Skoog-FAC, 8e Part 1-42-43 PDF

2A Selecting and Handling Reagents and Other Chemicals 21 Mastery of the tools of analytical chemistry will serve you well in chemistry courses and in related scientific fields. In addition, your ef...

2A Selecting and Handling Reagents and Other Chemicals 21 Mastery of the tools of analytical chemistry will serve you well in chemistry courses and in related scientific fields. In addition, your efforts will be rewarded with the considerable satisfaction of having completed an analysis with high stan- dards of good analytical practice and with levels of accuracy and precision consis- tent with the limitations of the technique. SELECTING AND HANDLING REAGENTS I2AJ AND OTHER CHEMICALS The purity of reagents has an important bearing on the accuracy attained in any analysis. It is therefore essential that the quality of a reagent be consistent with its intended use. 2A-l Classifying Chemicals Reagent Grade Reagent-grade chemicals conform to the mirurnum standards set forth by the Reagent Chemical Committee of the American Chemical Society (ACS)! and are used wherever possible in analytical work. Some suppliers label their products with the maximum limits of impurity allowed by the ACS specifications; others print actual concentrations for the various impurities. Primary-Standard Grade The qualities required of a primary standard, in addition to extraordinary purity, are discussed in Section 13A-2. Primary-standard reagents have been carefully analyzed by the supplier, and the assay is printed on the container label. The.. The National Institute of Standards National Institute of Standards and Technology is an excellent source for primary and Technology (NIST) is the current standards. This agency also provides reference standards, which are complex name of what was formerly the substances that have been exhaustively analyzed.? National Bureau of Standards. Special-Purpose Reagent Chemicals Chemicals that have been prepared for a specific application are also available. Included among these are solvents for spectrophotometry and high-performance liq- uid chromatography. Information pertinent to the intended use is supplied with these reagents. Data provided with a spectrophotometric solvent, for example, might include its absorbance at selected wavelengths and its ultraviolet cutoff wavelength. 2A-2 Rules for Handling Reagents and Solutions A high-quality chemical analysis requires reagents and solutions of known purity. A freshly opened bottle of a reagent-grade chemical can ordinarily be used with 'Committee on Analytical Reagents, Reagent Chemicals, 9th ed. Washington, DC: American Chemical Society, 2000. 2The Standard Reference Materials Program (SRMP) of the NIST provides thousands of reference materials for sale. The NIST maintains a catalog and price list of these materials at a Web site that is linked to the main NIST Web site at www.nist.gov. Standard reference materials may be purchased online. 22 CHAPTER 2 The Chemicals, Apparatus, and Unit Operations of Analytical Chemistry confidence; whether this same confidence is justified when the bottle is half empty depends entirely on the way it has been handled after being opened. The following rules should be observed to prevent the accidental contamination of reagents and solutions. 1. Select the best grade of chemical available for analytical work. Whenever pos- sible, pick the smallest bottle that will supply the desired quantity. 2. Replace the top of every container immediately after removal of the reagent; do not rely on someone else to do this. 3. Hold the stoppers of reagent bottles between your fingers; never set a stopper on a desk top. 4. Unless specifically directed otherwise, never return any excess reagent to a bot- tle. The money saved by returning excesses is seldom worth the risk of contam- inating the entire bottle. 5. Unless directed otherwise, never insert spatulas, spoons, or knives into a bottle that contains a solid chemical. Instead, shake the capped bottle vigorously or tap it gently against a wooden table to break up an encrustation; then pour out the desired quantity. These measures are occasionally ineffective, and in such cases a clean porcelain spoon should be used. 6. Keep the reagent shelf and the laboratory balance clean and neat. Clean up any spills immediately, even though someone else is waiting to use the same chem- ical or reagent. 7. Observe local regulations concerning the disposal of surplus reagents and solu- tions. CLEANING AND MARKING OF LABORATORY WARE A chemical analysis is ordinarily performed in duplicate or triplicate. Thus, each vessel that holds a sample must be marked so that its contents can be positively identified. Flasks, beakers, and some crucibles have small etched areas on which semipermanent markings can be made with a pencil. Special marking inks are available for porcelain surfaces. The marking is baked permanently into the glaze by heating at a high temperature. A saturated solution of iron (Ill) chloride, although not as satisfactory as the commercial preparation, can also be used for marking. Every beaker, flask, or crucible that will contain the sample must be thoroughly cleaned before being used. The apparatus should be washed with a hot detergent solution and then rinsed-initially with copious amounts of tap water and finally with several small portions of deionized water.' Properly cleaned glassware will be ~ Unless you are directed otherwise, coated with a uniform and unbroken film of water. It is seldom necessary to dry the do not dry the interior surfaces of glass- interior surface of glassware before use; drying is ordinarily a waste of time at best ware or porcelain ware. and a potential source of contamination at worst. An organic solvent, such as benzene or acetone, may be effective in removing grease films. Chemical suppliers also market preparations for eliminating such films. 3References to deionized water in this chapter and Chapter 37 apply equally to distilled water.