Other gravimetric effects

Liquids evaporate in a completely open crucible over a wide temperature range below their boiling point. If the crucible lid has a small hole in it, a so-called self-generated atmosphere is created in which the vapour molecules remain in equilibrium with the liquid phase right 

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The gravimetric estimation of vanadium in alkaline vanadate solutions has also been effected by precipitating as ammonium metavanadate in the presence of ammonium chloride.6 Precipitation is incomplete, however, unless the solution is quite saturated with ammonium chloride 7 the addition of alcohol is recommended.8 Other gravimetric processes which have been investigated include the precipitation of barium pyrovanadate,9 precipitation of silver metavanadate,10 precipitation of manganese pyrovanadate,11 and the use of cupferron.12... 

If there is introduced into the solution from some other source an ion that is in common with an ion of the insoluble solid, the chemical equilibrium is shifted to the left, and the solubility of that solid will be greatly decreased from what it is in pure water. This is called the 11 common-ion effect." This effect is important in gravimetric analysis, where one wishes to precipitate essentially all of the ion being analyzed for, by adding an excess of the "common-ion" precipitating reagent. There is a practical limit to the excess, however, which involves such factors as purity of precipitate and possibility of complex formation. You can calculate the solubility under a variety of conditions, as illustrated in the following problem. 

Firstly, when relating (gravimetrically determined) equilibrium film composition to solution composition, activity effects must be taken into account. These effects may cause solvent and other neutral molecules, as well as the target species, to enter/leave the film. The importance of medium effects cannot be overemphasised here. A special case is a co-ordination model, where favourable interaction between polymer and target species results in saturation of the film except at very low concentration. The film mass is then independent of solution composition. This situation is likely for systems where a strong, specific polymer/analyte interaction has been synthetically designed into the polymer. 

Most adsorption data were collected by volumetric method until microbalance of high sensitivity appeared few years ago. It can hardly say which method is superior to the other, and both methods need the value of the skeleton volume of sample adsorbent. This volume has to be subtracted from the whole volume of the sample container to obtain the volume of void space, which is used for the calculation of the amount adsorbed. The skeleton volume of sample adsorbent was directly used in the calculation of buoyancy correction in gravimetric method. This volume was usually determined by helium assuming the amount of helium adsorbed was negligible. If, however, helium adsorption cannot be omitted, error would yield in the skeleton volume and, finally, in the calculated amount adsorbed. However, the effect of helium adsorption would be much less for volumetric method if the skeleton volume is considerably less than the volume of void space, but the volume of void space cannot affect buoyancy correction. In this respect, helium adsorption would result in less consequence on volumetric method especially when the skeleton volume was determined at room temperature and pressures less than IS MPa. The skeleton volume (or density) was taken for a parameter in modeling process in some gravimetric measurements. However, the true value of skeleton volume (or density) can hardly be more reliable basing on a fitted parameter than on a measured value. Therefore, one method of measurement cannot expel the other up to now, and the consequence of helium adsorption in the measured amount adsorbed should be estimated appropriately. 

To separate aluminum from plastics, gravimetric, densiometric or electrostatic means can be employed. Gravimetric methods have been less favored, mainly because separation depends upon particle size as well as density. It is quite likely that some metal particles will be lighter than the plastic if only because of Uieir small size. On the other hand, it will always be denser. One firm which has used this approach uses an eddy current separator combined with a fluidized bed to effect removal [iS]. 

There are many methods for metal determination (Standard Methods, 1998 section 3000). Some, for example as gravimetric, titrimetric or colorimetric methods, are most effective at high metal concentrations. Others, for example atomic absorption (AA), inductively coupled plasma (ICP) or inductively coupled plasma mass spectrometry (ICPMS) are far more sensitive. The latter are used for typical textile applications, such as compliance testing for water quality or detection of trace impurities in high-volume raw materials. 

The theoretical gravimetric density of the effective hydrogen is 9.6 mass%. The thermogravimetric analysis for prepared Ca(BH4)2 shows that the total mass loss up to 800 K is 9.2 mass%, which supports the overall reaction of Eq. 15.13. On the other hand, the in situ X-ray diffraction analysis indicated that the dehydrogenation reaction of Ca(BH4)2 is as follows ... 

When the correction of an odor requires the removal of large gravimetric quantities of parent substances, other methods of separation must usually be employed, although in many cases activated carbon may still fill a need for the removal of residual traces. In this connection it may be of interest to relate an experience in which the roles were reversed A small dose of activated carbon effectively removed a very pungent odor from a fungicide but left a mild unpleasant odor that was subsequently corrected by a masking agent. 

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