible. The interference from the¹⁹F(p,d)¹⁸F reaction is eliminated by keeping the proton energy less than 8.2 MeV. Applications of this technique to studies of dissolved oxide species in molten alkali halide salts are discussed.     

Cyclische Kieselsäurederivate

Cyclic Silicic Acid Derivatives By reaction of SiCl₄ with oxygen at elevated temperatures a number of chlorsiloxanes consisting of more or less fused rings can be prepared of which those components of low molecular weight can be separated by combining distillation and crystallization. Methyl and ethyl esters of silicic acids are obtained by reaction with methyl or ethyl nitrite, resp. The compounds are characterized by silicon n.m.r. and mass spectroscopy.