Gravimetric analyses, difficulties

Methods based on gravimetric analysis (Table 7.2) are also simple and rapid, but they suffer from the same limitations as those of infrared spectrometric methods (Table 7.2). Gravimetric-based methods may be useful for oily sludge and wastewaters, which will present analytical difficulties for other, more sensitive methods. Immunoassay methods for the measurement of total petroleum hydrocarbon are also popular for field testing because they offer a simple, quick technique for in situ quantification of the total petroleum hydrocarbons. 

The major difficulties involved in making electrically conductive thermoplastic blends using polyaniline, polypyrrole, or their composites, are two-fold. The first is the thermal instability of doped polyaniline and polypyrrole at melt processing temperatures (1-3), The second is the chemical incompatibility of acidic conductive polyaniline with acid sensitive polymers such as the nylons. Conductive polyaniline is quite acidic and the adjustment of its acidity to neutral pH values eliminates its high conductivity (4,5). The authors present here thermal aging studies of conductivity and thermal gravimetric analysis - mass spectroscopy (TGA-MS) of Eeonomers which show pH independence of conductivity in acidic to neutral environments. The tunable surface properties of Eeonomer composites allows one to optimize the processibility as well as the electrical and mechanical properties of their blends with various thermoplastics. 

Since all condensed phosphates are ultimately degraded to monophosphate in hot solution, especially at low pH, the total phosphorus(V) content of a substance may readily be determined after hydrolysis either gravimetrically or titrimetrically (109). However, as soon as it is a question of estimating the content of separate components in mixtures of condensed phosphates insuperable difficulties are encountered if methods depending on precipitation, titration, or a combination of the two are used. Even a quantitative precipitation of monophosphate is impossible if polyphosphates with chain length of n = 3 or more arc present in the solution. The precipitating cation and the compound to be precipitated by it are partly kept in solution by the polyphosphate part of the polyphosphate is also carried down by the precipitate. Both of these effects depend in their extent in different ways on the nature and quantity of the substances present and the analysis gives a correct quantitative result only in isolated instances... 

The preparation of standards for the analysis of VOCs does present difficulties for many laboratories. Firstly, the common use of many solvents in laboratories means that contamination and high blanks are real problems. The analytical techniques used are highly sensitive, standard solutions are usually prepared in methanol, and therefore absorption of atmospheric contaminants readily occurs. Secondly, the volatile nature of the components may make quantitative transfer susceptible to losses. For these reasons, and because of the large number of components involved, many laboratories prefer to buy in solutions of standard mixtures, which are usually prepared gravimetrically under clean conditions. However, laboratories need to check the accuracy of such solutions, usually by comparison to solutions from other suppliers and by means of proficiency schemes, and take care to minimise evaporative losses over the time-scale of use. 

The use of internal standards makes it unnecessary to analyse all of the components of a mixture when evaluating process parameters, as it is sufficient to establish relative concentrations of the components and the standard. However, the introduction of a standard into the mbcture being analysed by gravimetric techniques leads to serious difficulties in laboratory analyses, especially in the analysis of gases clearly, such a solution is inapplicable to automatic production control. The standard may be a compound inert to a given reaction and incorporated in both the raw materials and the end products. Naturally, such compounds can be found only for a limited number of processes. For example, nitrogen contained in the air used for oxidation has been used as a standard for the oxidation of butane in a fluidized bed of a catalyst [163]. 

The deterrnination of the soluble or available sodium aluminate presents difficulties because sodium aluminate begins to hydrolyze to the insoluble alumina trihydrate in water. The degree of hydrolysis depends on concentration, temperature, and time. It is therefore necessary to use a method of analysis that simultaneously affords control of the hydrolysis and gives the amount of available sodium aluminate encoimtered. This is best done by extracting the soluble alumina using sodium hydroxide solutions and subsequently deterniining the alumina content by gravimetric methods or titration with EDTA or hydrochloric acid (16). 

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