Disadvantage gravimetric

A gravimetric feeder rehes on weighing the material to achieve the required rate. Feed accuracies of 1/4% are obtainable using a properly designed gravimetric feeder. A disadvantage of such a feeder is that it is usually mote expensive than a volumetric device. 

Gravimetric Methods.—The mixture of pentoxides of niobium and tantalum is redissolved in concentrated hydrofluoric acid and separated by Marignac s process (see p. 128) the potassium tantalum fluoride and potassium niobium oxyfluoride are then separately converted into the pentoxides as described above, and -weighed.1 This method has several disadvantages (1) The ratio of the solubilities of the two compounds on which the separation is based is only approximately 10 1, and the process is, therefore, necessarily inaccurate even when the recrystaflisation is repeated to a tedious extent the error approaches 1 per cent. (2) The concentration of the hydrofluoric add and of the potassium fluoride must be carefully controlled if the acidity is too low, an oxyfluoride of tantalum is precipitated if the acidity is too great, a normal fluoride of niobium is obtained. (3) Several platinum dishes are necessary. 

The major disadvantage of this technique is the time required for the refluxing and for sample workup after the extraction to complete the gravimetric determinations. Because bitumen and heavy oil may sometimes contain asphaltenes or heavy organic components that may be in solid form in the oil phase, interpretation of the fine solids component requires other information about solids composition. The advantage of the technique is that the sample can be relatively large, an important consideration in many situations where sample streams are quite heterogeneous. 

There are several nonspecific methods available that can determine the total amount of solvent(s) in a sample. Loss on drying (LOD) determines the amount of volatile components that are released from a sample under specific temperature and/or vacuum conditions. Thermal gravimetric analysis (TGA) measures the loss of volatile components from a sample over a temperature gradient. The advantage of these methods is that they give an estimate of the volatile component content of a sample relatively quickly. The disadvantages of these methods are that they do not speciate and cannot account for volatile components that are trapped in the lattice structure of the compound. By accepting the limitations of these methods, a total solvent amount can be... 

Analysis by gravimetry is usually more labor-intensive than other methods but is still used because of high reproducibility, simplicity and low cost of the necessary equipment. The disadvantage of gravimetric analysis is its non-specificity. There are no reagents that precipitate only particular surfactants. 

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