Volumetric determinations, analytical reagents

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. 

Tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Gerold Schwarzenbach first recognized their potential as analytical reagents in 1945. Since this original work, investigators throughout the world have described applications of these compounds to the volumetric determination of most of the metals in the periodic table. 

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. 

Six 25 ml volumetric flasks are filled with 10 ml of the analyte and then 1, 2, 3, 4, 5 and 6 ml of a standard solution containing 6.5 x 10 3 moll-1 of the same analyte. 5.00 ml of color-developing reagent is added to each flask and enough distilled water is added to bring each flask to exactly 25.0 ml. The absorbances of the five solutions were 0.236, 0.339, 0.425, 0.548, 0.630 and 0.745, respectively. Use a spreadsheet to obtain a graph of the data and extrapolate the data to obtain the information needed to determine the initial concentration of the analyte. From the data, estimate the uncertainty of the result. 

A standard solution (or a standard titrant) is a reagent of known concentration that is used to carry out a titrimetric analysis. A titration is performed by slowly adding a standard solution from a buret or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete. The volume or mass of reagent needed to complete the titration is determined from the difference between the initial and final readings. A volumetric titration process is depicted in Figure 13-1. 

Coulometric titrations, like their volumetric counterparts, require a means for determining when the reaction between analyte and reagent is complete. Generally, the end points described in the chapters on volumetric methods are applicable to coulometric titrations as well. Thus, for the titration of iron(II) just described, an oxidation/reduction indicator, such as 1,10-phenanthroline, can be used as an alternative, the end point can be determined potentiometrically. Potentioinetric or... 

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. 

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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