Analytical Precipitates

The appearance of a plateau for a compound on a TG curve does not necessarily imply that the compound is isothermally stable, in either a thermodynamic or practical sense, at all or any temperatures that lie on that plateau. 

If the curve obtained for a multistage reaction has no intermediate portion in which the mass remains constant with time over a range of temperature, one can make the reasonable inference that the reactions leading to the formation and to the subsequent decomposition of the intermediate are not independently sequential, but overlap at least partly. 

In the absence of a true plateau, one cannot determine from a curve for successive reactions exact values for either the initial or final temperatures of the plateau (7 or 7 ), or the stoichiometric mass level, although a reasonable inference as to the latter can often be made. 

the transfer of drying or ignition temperatures from a TG curve plateau to isothermal measurements appears to be questionable, although it is a widely used practice. 

The vapor pressure or the sublimation behavior of organic compounds can by determined conveniently using thermogravimetry. Ashcroft (48), using the Langmuir equation 

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Analyte Precipitant Precipitate Formed Precipitate Weighed... 

In a back titration, a known excess of EDTA is added to the analyte. Excess EDTA is then titrated with a standard solution of a second metal ion. A back titration is necessary if analyte precipitates in the absence of EDTA, if it reacts too slowly with EDTA, or if it blocks the indicator. The metal ion for the back titration must not displace analyte from EDTA. 

Representative analytical precipitations are listed in Table 27-1. A few common organic precipitants (agents that cause precipitation) are listed in Table 27-2. Conditions must be controlled to selectively precipitate one species. Potentially interfering substances may need to be removed prior to analysis. 

If we wish to calculate the solubility of barium sulfate in a system containing hydronium and acetate ions, we must take into account not only the solubility equilibrium but also the other three equilibria. We find, however, that using four equilibrium-constant expressions to calculate solubility is much more difficult and complex than the simple procedure illustrated in Examples 9-4, 9-5, and 9-6. To solve this type of problem, the systematic approach described in Section llA is helpful. We then use this approach to illustrate the effect of pH and complex fonna-tion on the solubility of typical analytical precipitates. In later chapters, we use this same systematic method for solution of problems involving multiple equilibria of several types. 

Analytical precipitations are usually performed in buffered solutions in which the pH is fixed at some predetermined and known value. The calculation of solubility under this circumstance is illustrated by the following example. 

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. 

When the solubility product involved is not low enough for quantitative analyte precipitation, a high concentration of a chemical species with similar properties to the analyte is generally added, leading to analyte coprecipitation and quantitative analyte collection. 

Although preliminary separations may be required, in other instances the precipitation step in gravimetric analysis is sufficiently selective that other separations are not required. The pH is important because it often influences both the solubility of the analytical precipitate and the possibility of interferences from other substances. For example, calcium oxalate is insoluble in basic medium, but at low pH the oxalate ion combines with the hydrogen ions to form a weak acid. 8-Hydroxy-quinoline (oxine) can be used to precipitate a large number of elements, but by controlling pH, we can precipitate elements selectively. Aluminum ion can be precipitated at pH 4, but the concentration of the anion form of oxine is too low at this pH to precipitate magnesium ion. 

While all colloidal precipitates cause difficulties in analytical precipitates, some are worse than others. There are two types of colloids, hydrophilic and hydrophobic. Hydrophilic means water loving, and these colloids have a strong affinity for water. A solution of a hydrophilic colloid is therefore viscous. A hydro-phobic colloid has little attraction for water. A solution of this type of colloid is called a sol. 

Standard solutions - Prepare at least three standard solutions by combining appropriate volumes of stock standard solutions or intermediate standard solutions in volumetric flasks. Dilute to volume with 1 % nitric acid. Chemical compatibility (i.e., of analytes, acids, etc.) must be considered to avoid the formation of analyte precipitates when mixing single element stock solutions to prepare standard solutions. 

Analyte Precipitation - Certain substances such as colloids that precipitate from solution affect the nature of the sample. Similarly, if the concentration of a compound is near the saturation point, it can precipitate if the storage conditions differ greatly from those at sampling. In some environmental samples, elements may precipitate due to changes in their oxidation state or sulfide formation that may lead to the scavenging or coprecipitating of other elements in a sample. 

Mobile phases for argentation chromatography. It is necessary to add acetonitrile to the CO2 to facilitate elution of TG. Acetonitrile modifies the interactions between the analytes and the stationary phase and also improves the solubility of the analytes in the mobile phase. Although the solubility of TG in supercritical CO2 is, in general, good, the presence of a polar modifier may diminish the risk of analyte precipitation as the pressure is released in the restrictor. 

TG has been applied extensively to the study of analytical precipitates for gravimetric analysis [9]. One example is calcium oxalate, as illustrated in Figure 2. Information such as extent of hydration, appropriate drying conditions, stability ranges for intermediate products, and reaction mechanisms can all be deduced from appropriate TG curves. Figure 2 also includes the first derivative of the TG curve, termed the DTG cmve, which is capable of revealing fine details more clearly. 

The estimation of protein mass by centrifuging the precipitated protein in a calibrated tube and estimating its volume has been seriously proposed and used (185, 186, 187). This technique has been revived several times for use with common analytical precipitates and has been rather carefully studied. Its popularity has always been short lived because it has never been sufficiently easy to standardize properly the conditions of precipitating and centrifuging to make it truly quantitative. Variations of 20% between operators would be considered reasonable, though a single operator working with rather uniform samples... 

In this approach, the sample is initially mixed with a reagent and the analyte precipitates as an insoluble salt. The precipitate is accumulated, washed and then dissolved for introduction into the ICP-MS apparatus. Analytes are precipitated as either hydroxides or with organic reagents (e.g. ammonium pyrrolidinedithiocarbamate, APDC, sodium diethyldithiocarbamate, NaDDTC). A common problem of the precipitation and co-precipitation methods is that analytes are not quantitatively separated from the matrix. 

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