Continuous gravimetric procedure

Continuous procedures for determining the adsorption isotherm 70 The continuous gravimetric procedure 70... 

Transfer the hydrolysed solution after extraction of the fatty alcohol or ethoxylate to a 500 ml volumetric flask, dilute to volume and mix. Use water for this dilution if the gravimetric procedure is to be followed, or acetone for the volumetric procedure. Continue as follows. The volumetric procedure is preferred if other surfactants are present. 

A gravimetric procedure based upon precipitation with carbon disulphide is possible, but the precipitate must be dried in a vacuum desiccator any method involving the use of heat, however mild, results in a continuing loss. 

Thermoanalytical methods essentially encompass such techniques that are based entirely on the concept of heating a sample followed by well-defned modified procedures, such as gravimetric analysis, differential analysis and titrimetric analysis. In usual practice, data are generated as a result of continuously recorded curves that may be considered as thermal spectra . These thermal spectra also termed as thermograms, often characterize a single or multicomponent system in terms of ... 

The degree of hydration of the products from these preparations and the water content given by analytical procedures depends upon the heat treatment (method and history) of the product. A sample subjected to TGA (thermal gravimetric analysis) looses water almost continually from room temperature until it becomes the completely anhydrous heteropolytungstate salt at about 400°C. On the other hand, these crystals lose some lattice water rapidly upon removal from the mother liquor and exposure to air even at room temperature. 

Polyfvinyl acetate) (PVAc) latexes produced by batch and continuous emulsion polymerization were used in this study. Details for the apparatus and the polymerization procedure can be found in Penlidis et al. (6,12,K3). Samples taken during the reaction were subsequently analyzed to follow conversion- and particle growth-time histories. The batch experimental runs were designed to yield similar conversion-time histories but different particle sizes. Conversion was measured both off-line, by gravimetric analysis, and on-line using an on-line densitometer (a U-tube DPR-YWE model with a Y-mode oscillator with a PTE-98 excitation cell and a DPR-2000 electronic board by Anton Paar, Austria). A number of runs were repeated to check for reproducibility of the results. Four batch runs are described in Table I below and their conversion histories are plotted in Figure 1. 

In principle, a continuous procedure can be used to construct the isotherm under quasi-equilibrium conditions the pure adsorptive is admitted (or removed) at a slow and constant rate and a volumetric or gravimetric technique used to follow the variation of the amount adsorbed with increase (or decrease) in pressure. A carrier gas technique, making use of conventional gas chromatrographic equipment, may be employed to measure the amount adsorbed provided that the adsorption of the carrier gas is negligible. In all types of measurement involving gas flow it is essential to confirm that the results are not affected by change in flow rate and to check the agreement with representative isotherms determined by a static method. 

The amount of adsorbed gas can be measured either gravimetrically or volumetrical-ly, and a variety of types of apparatus have been designed and used. A continuous procedure may also be used. In this method a nonadsorptive carrier gas, such as helium, is passed through the sample at a known rate with varying concentrations of nitrogen introduced in stages. The amount adsorbed can be determined by conventional gas chromatography. 

The time required to determine Icorr by Tafel extrapolation is approximately 3 h, which corresponds to the approximate time required for experimental setup and generation of a cathodic polarization curve at a commonly employed, slow scan rate of 600 mV/h. In comparison, a comparable gravimetric evaluation (mass-loss measurement) on a corrosion-resistant metal or alloy could take months, or longer. A limitation of the Tafel extrapolation method is the rather large potential excursion away from Ecorr, which tends to modify the WE surface, such that if the measurement is to be repeated, the sample should be re-prepared following initial procedures and again allowed to stabilize in the electrolyte until a steady-state Ecorr is reached. Consequently, the Tafel extrapolation method is not amenable to studies requiring faster, or even continuous, measurements of Icorr. 

Gravimetric measurements require corrections to account for buoyancy effects while volumetric measurements must be corrected for the sample volume. It is often not realised that these corrections are equivalent. Helium is used to determine both corrections since it is assumed not to be adsorbed. The degas procedure for gravimetric instruments involves continuous monitoring of mass in relation to time, which allows outgas rate to be specified and the equilibrium state and any sample mass losses... 

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