Weighing form

Table 11.26 Heating Temperatures, Composition of Weighing Forms, and Gravimetric Factors...

Element Thermal stability range, °C Final heating temperature, °C Composition of weighing form Gravimetric factors... 

Gravimetry Chemical reaction Precipitation Weighing form -Balance - - Result ... 

A noticeable difference occurring between the stoichiometric equivalence point of a reaction and the observed end-point. 4. Volatalization of weighing forms on ignition, and... 

During precipitation of metal cupferronates from mineral acid solutions, some nitrosophenylhydroxylamine is always coprecipitated. If attempts are made to wash out this water insoluble free acid with an organic solvent or ammonia, some metal is also lost and the remaining precipitate is often hydrolyzed. Consequently, cupferronates cannot be used as final weighing forms, and they serve only for purposes of precipitation and separation. These precipitates often have distinct colours which can form the basis of their estimation after extraction with suitable organic solvents.85... 

Reagent Conditions Heat treatment Weighing form Ref. 

Adding oxalate ions to a solution of lanthanide ions affords quantitative precipitation of the oxalate this is used in traditional quantitative analysis of lanthanides, as on ignition they are converted into the oxide, the weighing form. 

The oxalates are very insoluble, affording quantitative precipitation of the lanthanide, a fact made use of in traditional qnantitative analysis on ignition, they are obtained as the oxide, the weighing form. In the solid state, stmctures show eight-coordination in the hexahydrates and nine-coordination in the nonahydrates such as Nd2(C204)3-9H20. 

Calcium oxalate normally is precipitated as the monohydrate from hot solution. The thermolysis curve shows several plateaus, corresponding to the monohydrate from room temperature to 100°C, anhydrous calcium oxalate from 226 to 398°C, calcium carbonate from 420 to 660°C, and calcium oxide above 840 to 850°C. Sandell and Kolthoff concluded that the monohydrate is not a reliable weighing form because of its tendency to retain excess moisture. Coprecipitated ammonium oxalate also remains undecomposed, so the results are usually 0.5 to 1.0% too high when the precipitate is dried at 105 to 110°C. Anhydrous calcium oxalate also is unsuitable as a weighing form because of its hygroscopicity. 

Willard and Boldyreff concluded that calcium carbonate is an excellent weighing form if the oxalate is ignited at a temperature of 500 25 C. The necessity of this closeness of temperature control is evident from the following consideration. The minimum temperature is determined by the rate of the irreversible decomposition... 

One can easily summarize the requirements that should be fulfilled before the titanium-based ceramic materials can be obtained through an aqueous solution synthesis method. First, the precursor compound should possess good solubility in water, and preferably it should be stable over a wide pH range. In ideal case such compound should be a weighing form for titanium however, from the practical considerations it is sufficient to have a stock solution stable for a reasonably long period of time. Second, the reagent should be non-toxic, relatively cheap and its impact on the environment should be small. Its composition and chemistry should be simple and the reactions with other cations that will be introduced to the system must be well-predictable. The tendency to form precipitates with many cations, like in the case of oxalate ions, must be avoided. Finally, from an industrial point of view, the overall process should be cost effective and environmentally benign. 

Simple crucibles serve only as containers. Porcelain, aluminum oxide, silica, and platinum crucibles maintain constant mass—within the limits of experimental error—and are used principally to convert a precipitate into a suitable weighing form. The solid is first collected on a filter paper. The filter and contents are then transferred to a weighed crucible, and the paper is ignited. 

After filtration, a gravimetric precipitate is heated until its mass becomes constant. Heating removes the solvent and any volatile species carried down with the precipitate. Some precipitates are also ignited to decompose the solid and form a compound of known composition. This new compound is often called the weighing form. 

The temperature required to produce a suitable weighing form varies from precipitate to precipitate. Figure 12-6 shows mass loss as a function of temperature for several common analytical precipitates. These data were obtained with an automatic thermobalance, an instrument that records the mass of a substance continuously as its temperature is increased at a constant rate (Figure 12-7). Heating three of the precipitates—silver chloride, barium sulfate, and aluminum oxide—simply causes removal of water and perhaps volatile electrolytes. Note the vastly different temperatures required to produce an anhydrous precipitate of constant mass. Moisture is completely removed from silver chloride at temperatures higher than 110°C, but dehydration of aluminum oxide is not complete until a temperature greater than 1000°C is achieved. Aluminum oxide formed homogeneously with urea can be completely dehydrated at about 650°C. 

Boldface type indicates that gravimetric analysis is the preferred method for the element or ion. The weighed form is indicated in parenthe.ses. 

Weighing form In gravimetric analysis, the species collected whose mass is proportional to the amount of analyte in the sample. Weight The attraction between an object and its suiToundings, teiTestrially, the earth. 

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