Transient freezing of liquids in forced laminar flow inside a parallel plate channel

A simple analytical approximative solution was given for calculating the time dependent development of the ice-layers at the cooled walls inside a parallel plate channel. By ignoring the effect of acceleration, resulting from converging ice-layers in the axial direction, an analytical solution for the variation of the ice-layer thickness with time and axial position could be obtained. The approximative solution was checked by numerical calculations and good agreement was found.Es wurde ein analytisches Näherungsverfahren entwickelt, das es ermöglicht, die zeitliche Entwicklung der Erstarrungsfronten im gekühlten, ebenen Kanal zu bestimmen. Die Methode liefert unter Vernachlässigung der Beschleunigungsterme durch die konvergenten Eisschichten eine exakte Lösung der Phasengrenzbeziehung. Das Näherungsverfahren wurde mittels numerischer Berechnungen überprüft und stimmt bis zu Wandunterkühlungsverhältnissen vonB=10 sehr gut mit der numerischen Lösung überein.     

Heat transfer and solidification of a laminar liquid flow in a cooled parallel plate channel: The stationary case

A simple numerical model is presented to predict the steady-state ice layers on the cooled walls inside a parallel plate channel for arbitrary entrance velocity profiles. The effect of two different entrance velocity distributions (a parabolic velocity distribution and a slug flow) on the shape of the ice-layers are examined. The quality of an approximative solution given in literature was checked by comparing with the numerical results. For the case of a fully developed parabolic velocity distribution at the entrance of the cooled channel the results are compared with experimental findings of Kikuchi [8]. A generally good agreement was found.Es wurde ein einfaches numerisches Modell entwickelt, das es ermöglicht, die stationären Erstarrungsfronten an den Kanalwänden für beliebige Verteilungen des Eintrittsgeschwindigkeitsprofils zu berechnen. Als Beispiele wurden ein voll ausgebildetes Parabelprofil und ein Pfropfenprofil am Eintritt in die Kühlstrecke untersucht. Mit Hilfe der numerischen Lösung konnte die Güte einer aus der Literatur bekannten Näherungslösung zur Berechnung der Erstarrungsfronten überprüft werden. Für den Fall des Parabelprofils am Kanaleintritt wurde die Rechnung mit Meßwerten von Kikuchi [8] verglichen. Es zeigte sich eine gute Übereinstimmung zwischen Theorie und Experiment.     

Freezing in turbulent flow inside tubes and channels

A simple and quite flexible numerical model is presented to predict the steady state ice-layer formation inside a cooled two dimensional channel or a tube containing a turbulent flow. The effects of arbitrary entrance velocity distributions upon the shape of the ice-layers are examined. The presented numerical scheme is verified by comparing the predicted ice-layers with measurements and generally good agreement was found.     

Overestimation of geogenic arsenic in landfill leachate: interference of humic and phosphinic acid in a commercial test kit

Large-scale groundwater contamination by arsenic (As) requires economical yet reliable analytical procedures to monitor the quality of well water and the effluent of As elimination processes. The silver diethyldithiocarbamate (AgDDTC) method provides the basis of a series of colourimetric tests used as a compromise between semi-quantitative field-test kits and expensive instrumental analytics. This study aimed at understanding the significant As overestimation by the AgDDTC method observed in landfill leachate samples. Results of the AgDDTC test were up to threefold the levels obtained by reference analyses. Overestimation broadly agreed with the time course of chemical oxygen demand and total phosphorus in the leachate. We hypothesised that the interference was caused by reduced P species or humic acid (HA), respectively. To test this, As-AgDDTC determinations from As standards were conducted with phosphonic (H₃PO₃) and phosphinic acid (H₃PO₂) as well as two commercial HA as matrix components. While H₃PO₃ showed no effect, H₃PO₂ in concentrations of 0.06 to 28 mg P L^?1 yielded As recoveries of up to 130%. This was probably due to the evolution of phosphine (PH₃) as suggested by the absorption spectra recorded after reacting pure PH₃ with AgDDTC in pyridine. The HA in concentrations of 0.6–320 mg dissolved organic carbon L^?1 gave As recoveries of up to 600%, possibly due to a chromophoric reaction of volatile HA digestion products formed during the reduction step. Significant As overestimation in the presence of the above-named compounds may require pre-oxidation of the samples. To recognise interference effects, the acquisition of full absorbance spectra may be recommendable in lieu of fixed wavelength analyses.     

P−P Condensation and P−N/P−P Bond Metathesis: Facile Synthesis of Cationic Tri‐ and Tetraphosphanes

[L_C^RP((PhP)₂C₂H₄)][OTf] ( 4 a,b [OTf]) and [L_C^iPrP(PPh₂)₂][OTf] ( 5 b [OTf]) were prepared from the reaction of imidazoliumyl‐substituted dipyrazolylphosphane triflate salts [L_C^RP(pyr)₂][OTf] ( 3 a,b [OTf]; a : R=Me, b =iPr; L_C^R=1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐yl; pyr=3,5‐dimethylpyrazol‐1‐yl) with the secondary phosphanes PhP(H)C₂H₄P(H)Ph) and Ph₂PH. A stepwise double P?N/P?P bond metathesis to catena‐tetraphosphane‐2,3‐diium triflate salt [(Ph₂P)₂(L_C^MeP)₂][OTf]₂ ( 7 a [OTf]₂) is observed when reacting 3 a [OTf] with diphosphane P₂Ph₄. The coordination ability of 5 b [OTf] was probed with selected coinage metal salts [Cu(CH₃CN)₄]OTf, AgOTf and AuCl(tht) (tht=tetrahydrothiophene). For AuCl(tht), the helical complex [{(Ph₂PPL_C^iPr)Au}₄][OTf]₄ ( 9 [OTf]₄) was unexpectedly formed as a result of a chloride‐induced P?P bond cleavage. The weakly coordinating triflate anion enables the formation of the expected copper(I) and silver(I) complexes [( 5 b )M(CH₃CN)₃][OTf]₂ (M=Cu, Ag) ( 10 [OTf]₂, 11 [OTf]₂).     

Half-sandwich ruthenium complexes of pentafluorobenzenethiolato ligands

A synthetic study of ruthenium complexes containing pentafluorobenzenethiolato ligand is presented. The bis(triphenylphosphine) complex CpRu(PPh₃)₂SC₆F₅ (1) is prepared from CpRu(PPh₃)₂Cl and C₆F₅S⁻ in high yield. This complex is readily reacted with CO gas to give the mixed carbonyl-phosphine complex CpRu(PPh₃)(CO)SC₆F₅ (2) and with NOBF₄ at room temperature to give [CpRu(PPh₃)(NO)SC₆F₅]BF₄ (3). The one-pot reaction of CpRu(PPh₃)₂Cl, dppa ligands, and C₆F₅S⁻ produces CpRu(dppa)SC₆F₅ [dppa = bis(diphenylphosphino)methane: dppm (4); bis(diphenylphosphino)ethane: dppe (5)]. Complexes (1)–(5) have been characterized by spectroscopic techniques (i.r., ¹H-n.m.r., ³¹P-n.m.r.) and by elemental analysis. The X-ray structural analysis of (5) is reported. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hexaphenylhexaphosphinane benzene solvate

In the molecular crystal of hexaphenylhexaphosphinane benzene solvate, C₃₆H₃₀P₆·C₆H₆, representing the trigonal form of phosphobenzene as a solvate, the six‐membered ring of P atoms adopts a chair conformation wherein the six phenyl groups are located in equatorial positions. The molecules [molecular symmetry (C_3i)] are stacked infinitely along the c‐axial direction.     

Visible-Light-Triggered Photoswitching of Diphosphene Complexes

Although diphosphene transition metal complexes are known to undergo E to Z isomerization upon irradiation with UV light, their potential for photoswitching has remained poorly explored. In this study, we present diphosphene complexes capable of reversible photoisomerizations through haptotropic rearrangements. The compounds [( 2 -κ²P,κ⁶C)Mo(CO)₂][OTf] ( 3 a [OTf]), [( 2 -κ²P,κ⁶C)Fe(CO)][OTf] ( 3 b [OTf]), and [( 2 -κ²P)Fe(CO)₄][OTf] ( 4 [OTf]) were prepared using the triflate salt [(L_C)P=P(Dipp)][OTf] ( 2 [OTf) as a precursor (L_C=4,5-dichloro-1,3-bis(2,6-diisiopropylphenyl)-imidazolin-2-yl; Dipp=2,6-diisiopropylphenyl, OTf=triflate). Upon exposure to blue or UV light (λ=400 nm, 470 nm), the initially red-colored η²-diphosphene complexes 3 a , b [OTf] readily undergo isomerization to form blue-colored η¹-complexes [( 2 -κ¹P,κ⁶C)M(CO)_n][OTf] ( 5 a , b [OTf]; a : M=Mo, n=2; b : M=Fe, n=1). This haptotropic rearrangement is reversible, and the (κ²P,κ⁶C)-coordination mode gradually reverts back upon dissolution in coordinating solvents or more rapidly upon exposure to yellow or red irradiation (λ=590 nm, 630 nm). The electronic reasons for the reversible visible-light-induced photoswitching observed for 3 a , b [OTf] are elucidated by DFT calculations. These calculations indicate that the photochromic isomerization originates from the S₁ excited state and proceeds through a conical intersection.     

Saccharified Uranyl Ions: Self-Assembly of UO22+ into Trinuclear Anionic Complexes by the Coordination of Glucosamine-Derived Schiff Bases

The reaction of UO₂(OAc)₂ ⋅ 2H₂O with the biologically inspired ligand 2-salicylidene glucosamine (H₂ L¹ ) results in the formation of the anionic trinuclear uranyl complex [(UO₂)₃(μ₃-O)( L¹ )₃]²⁻ ( 1 ²⁻), which was isolated in good yield as its Cs-salt, [Cs]₂ 1 . Recrystallization of [Cs]₂ 1 in the presence of 18-crown-6 led to formation of a neutral ion pair of type [M(18-crown-6)]₂ 1 , which was also obtained for the alkali metal ions Rb⁺ and K⁺ (M=Cs, Rb, K). The related ligand, 2-(2-hydroxy-1-naphthylidene) glucosamine (H₂ L² ) in a similar procedure with Cs⁺ gave the corresponding complex [Cs(18-crown-6)]₂[(UO₂)₃(μ₃-O)( L² )₃ ([Cs(18-crown-6)]₂ 2 ). From X-ray investigations, the [(UO₂)₃O( Lⁿ )₃]²⁻ anion (n=1, 2) in each complex is a discrete trinuclear uranyl species that coordinates to the alkali metal ion via three uranyl oxygen atoms. The coordination behavior of H₂ L¹ and H₂ L² towards UO₂²⁺ was investigated by NMR, UV/Vis spectroscopy and mass spectrometry, revealing the in situ formation of the 1 ²⁻ and 2 ²⁻dianions in solution.