Search Results

You are looking at 1 - 10 of 91 items for :

  • "gravimetry" x
  • All content x
Clear All

Abstract  

Water adsorption at temperatures of 286 and 296 K on silicalite-I, ZSM-5 (Si/Al=16), ZSM-48 (Si/Al=50) and AlPO4-5 is followed by gravimetry with a quasi-equilibrium continuous adsorptive introduction. The results show that all of these samples are characterized by a continuous distribution of strongly energetic water adsorption sites (from 60 to 120 kJ·mol−1) for which the adsorption is irreversible at the experimental temperature. This probably justifies the presence of hysteresis on desorption at very low relative pressure values. Adsorption of water in these systems firstly occurs by site. This is then followed by cluster formation and it is suggested that it is the ability of the adsorbent to build up these clusters within the microporous structure which determines intracrystalline uptake. It is put forward that the zeolites, silicalite-I and ZSM-5, do not accommodate cluster formation within its microporous network. However, an external flexible microporous structure, containing Lewis sites, may be present for large crystals. This flexible secondary structure may then be able to opened (swelled) at high relative pressures. On the other hand, for the aluminophosphate AlPO4-5, it is believed that a change in the aluminium coordination on the formation of a crystal hydrate together with capillary condensation results in a large step in the adsorption isotherm, which is itself preceded by a smaller step, revealing a brutal densification of the adsorbed phase.

Restricted access

Abstract  

The kinetics of thermal decomposition of a series of uranyl nitrate complexes with N-alkylcaprolactams (alkyl=C2H5, C4H9, C6H13, C8H17, C10H21 or C12H25) was studied by means of non-isothermal gravimetry under a nitrogen atmosphere. From the TG-DTG curves, the kinetic parameters relating to the loss of two molecules of coordinated ligand were obtained by employing two groups of methods: (I) a group of conventional methods involving the Coast-Redfern, Freeman-Carroll, Horowitz-Metzger, Dharwadkar-Karkhanavala and Doyle (modified by Zsakó) equations; (II) a new method were suggested by J. Máleket al.. The results obtained using two types of methods were compared, and it emerged that the results of method II were much more meaningful and reasonable in this work. Additionally, the effects of the molecular structure of the ligands on the kinetic data and models were studied and are discussed.

Restricted access

Abstract  

Adsorption isotherms of n-butane on a granulated activated carbon were measured by two different but complementary experimental methods: calorimetry and gravimetry. Adsorption heats were determined in different ways. For the system studied, the experimental results prove that the adsorbent offers a homogeneous site distribution. Besides, there can be differences between the adsorption heat values which might come from the way they are obtained (by calculation or direct measurements).

Restricted access

Abstract  

The amorphous content of different Desferal samples was quantified by recording its recrystallization using isothermal microcalorimetry in a static as well as in a flowing humid atmosphere. Furthermore water vapor sorption gravimetry was performed for the same purpose. These analytical methods result in a quantitative signal directly dependent on the content of the amorphous phase (recrystallization, water sorption equilibrium). Their sensitivity allows the detection of amorphous content below 1%. Methods are compared and advantages and disadvantages are discussed.

Restricted access

Abstract  

Jäntti published in 1970 a method to obtain equilibrium values at an early stage of gravimetric sorption measurements. In former papers the authors criticised and extended that method. In the present work we discuss problems of its applicability on practical measurements.

Restricted access

Abstract  

The interaction between samples of metallic zinc and water vapour was studied gravimetrically, both in the absence and in the presence of oxygen. The experimental total mass gain vs. time curves exhibited two plateaus, whose heights increased with, elevations both of relative humidity and of temperature. The amount of product retained on the surface after desorption was also determined as a function of time. The product was identified as hydrated zinc oxide. In the runs conducted without oxygen, the retained product curves displayed a time delay with respect to the total mass gain curves. In the presence of oxygen, however, there was practically only one chronogravimetric curve. This behaviour is interpreted on the basis of a common mechanism involving the formation of an intermediate precursor oxide, which is more readily formed in the presence of oxygen than in its absence. A set of mathematical equations was derived, from which the rate constants for both processes were obtained. The second step was ascribed to a further weak adsorption of water.

Restricted access

Abstract  

We will review the application of acoustic wave mass sensors in chemical and biological sensing with focus on quartz crystal microbalance and surface acoustic wave devices. In chemical sensing, it is unlikely that a single sensor will display a selective and reversible response to a given analyte in a mixture. Alternative strategies such as use of sensor arrays and sampling devices will be discussed to improve performance. We will also discuss applications of quartz crystal microbalance as biosensor in the liquid phase.

Restricted access

Abstract  

Instead of reading the equilibrium value, the deflection of the balance as a function time can be measured and the equation of motion

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$T = J\ddot \alpha + k\dot \alpha + C\alpha$$ \end{document}
can be used to calculate the unknown torque T and to relate the other quantities in the equation to the actual instrument-constants. In this way, balance reading could be much faster and weighing errors due to faulty instrument and environmental influences can be smaller than those in equilibrium position. This enables the use of microbalances for the observation of fast chemical or thermal processes and to use it as fast checkweigher for control of sorting machines. In the present paper we present results from calculations of a simulation procedure.

Restricted access

Abstract  

The short survey concerns the discovery of adsorbents and the measurement of the adsorptivity. In the investigation of adsorption very sensitive instruments are needed, developed only recently. Two methods, however, are very old: the volumetric and the gravimetric measurement of the adsorbed amount. In the Bible we find a thorough description of a volumetric adsorption experiment. The systematic research began in 1773 when Scheele, Fontana and Priestley observed the adsorption of air by charcoal. The volumetric apparatus of Brunauer, Emmett and Teller set the prototype for many instruments devoted to measure adsorption isotherms. The first gravimetric adsorption measuring instruments were hygrometers, described by Nicholas of Cues in 1450, Alberti and Leonardo da Vinci. In 1833 Talabot installed 39 drying balances in a Lyon laboratory for humidity control of raw silk imported from China. In 1912 Emich described an electronic beam microbalance to investigate adsorption and a coil spring balance. Today isotherms are measured gravimetrically by means of electro-dynamically compensating microbalances. Also oscillating systems are being used which allow weighing down to the zeptogram region.

Restricted access