Analytical Chemistry Lecture Notes (PDF)

Lect-2 Chemicals, Apparatus, and Unit Operations of Analytical Chemistry  Chemicals, Apparatus, and Unit Operations of Analytical Chemistry At the heart of analytical chemistry is a core set of operations and equipment. This set is necessary for laboratory work in the discipline...

Lect-2 Chemicals, Apparatus, and Unit Operations of Analytical Chemistry  Chemicals, Apparatus, and Unit Operations of Analytical Chemistry At the heart of analytical chemistry is a core set of operations and equipment. This set is necessary for laboratory work in the discipline and serves as the foundation for its growth and development. In this chapter, we shall introduce the tools, techniques, and chemicals that are used by analytical chemists.  Outline Selecting and handling reagents and other chemicals, Cleaning and marking of laboratory ware, Evaporating liquids  Selecting and Handling Reagents 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.  Selecting and Handling Reagents and Other Chemicals 1. Classifying Chemicals a. Reagent Grade b. Primary Standard Grade c. Special Purpose Reagent Chemicals a. Reagent Grade  Reagent-grade chemicals conform to the minimum standards set forth by the Reagent Chemical Committee of the American Chemical Society (ACS) and are used whenever possible in analytical work.  Some suppliers label their products with the maximum limits of impurity allowed by the ACS specifications while others print actual concentrations for the various impurities  Selecting and Handling Reagents and Other Chemicals 1. Classifying Chemicals b. Primary Standard Grade  The primary standard chemicals require extraordinary pure qualities  Primary-standard reagents have been carefully analyzed by the supplier, and the results are printed on the container label.  The National Institute of Standards and Technology (NIST) is an excellent source for primary standards.  This agency also prepares and sells reference standards, which are complex substances that have been exhaustively analyzed  Selecting and Handling Reagents and Other Chemicals 1. Classifying Chemicals c. Special-Purpose Reagent Chemicals  Chemicals that have been prepared for a specific application  Included among these are solvents for spectrophotometry and high-performance liquid chromatography.  Information pertinent to the intended use is supplied with these reagents.  Selecting and Handling Reagents and Other Chemicals 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 usually be used with 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  Selecting and Handling Reagents and Other Chemicals 2. Rules for Handling Reagents and Solutions  Following are the rules to prevent the accidental contamination of reagents and solutions: i. Select the best grade of chemical available for analytical work. Whenever possible, pick the smallest bottle that is sufficient to do the job. ii. Replace the top of every container immediately after removing reagent. Do not rely on someone else to do so. iii. Hold the stoppers of reagent bottles between your fingers. Never set a stopper on a desk top.  Selecting and Handling Reagents and Other Chemicals 2. Rules for Handling Reagents and Solutions iv. Unless specifically directed otherwise, never return any excess reagent to a bottle. v. 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. vi. Keep the reagent shelf and the laboratory balance clean and neat. Clean up any spills immediately. vii. Follow local regulations concerning the disposal of surplus reagents and solutions  Cleaning and Marking of Laboratory Ware 1. Marking – A chemical analysis is usually performed in duplicate or triplicate. – 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 semi-permanent markings can be made with a pencil. – Special marking inks are available for porcelain surfaces.  Cleaning and Marking of Laboratory Ware 2. Cleaning – 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 large amounts of tap water and finally with several small portions of deionized water. – Properly cleaned glassware will be coated with a uniform and unbroken film of water. – Unless you are directed otherwise, do not dry the – interior surfaces of glassware or porcelain ware.  Evaporating Liquids It is often necessary to reduce the volume of a solution that contains a nonvolatile solute. Some unwanted substances can be eliminated during evaporation. Evaporation is frequently difficult to control because of the tendency of some solutions to overheat locally.  Evaporating Liquids Bumping is the sudden, often violent boiling that tends to spatter solution out of its container. It can cause partial loss of the solution. Careful and gentle heating will minimize the danger of such loss. Glass beads may also minimize bumping if their use is permissible Chemicals are used for evaporation. For example, chloride and nitrate can be removed from a solution by adding sulfuric acid and evaporating until copious white fumes of sulfur trioxide are observed (this opera tion must be performed in a hood)  Evaporating Liquids Organic constituents can frequently be eliminated from a solution by adding sulfuric acid and heating to the appearance of sulfur trioxide fumes (in a hood). This process is known as wet ashing. Wet ashing is the oxidation of the organic constituents of a sample with oxidizing reagents such as nitric acid, sulfuric acid, hydrogen peroxide, aqueous bromine, or a combination of these reagents THANKS