Equations for calculation of the pH of buffer solutions containing sodium or potassium dihydrogen phosphate, sodium hydrogen phosphate, and sodium chloride at 25°C

Published thermodynamic data measured in aqueous mixtures of sodium or potassium dihydrogen phosphate with hydrogen phosphate and chloride at 25°C were used to test recently developed methods for calculation of the pH of phosphate buffer solutions. Equations for ionic activity coefficients are used in these methods. It is shown that all data used in the tests up to an ionic strength of about 0.5 mol-kg^-1 can be accurately predicted by the two methods recommended. In one of these methods, equations of the Hückel type are used for ionic activity coefficients and in the other equations of the Pitzer type. Several sets of phosphate buffer solutions are recommended,e.g., for calibrations of glass electrode cells. In the recommended sets, the pH of the buffer solutions can be calculated either by the Hückel or Pitzer method, and the pH predictions of these methods agree in most cases within 0.005 at least up to ionic strengths of about 0.2 mol-kg^-1. The pH values of the two primary pH standards endorsed by IUPAC based on aqueous mixtures of KH₂PO₄ and Na₂HPO₄,i.e., pH values of 6.865 and 7.413, can also be accurately predicted by the equations recommended in this study.     

On the 1/n expansion

The 1/n expansion is considered for then-component non-linear -model (classical Heisenberg model) on a lattice of arbitrary dimensions. We show that the expansion for correlation functions and free energy is asymptotic, for all temperatures above the spherical model critical temperature. Furthermore, the existence of a mass gap is established for these temperatures andn sufficiently large.Supported in part by the National Science Foundation under Grant PHY 79-16812     

In situ reaction mechanism studies on the atomic layer deposition of Al2O3 from (CH3)2AlCl and water

Reaction mechanisms between dimethylaluminum chloride and deuterated water in the atomic layer deposition (ALD) of Al2O3 were studied at 150-400 degrees C using a quartz crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS). The observed reaction byproducts were DCl and CH3D. QMS showed that about one-third of the chlorine, and half of the methyl ligands were released during the (CH3)2AlCl pulse. The growth rate deduced from the QMS and QCM data was in qualitative agreement with the previously published growth rate from ALD film growth experiments.     

Expanding the Scope of the Cleavable N-(Methoxy)oxazolidine Linker for the Synthesis of Oligonucleotide Conjugates

Oligonucleotides modified by a 2′-deoxy-2′-(N-methoxyamino) ribonucleotide react readily with aldehydes in slightly acidic conditions to yield the corresponding N-(methoxy)oxazolidine-linked oligonucleotide-conjugates. The reaction is reversible and dynamic in slightly acidic conditions, while the products are virtually stable above pH 7, where the reaction is in a ‘‘switched off-state’’. Small molecular examinations have demonstrated that aldehyde constituents affect the cleavage rate of the N-(methoxy)oxazolidine-linkage. This can be utilized to adjust the stability of this pH-responsive cleavable linker for drug delivery applications. In the present study, Fmoc-β-Ala-H was immobilized to a serine-modified ChemMatrix resin and used for the automated assembly of two peptidealdehydes and one aldehyde-modified peptide nucleic acid (PNA). In addition, a triantennary N-acetyl-d-galactosamine-cluster with a β-Ala-H unit has been synthesized. These aldehydes were conjugated via N-(methoxy)oxazolidine-linkage to therapeutically relevant oligonucleotide phosphorothioates and one DNA-aptamer in 19–47% isolated yields. The cleavage rates of the conjugates were studied in slightly acidic conditions. In addition to the diverse set of conjugates synthesized, these experiments and a comparison to published data demonstrate that the simple conversion of Gly-H to β-Ala-H residue resulted in a faster cleavage of the N-(methoxy)oxazolidine-linker at pH 5, being comparable (T_0.5 ca 7 h) to hydrazone-based structures.     

Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature. An erratum to this article is available at .     

Expanding the hydride chemistry: antiperovskites A3MO4H (A = Rb,Cs; M = Mo,W) introducing the transition oxometalate hydrides

The four compounds A₃MO₄H (A = Rb, Cs; M = Mo, W) are introduced as the first members of the new material class of the transition oxometalate hydrides. The compounds are accessible via a thermal synthesis route with carefully controlled conditions. Their crystal structures were solved by neutron diffraction of the deuterated analogues. Rb₃MoO₄D, Cs₃MoO₄D and Cs₃WO₄D crystallize in the antiperovskite-like K₃SO₄F-structure type, while Rb₃WO₄D adopts a different orthorhombic structure. ²H MAS NMR, Raman spectroscopy and elemental analysis prove the abundance of hydride ions next to oxometalate ions and experimental findings are supported by quantum chemical calculations. The tetragonal phases are direct and wide band gap semiconductors arising from hydride states, whereas Rb₃WO₄H shows a unique, peculiar valence band structure dominated by hydride states.

The synthesis, structures and electronic properties of the first four heteroanionic compounds containing both hydride and transition oxometalate ions are reported.     

The vane method and kinetic modeling: shear rheology of nanofibrillated cellulose suspensions

We conduct rheological characterization of nanofibrillated cellulose (NFC) suspensions, a highly non-Newtonian complex fluid, at several concentrations. Special care is taken to cope with the prevalent problems of time scale issues, wall depletion and confinement effects. We do this by combining the wide-gap vane geometry, extremely long measurement times, and modeling. We take into account the wide-gap related stress heterogeneity by extending upon mainstream methods and apply a gap correction. Furthermore, we rationalize the experimental data through a simple viscous structural model. With these tools we find that, owing to the small size of the particles subjected to Brownian motion, the NFC suspensions exhibit a critical shear rate, where the flow curve experiences a turning point. This makes the steady state of these suspensions at low shear rates non-unique. To optimize various mixing and pumping applications, such history dependent tendency of NFC suspensions to shear band needs to be taken into account.     

Continuous‐Flow Microwave Synthesis of Metal–Organic Frameworks: A Highly Efficient Method for Large‐Scale Production

Metal–organic frameworks are having a tremendous impact on novel strategic applications, with prospective employment in industrially relevant processes. The development of such processes is strictly dependent on the ability to generate materials with high yield efficiency and production rate. We report a versatile and highly efficient method for synthesis of metal–organic frameworks in large quantities using continuous flow processing under microwave irradiation. Benchmark materials such as UiO‐66, MIL‐53(Al), and HKUST‐1 were obtained with remarkable mass, space–time yields, and often using stoichiometric amounts of reactants. In the case of UiO‐66 and MIL‐53(Al), we attained unprecedented space–time yields far greater than those reported previously. All of the syntheses were successfully extended to multi‐gram high quality products in a matter of minutes, proving the effectiveness of continuous flow microwave technology for the large scale production of metal–organic frameworks.