Solubility gravimetric method

Solubility Considerations An accurate precipitation gravimetric method requires that the precipitate s solubility be minimal. Many total analysis techniques can routinely be performed with an accuracy of better than 0.1%. To obtain this level of accuracy, the isolated precipitate must account for at least 99.9% of the analyte. By extending this requirement to 99.99% we ensure that accuracy is not limited by the precipitate s solubility. 

Gravimetric methods based on precipitation or volatilization reactions require that the analyte, or some other species in the sample, participate in a chemical reaction producing a change in physical state. For example, in direct precipitation gravimetry, a soluble analyte is converted to an insoluble form that precipitates from solution. In some situations, however, the analyte is already present in a form that may be readily separated from its liquid, gas, or solid matrix. When such a separation is possible, the analyte s mass can be directly determined with an appropriate balance. In this section the application of particulate gravimetry is briefly considered. 

A number of gravimetric methods, such as the determination of Ct in a soluble salt, have been part of the standard repertoire of experiments for introductory courses in analytical chemistry. Listed here are additional experiments that may be used to provide practical examples of gravimetry. 

Enzymatic Gravimetric Methods for TDF, SDF, and IDF. These methods use an a-amylase and protease to remove starch and reduce protein. They differ from each other in the conditions for gelatinization of starch. Elimination of detergent permits recovery of soluble fiber, which is not possible with the detergent methods. 

Solubility Gravimetric or volumetric methods Abramowitz and Yalkowsky [10],... 

Total dietary fiber (TDF) content of potato dry matter is determined according to the AACC (2000) method 32-05 following the total dietary fiber assay procedure (Megazyme k-TDFR 01/05). This is a gravimetric method that is simpler and faster than other analysis methods. In addition to total dietary fiber content, both soluble and insoluble dietary fiber content can be determined by this method. 

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. 

There are a number of well-documented, validated methods for the analysis of total fibre in food products in the AOAC manual (993.19, 993.23, 991.42, 991.43 and 958.29). Most of these are gravimetric methods where the fibre is precipitated with ethanol and washed and the fibre content calculated after allowances are made for any protein and/or ash that the precipitate contains. These methods rely on precipitation of the fibre with alcohol for their quantification, but as inulin is soluble in mixtures of ethanol and water it cannot be detected in these tests. The most recent method (2001.03) includes an HPLC stage to allow for detection of the presence of maltodextrins, which are poorly digested in the body, but this will not detect inulin. 

The use of ATR-FTIR spectroscopy is not a requirement for the determination of the metastable zone. If the goal is the determination of metastable zone or solubility curve alone, then there are less technically complicated methods, such as the gravimetric method for solubility measurement and observation by eye for detection of the metastable limit. Even for the automation of the system, ATR-FTIR spectroscopy is not a requirement. Automated determination... 

Fig. 1 Solubility of CO2 in several polymers (at 200°C, a gravimetric method). (From Ref 1)...

Table 13.1 lists the detection limits (King, 1984) of a number of spectroscopic and chromatographic analytical methods employed in supercritical extraction studies. A gravimetric method requires a few milligrams of material for analysis TLC analysis can detect materials at solubility levels one billion times lower If we are measuring the threshold pressure at which the compound dissolves at a detectable level, an analytical method that can identify 10 g will result in the identification of a lower threshold pressure than can be ascertained in the gravimetric method. The threshold pressure is therefore a function of the sensitivity of the analytical method. By itself, it is certainly not the critical criteria for determining the feasibility of a potential SCF process. Nevertheless, the work of Stahl provides us with a wealth of solubility information that we briefly consider here. 

In the literature, there are reports of work concerned with the measurement of oxoacidic properties of ionic melts by gravimetric measurements of the solubility of acidic gases in these media [76, 77], The solubility of sulfur(VI) oxide in molten sodium phosphates was determined by the gravimetric method [76], A correlation was obtained between the melt basicities and the solubility of gaseous acid S03 in them. Iwamoto reported the estimation of the basic properties of molten salt by measurements of acidic gas solubilities (carbon dioxide and water) in them [77], However, similar methods cannot be used widely, owing to two factors. The first of these consists in the fact that the solubility of any gas in a liquid phase obeys Henry s law. Let us consider the following system of reactions ... 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

The solubility of gases in solids imposes a serious limitation on the thermo-gravimetric method, as discussed by Guiochon (50). It is difficult to eliminate or even measure and is generally unknown. This was shown by the heating of solid ammonium nitrate initially containing I % nitric acid at 200°C for 3 h. At the end of this period, the sample contained 0.6% nitric acid. This acid has no catalytic effect on the decomposition of the sample, which gives no nitric acid under these conditions, so that only the slowness of its evaporation... 

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). 

The normal salts of several carboxylic acids such as oxalic, benzoic, and mandelic acids are insoluble and useful gravimetric methods were based on this property. It is also worth mentioning that sodium tetraphenylmetaborate is soluble, whereas its potassium salt is insoluble, and that selective gravimetric and titrimetric methods are thus possible. 

AOAC enzymatic gravimetric method The samples are dried, with fat extracted if necessary, gelatinized with heat-stable a-amylase and enzymatically digested with protease and amyloglucosidase to remove protein and starch. The soluble dietary fiber is then precipitated using ethanol, the residue filtered and washed, and corrections made for indigestible protein and ash. This method is widely used in the EC and the USA. 

A very accurate and reliable gravimetric method is to precipitate the phosphorus as quinolinium phosphomolybdate, (C9H7N)3PMOi204q in the presence of citric acid. This is done from a boiling acid solution of the sample, to which has been added a reagent solution of quinoline + molybdate -I- citric acid. The quinolinium compound is less soluble and after drying at 250°C has a more constant composition than the ammonium salt. 

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