Studies on the polymer + plasticizer membrane-electrode-VIII Use of the PVC + tricresyl phosphate membrane as indicating electrode in potentiometric precipitation titration

Results obtained by use of the PVC + tricresyl phosphate membrane-electrode in potentiometric precipitation titration of some halides and pseudohalides are given. Solutions of Ag(+), Tl(+) and Hg(2)(2+) were used as titrants, to which the membrane is responsive. The fast response to Ag(+) ions allowed the determination of iodide by automatic recording of the potentiometric curve (or its derivative).     

Hyperfine fields on59Co in ReCo2

We report⁵⁷Fe Mössbauer measurements in the diluted antiferromagnetic (AF) Fe_xZn_1?xF₂ withx=0.25 at temperatures between 4.2 and 28 K. DC susceptibility measurements show a spin-glass (SG) phase at low temperatures forx?0.3. Our Mössbauer spectra show a competitive coexistence of an SG phase and AF order. We propose an interpretation in terms of clusters in the AF order and some spins exhibiting SG behavior.     

A lower estimate for the achromatic number of irreducible graphs

The achromatic number of a graph is the largest number of independent sets its vertex set can be split into in such a way that the union of any two of these sets is not independent. A graph is irreducible if no two vertices have the same neighborhood. The achromatic number of an irreducible graph with n vertices is shown to be $\text{≥(}\text{1}\text{2}\text{?0(1))log}\text{n?}\text{log log}\text{n,}$ while an example of Erdös shows that it need not be log n/log 2+2 for any n. The proof uses an indiscernibility argument.     

Multiple spin-echoes in a photo-excited triplet state

A Carr-Purcell multiple spin-echo, as modified by Meiboom and Gill, is described for the 3598 MHz zero-field transition of phosphorescent quinoline C₉ND₇ in a durene host. The results show that the dephasing of the electron spins through spectral diffusion may be largely eliminated by the multiple echo technique.     

Soil analysis by thin-film energy-dispersive x-ray fluorescence

Energy-dispersive x-ray fluorescence is advantageous for trace analysis of soils present as thin films. A target thickness of about 2 mg cm^-2 provides a compromise between optimal sensitivity and minimal absorption effect or optimal accuracy. Sample preparation involves only suspending the finely ground soil in water and drying this suspension on a thin mylar foil glued on a ring that fits into the x.r.f. spectrometer. The “effective sample weight” present in the exciting beam area is computed from the scatter peaks, a method that cancels out target heterogeneity problems. High accuracy is demonstrated for many elements in reference soil and rock materials; a precision around 5% and a detection limit around 10 ppm can be achieved. As an illustration, results for 16 trace elements and preliminary interpretation are given for a series of pedologically important soil samples from Brasil.     

Konformative Beweglichkeit Flexibler Ringsysteme—XIII: Untersuchungen mit Hilfe der Protonenresonanzspektroskopie die Konformation des Ungesättigten Siebenringes im Benzocyclohepten und in symmetrisch substituierten derivaten

The confromations of the unsaturated seven membered ring in 4,4,6,6-tetradeuterium-1,2-benzocycloheptene-(1) ( 1 ) and five benzocycloheptene derivatives were determined by NMR spectroscopy. For all investigated compounds at ?80°C only one conformer was present in detectable quantity. By analysis of the NMR data – molecular symmetry, coupling constants and chemical shift – it can be shown that the conformation is always the chair form. The free conformational enthalpy of both the other conformations with boat or twist form of the ring is for all six compounds more than 1.8 kcal/mole. The experimental results agree with those from model calculations: thus for benzocycloheptene, the 5,5-dimethyl derivative ( 2 ) and the 4,4,6,6-tetramethyl derivative ( 4 ) the lowest energy was found for the chair conformation; the second most stable conformations were found to be the boat for 1 and 4 , and the twist form for 2 .     

Effect of vacuum conditions on breakdown voltage between electrodes in vacuum

The electrical breakdown in vacuum was studied in the configuration: wire —plane. Measurements were performed in two different experimental systems: with technical vacuum and with the clean one. It was found that the value of breakdown voltage decreases with increasing vacuum and that at the same pressure the insulating properties of electrodes are better in the technical vacuum than in the oilless one.     

Secondary electron emission from thin self-supporting films of silver

The paper deals with the measurement of secondary electron emission and the scattering of primary electrons on thin self-supporting films of Ag. The measured values of the practical range agree with the older results obtained by Kanter. New results are those concerning the maximum depth from which the primaries are scattered back to the front surface and the maximum range of secondaries, which has been found to be about 300 å.     

Excitations in the antiferromagnetic Heisenberg chain

Non-singlet excitations of the antiferromagnetic Heisenberg chain of N atoms with spin $\text{1}\text{2}$ are examined. It is found that the energy of the lowest lying excitations for total spin S and wave number q converges to $\text{(}\text{π}\text{2}\text{)|}\text{sin}\text{q |}\text{as}\text{N → ∞,}\text{if only}\text{S?}\text{ln}\text{N}$.