Gravimetric determinations, analytical reagents

TABLE 11.20 Elements Precipitated by General Analytical Reagents This table includes the more common reagents used in gravimetric determinations. The lists of elements precipitated are not in all cases exhaustive. The usual solvent for a precipitating agent is indicated in parentheses after its name or formula. When the symbol of an element or radical is italicized, die element may be quantitatively determined by the use of the reagent in question. ... 

Our review on the use of oximes and hydroxamic acids in inorganic analytical chemistry showed that these reagents are/were most frequently used for gravimetric determinations, determinations based on complexation, spectrophotometric determinations and separations, while their use for column separations, as electrode sensors, as supporting electrolytes or compounds that enhance sensitivity of determination is less common. Additionally, it was noticed that the analytical chemistry of anions is less advanced than that of cations and for this reason this chapter was limited to analytical chemistry of metallic cations. 

Derivatives of quinoxaline-2-carboxylic acid have been tested for ant-helminitic activity, and quinoxaline-2-carboxylic acid and its 3-chloro and 3-0X0 derivatives have been used as analytical reagents in gravimetric analysis. Quinoxaline-2-carboxylic acid has also been used for the amperometric determination of Cu, Zn, Co, and Ni. ... 

All reagents and solvents that are used to prepare the sample for analysis should be ultrapure to prevent contamination of the sample with impurities. Plastic ware should be avoided since these materials may contain ultratrace elements that can be leached into the analyte solutions. Chemically cleaned glassware is recommended for all sample preparation procedures. Liquid samples can be analyzed directly or after dilution when the concentrations are too high. Remember, all analytical errors are multiplied by dilution factors therefore, using atomic spectroscopy to determine high concentrations of elements may be less accurate than classical gravimetric methods. 

Oximes, hydroxamic acids and related species are often used as reagents in inorganic analytical chemistry for precipitation, gravimetric and volumetric determinations as well as for preconcentration, extraction, derivatizations and subsequent determination of analyte using instrumental techniques. A brief review of analytical chemistry in general and of these species in particular follows. 

Two other types of analytical methods are based on mass. In gravimetric titrimetry, which is described in Section 13D, the mass of a reagent, of known concentration, required to react completely with the analyte provides the information needed to determine the analyte concentration. Atomic mass spectrometry uses a ma.ss. spectrometer to separate the gaseous ions formed from the elements making up a sample of matter. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the suiface of an ion detector. This technique is described briefly in Chapter 28. 

The concentration range of the analyte may well limit the number of feasible methods. If, for example, we wish to determine an element present at the parts-per-billion or parts-per-million level, gravimetric or volumetric methods can generally be eliminated, and spectrometric, potentiometric, and other more sensitive methods become likely candidates. For components in the parts-per-billion and parts-per-million range, even small losses resulting from coprecipitation or volatility and contamination from reagents and apparatus become major concerns. In contrast, if the analyte is a major component of the sample, these considerations are less important, and a classical analytical method may well be preferable. 

Once a sample is in solution, the solution conditions must be adjusted for the next stage of the analysis (separation or measurement step). For example, the pH may have to be adjusted, or a reagent added to react with and mask interference from other constituents. The analyte may have to be reacted with a reagent to convert it to a form suitable for measurement or separation. For example, a colored product may be formed that wUl be measured by spectrometry. Or the analyte will be converted to a form that can be volatilized for measurement by gas chromatography. The gravimetric analysis of iron as FeaOa requires that all the iron be present as iron(in), its usual form. A volumetric determination by reaction with dichromate ion, on the other hand, requires that all the iron be converted to iron(II) before reaction, and the reduction step will have to be included in the sample preparatioii. 

In the early years of chemistry, most analyses were carried out by separating the components of interest (the analytes) in a sample by precipitation, e.straction. or distillation. For qualitative analyse.s, the separated components were then treated with reagents that yielded products that could be reeogni/ed by their colors. their boiling or melting points, their solubilities in a series of solvents, their odors, their optical activities, or their refractive indexes. I or quantitative analyses, the amount of analyte was determined by gravimetric or by volumetric measurements. 

In gravimetric incasuremcnts. the mass of the analyte or some compound produced from the analyte was determined. In volumetric, also called mrimeiric. procedures, the volume or mass of a standard reagent required to react completely with the analyte was measured. 

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