Photometrische Bestimmung von NH4OH in NH4F-L?sungen und NH4F-HF-Puffergemischen

Zusammenfassung Zur reproduzierbaren Herstellung von NH₄F-HF-Ätzlösungen muß der Gehalt an NH₄OH in den käuflichen 40% igen NH₄F-Lösungen ermittelt werden. Hierzu wurde eine colorimetrische Methode unter Verwendung von pH-Farbindicatoren der Phenolgruppe ausgearbeitet, die geringe pH-Wertabweichungen (0,007 pH) vom Soll (0% NH₄OH) zu messen erlaubt, wobei der Sollwert durch stabile Citronensäure-Phosphatpuffer simuliert wird. Durch Extinktionsmessung gelingt es, 0,01% NH₄OH in NH₄F-Lösungen zu erfassen. Die gefundenen Werte liegen zwischen 0,1–0,3%.

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Photometric determination of NH₄OH in NH₄F solutions and NH₄F-HF buffer mixtures

To obtain NH₄F-HF etching agents with constant etching behaviour the concentration of NH₄OH in commercially available 40% NH₄F solutions has to be measured. By application of pH-indicators of the phenol type very small pH-deviations (0.007 pH) from the specified value (0% NH₄OH) can be measured. The specified value is simulated by the stable McIlvaine buffer. By determining the differences in optical absorption 0.01% NH₄OH can be measured in NH₄F solutions. Values found in practice are between 0.1– 0.3%.

     

DSC investigation of the thermal behaviour of (NH4)2SO4, NH4HSO4 and NH4NH2SO3

Gaseous products evolved from (NH₄)₂SO₄, NH₄HSO₄ and NH₄NH₂SO₃ during successive heating and cooling cycles were flushed with inert gas into analyzer Dräger tubes hooked tightly to the terminal port of the DSC cell base. This simple procedure allowed the starting temperature of the decomposition to be determined and the amount of the individual gases in the mixture to be identified and even estimated. NH₄NH₂SO₃ at 523 K in humid air produced HNH₂SO₃ initially and, on further cycling, (NH₄)₂SO₄ and NH₄HSO₄ also appeared. The ΔH_f values for NH₄HSO₄ were (kJ mole^?1): in an airtight sample holder 12.67, in a dry argon atmosphere 11.93, and in a static air atmosphere 10.92. Endothermic peaks for (NH₄)₂SO₄ and 498 and 411 K represented the incongruent melting point and the polymorphic transition of (NH₄)₂SO₄·NH₄HSO₄. After the first heating in air to 530 K, (NH₄)₂SO₄ and NH₄HSO₄ exhibited closely similar cyclic DSC curves. The endothermic peaks at about 393–420 K may be assigned to different combinations of (NH₄)₂SO₄ and NH₄HSO₄.     

Internal-energy-selected measurements on the relative cross section of the ion-molecule reaction NH3+(NH3, NH2)NH4+

A photoelectron-photoion coincidence technique is used to measure the internal-energy dependence of the ion-molecule reaction NH₃⁺(E_int+NH₃ → NH₄⁺ + NH₂ at thermal collision energy. The range in which the internal energy is varied, is enlarged by including in the experiment the electronically excited state of the NH₃⁺ ion. Special attention is paid to the possible influence of the product's kinetic energy on the measurements. The experimental results are analysed using a modified statistical model and compared with previous data.     

Mössbauer spectroscopy of 57Fe in NH4MnCl3, NH4FeCl3 and NH4CoCl3

The Mössbauer spectra of ⁵⁷Fe doped NH₄MnCl₃ reveal that this compound undergoes a crystallographic phase transition at 258°K and becomes magnetic at 105°K. No crystallographic transition appeared either in NH₄FeCl₃ or in ⁵⁷Fe:NH₄CoCl₃; the latter orders magnetically around 27°K.     

A re-interpretation of the crystal structure of ‘(NH4)2(NH3)[Ni(NH3)2Cl4]’ in terms of (NH4)2−2z[Ni(NH3)2]z[Ni(NH3)2Cl4]

A re-interpretation and re-evaluation of single-crystal X-ray diffraction data of a previously reported ‘(NH₄)₂(NH₃)[Ni(NH₃)₂Cl₄]’ (J. Solid State Chem. 162 (2001) 254) give a new formula (NH₄)_2−2z[Ni(NH₃)₂]_z[Ni(NH₃)₂Cl₄] with z=0.152. This new formula results from defects in an idealized ‘(NH₄)₂[Ni(NH₃)₂Cl₄]’ basic structure, where two adjacent NH₄⁺ cations are replaced by one Ni(NH₃)₂²⁺ unit. Cl⁻ anions from the basic structure complete the coordination sphere of the new Ni²⁺ to [Ni(NH₃)₂Cl₄]²⁻.     

Phase relations of NH4ClO4 and NH4BF4 to high pressures

The high pressure phase diagrams of NH₄ClO₄ and NH₄BF₄ were studied by means of differential thermal analysis and volumetric techniques. The high temperature portions of these diagrams are intermediate between those of the corresponding potassium and rubidium salts, but at low temperatures the onset of hydrogen bonding causes the appearance of phases which are unique to the ammonium compounds.     

Laser fluorescence of NH2 and rate constant measurement of NH2 + NO

Laser induced fluorescence from the òA₁ state of the NH₂ radical, obtained by different methods, has been observed with a tunable cw dye laser as excitation source. Using pulsed photolysis of NH₃ to produce NH₂, the fluorescence technique has been employed in a first gas kinetic application to measure the rate constant of the aeronomically interesting NH₂ + NO reaction at 298 K. A value of (2.1 ± 0.2) × 10^-11 cm³ molecule^-1 s^-1 has been obtained.     

Hindered rotation of the ammonium ion in (NH4)2SiF6 and (NH4)2SnCl6

The heat capacity due to the hindered rotation of the ammonium ion has been computed for (NH₄)₂SiF₆ and (NH₄)₂SnCl₆ and compared to that derived from the observed heat capacity. The torsional frequencies for (NH₄)₂SiF₆ and (NH₄)₂SnCl₆ are 226 cm^?1 and 190 cm^?1 respectively, and the barriers to rotation are 3210 calories/mole and 1470 calories/mole, respectively.     

Analysis of (NH4)2SO4/(NH4)H2PO4 mixtures by thermogravimetry and X-ray diffraction

(NH₄)₂SO₄ and (NH₄)H₂PO₄ are the principal components in the powder material used in fire extinguishers. In this paper the mutual influence in their thermal decomposition is investigated by thermogravimetry. Two methods for the quantification of both salts in mixtures (NH₄)₂SO₄/(NH₄)H₂PO₄ are proposed. The first employs thermogravimetry and is based on the measurement of the mass fraction in the 500-550 °C interval, once (NH₄)₂SO₄ has totally decomposed to yield gaseous products. The second uses some selected peaks in the X-ray diffractogram.     

On the temperature dependence of proton magnetic resonance lineshapes of tunnelling ammonium in NH4ClO4 and NH4PF6

The proton magnetic resonance lineshapes of NH₄ClO₄ and NH₄PF₆ at liquid helium temperature are narrower than those usually expected for rapidly tunnelling ammonium ions. The narrowing is temperature-dependent, suggesting a kinetic process that involves a modulation of the tunnelling states. A possible “exchange-narrowing” mechanism involving fluctuating molecular torsional states brought about by external lattice perturbations is considered, and is seen to lead to a satisfactory simulation of the observed low-temperature lineshapes. Activation energies of 0.9 ± 0.1 K and 1.7 ± 0.5 K governing the exchange process are derived for the cases of NH₄ClO₄ and NH₄PF₆, respectively.     

Melting curve and high-pressure polymorphism of NH4HF2

The melting curve of NH₄HF₂ I rises from 125.2°C at atmospheric pressure to a triple point II/I/liquid at 9.3 kbar, 220°C. The I/II phase boundary is terminated at a triple point III/I/II at ∼45 kbar, 295°C. The melting curve of the new phase NH₄HF₂ II passes through a broad maximum at ∼39 kbar, 306°C, and is terminated at a triple point III/II/liquid at 46.3 kbar, 301°C. The melting curve of NH₄HF₂ III rises with pressure. The NH₄HF₂ III may be a dense hydrogen-bonded phase. Liquid NH₄HF₂ appears to be anomalous in several respects, and has a high compressibility relative to the solid phases.     

Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6, a NMR and DFT study

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH₄)₂NbOF₅ and (NH₄)₃NbOF₆ were studied by ¹⁹F, ¹H NMR and DFT calculations. Equatorial fluorine atoms (F_eq) in [NbOF₅]²⁻ and [NbOF₆]³⁻ are characterized by high ¹⁹F NMR chemical shifts while axial fluorine atoms (F_ax) have those essentially lower. The high-temperature ionic mobility in (NH₄)₂NbOF₅ does not stimulate the ligand exchange F_eq ↔ F_ax, whereas it is observed in (NH₄)₃NbOF₆ as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF₆]³⁻ into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH₄)₂NbOF₅ is accompanied by transition of anionic sublattice to a rigid state. The ¹⁹F and ¹H NMR data corroborate the independent motions of NH₄ groups and anionic polyhedra in (NH₄)₂NbOF₅ while they are coordinated in (NH₄)₃NbOF₆.     

Determination of the rate constant for the reaction NH3 + OH → NH2 + H2O

The rate constant for the reaction NH₃ + OH → NH₂ + H₂O was determined by the comparison of the calculated induction period data with experiments by the shock tube technique in the range 1360–1840 K, for NH₃-H₂-O₂-Ar mixtures. The rate constants can be represented by the expression k = 10^12.49±0.04exp[(?1.95±0.15) kcal/,RT] cm³ mol^?1 s^?1.     

Preparation of high-purity titanium salts from the system (NH4)2TiF6-(NH4)3FeF6-NH4F-H2O

Influence of NH₄F concentration on the conditions of separation of Ti(IV) and Fe(III) salts by fractional hydrolysis under the action of ammonia in the system (NH₄)₂TiF₆-(NH₄)₃FeF₆-NH₄F-H₂O was studied. The coprecipitation of (NH₄)₃TiO₂F₅ and (NH₄)₃FeF₆ as a result of crystallochemical substitution of Fe⁺³ for Ti⁺⁴ makes it possible to reduce the content of Fe(III) salts in the system. After separation of a precipitate containing up to 25% of Ti from solutions with NH₄F concentrations from 10.6 to 22.3% and subsequent hydrolysis, the remaining titanium precipitates. The precipitate is annealed up to 800°C in the presence of water vapor to form TiO₂ with a whiteness above 92%. Original Russian Text ? N.G. Bakeeva, P.S. Gordienko, E.V. Pashnina, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 1, pp. 3–8.     

Thermal behaviour of (NH4)3VO2F4 and Na(NH4)2VO2F4

The thermal behaviour of (NH₄)₃VO₂F₄ and Na(NH₄)₂VO₂F₄ was investigated using TG, DTA and DSC techniques. The occurrence of a first order phase transition with the onset of decomposition in both the compounds is confirmed. The temperature, energetics and hysteresis of the transition are obtained. A possible path for the thermal degradation is given for both the compounds, and the residues are identified.     

Orientation disorder in ammonium elpasolites: Crystal structures of (NH4)3AlF6, (NH4)3TiOF5 and (NH4)3FeF6

Crystal structures of the known (NH₄)₃AlF₆(I) and (NH₄)₃FeF₆(III) and new (NH₄)₃TiOF₅(II) elpasolites were refined by localizing anions (F⁻, O²⁻) in four acceptable positions of the cubic system Fm3m (Z=4) with a=8.9401(3), 9.1104(3), 9.110(1) Å, respectively. According to the refinement data and a rather large entropy change due to fluorine (oxygen) octahedra disordering in the above compounds and in (NH₄)₃WO₃F₃(IV) elpasolite, it was found that fluorine (oxygen) atoms are randomly distributed in two ways, in general 192l position or in mixed 24e + 96j one. Statistics in fluorine (oxygen) distribution is, probably, the result of domain structure of the crystals.     

Protonic conduction in NH4H2PO4

The conductivity of ammonium dihydrogen phosphate has been measured as a function of temperature and dopant concentration. A previously disputed break in the log conductivity vs reciprocal temperature plots has been observed. The activation energy is in agreement with previous work on NH₄H₂PO₄. In addition, the conductivity vs concentration of NH₄HSO₄ plot is linear, permitting the calculation of the L defect mobility and indicating that the proton is the conducting species. It is concluded that the mechanism of conduction is the same as previously proposed for KH₂PO₄ and KH₂AsO₄.     

Crystal structure of [Pd(NH3)4][Rh(NH3)(NO2)5]

An XRD analysis is used to study the single crystal of [Pd(NH₃)₄][Rh(NH₃)(NO₂)₅] double complex salt at T = 150(2) K. Crystallographic characteristics are as follows: a = 7.6458(5) ?, b = 9.8813(6) ?, c = 9.5788(7) ?, β = 109.469(2)°, V = 682.30(8) ?³, P2₁/m space group, Z = 2, d _x = 2.553 g/cm³. The geometry of the complex [Rh(NH₃)(NO₂)₅]²⁻ anion is described for the first time: Rh-N(NO₂) distances are 2.020(4)–2.060(3) ?, Rh-N(NH₃) 2.074(4) ?, N(NO₂)-Rh-N(NH₃) trans-angle is 178.8(2)°.     

The electronic structure of the [(NH3)5CoO2Co(NH3)5]5+ ion

SCCC MO calculations of the [(NH₃)₅CoO₂Co(NH₃)₅]⁵⁺ cation were carried out. On the basis of the results obtained, the magnetic properties and bond characters of the compound are discussed. Finally, the electronic structure of the O₂^? group as a ligand is analyzed.     

杂化钙钛矿(NH3C6H12NH3)CuCl4的制备与表征

采用低温溶液法合成了含有二铵阳离子结构的新型二维层状结构的有机/无机杂化钙钛矿材料（NH₃C₆H₁₂NH₃） CuCl₄。采用元素分析、红外光谱、X射线衍射和紫外-可见光吸收光谱等手段对其结构与性能进行了表征。结果表明该材料的紫外-可见吸收光谱吸收峰位于285 nm和387 nm,层间距为1.18 nm。二铵阳离子的引入,使有机层⁺NH₃C₆H₁₂NH₃+与2个相邻的无机框架CuCl₄^2-分别通过较强的氢键结合在一起,排列更为规整,热稳定性更高。与单铵阳离子结构的杂化钙钛矿材料相比,由于不存在两层有机分子层间较弱的范德华力,（NH₃C₆H₁₂NH₃） CuCl₄材料的电阻率为1.36×10⁵ Ω·cm,比单胺结构的杂化钙钛矿材料的电阻率低3个数量级。