Elemental determinations, analytical reagents

The combination of oxidi2ing effect, acidic strength, and high solubiHty of salts makes perchloric acid a valuable analytical reagent. It is often employed in studies where the absence of complex ions must be ensured. The value of wet ashing techniques, in which perchloric acid is used to destroy organics prior to elemental analysis for the determination of trace metals in organics, has been well estabHshed (see Trace and residue analysis). 

Bromine is used as an analytical reagent to determine the amount of unsaturation in organic compounds because carbon—carbon double bonds add bromine quantitatively, and for phenols which add bromine in the ortho and para positions. Standard bromine is added in excess and the amount unreacted is deterrnined by an indirect iodine titration. Bromine is also used to oxidize several elements, such as T1(I) to T1(III). Excess bromine is removed by adding phenol. Bromine plus an acid, such as nitric and/or hydrochloric, provides an oxidizing acid mixture usefiil in dissolving metal or mineral samples prior to analysis for sulfur. 

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. ... 

Metal concentrations are determined using molecular spectrophotometric, atomic spectrometric, and electrochemical techniques. All of these require samples to be homogenous, or at least to contain the smallest possible amounts of organic matter that could interfere with the metal determination by interacting with the metal ions and the analytical reagents. Traditionally, decomposition of the sample in elemental analysis requires it to be mineralized in order to remove the organic content.1 Sample decomposition for total element determination therefore appears to be the recommended procedure on every occasion. 

Spot tests may prove ultimately to be the most, important example of determinations in which traces are major constituents. The technique is well known15 apd has proved very valuable in analytical chemistry. As often carried out, a reagent (specific if possible) is made to react in or on filter paper with the element sought, usually present as a trace. The results are normally qualitative or semiquantitative, it often being difficult to make them quantitative by methods other than x-ray emission spectrography.16 With this technique, however, not only is it possible... 

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. 

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