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Chapter 2 Chemical Apparatus, and Unit Operation of Analytical Chemistry. Classifying Chemicals
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Chapter 2Chemical Apparatus, and Unit Operation of Analytical Chemistry Classifying Chemicals 1. Reagent Grade: Reagent grade chemical conform to the minimum standards set forth by the Reagent Chemical committee of the American Chemical Society and are used wherever possible in analytical work. 2. Primary Standard Grade: Extraordinary purity is required for a primary standard. Primary standard reagent is carefully analyzed and the assay is printed on the container label. 3. Special-Purpose Reagent: chemicals that have been prepared for a specific application. Included among these are solvents for spectrophotometry and high-performance liquid chromatography. 3. Special-Purpose Reagent: chemicals that have been prepared for a specific application. Included among these are solvents for spectrophotometry and high-performance liquid chromatography. 2. Primary standard Grade: Extraordinary purity is required for a primary standard. Primary standard reagent is carefully analyzed and the assay is printed on the container label.
Rules for Handling Reagents and Solutions 1. Select the best grade of chemical available for analytical work. 2. Replace the top of every container immediately after removal of the reagent. 3. Hold the stoppers of reagent bottles between your fingers. 4. Never return any excess reagent to a bottle. 5. Never insert spatulas, spoons, or knives into a bottle that contains a solid chemicals. 6. Keep the reagent shelf and the laboratory balance clean and neat. 7. Observe regulations concerning the disposal of surplus reagents and solutions.
Cleaning and Marking Laboratory Ware 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 tap water and finally with several small portions of deionized water. Organic solvents such as benzene or acetone may be used to remove grease films. A chemical analysis is ordinarily performed in duplicate or triplicate. Each vessel that holds a sample must be marked so that its content can be positively identified. Flask, beaker and some crucibles have small etched areas on which semi permanent mark can be made with a pencil.
Types of Analytical Balances An analytical balance is a weighing instrument with a maximum capacity that ranges from 1 g to a few kilograms with a precision of at least 1 part in 105 at maximum capacity. Macrobalances have a maximum capacity ranging between 160 and 200 g; measurement can be made with a standard deviation of 0.1mg. Semimicroanalytical balances have a maximum load of 10 to 30 g with a precision of 0.01mg. Microanalytical balance has a capacity of 1 to 3 g and a precision of 0.001mg.
Desiccators and Desiccants Oven drying is the most common way of removing moisture from solids. This approach is not appropriate for substances that decompose or for those from which water is not removed at the temperature of the oven. Dried material are stored in desiccator while they cool so as to minimize the uptake of moisture. The base section of the desiccator contains a chemical drying agent (desiccants) such as anhydrous calcium chloride, calcium sulfate, magnesium perchlorate or phosphorus pentoxide.
Weighing by Difference Weighing by difference is a simple method for determining a series of sample weights. First the bottle and its contents are weighed. One sample is then transferred from the bottle to a container; gentle tapping of the bottle with its top and slight rotation of the bottle control over the amount of sample removed. Following transfer, and its residual contents are weighed. The mass of the sample is the difference between the two weighings.
Simple Crucibles Simple crucibles serve only as containers. Porcelain, aluminum oxide, silica and platinum crucibles maintain constant mass and are used principally to convert a precipitate into a suitable weighing form. The solid is first collected on filter paper. The filter and contents are then transferred to a weighed crucible, and the paper is ignited. Filtering Crucibles Filtering crucibles serve not only as containers but also as filters. A vacuum is used to hasten the filtration, a tight seal between crucible and filtering flask is accomplished with any of the several types of rubber adapters.
Sintered-glass Crucibles Sintered-glass crucibles are manufactured in fine, medium, and coarse porosities. The upper temperature limit for sintered glass crucible is ordinarily about 200oC. Filtering crucibles made entirely of quartz can tolerate substantially higher temperatures. Filter Paper Paper is an important filtering medium. Ashless paper is manufactured from cellulose fibers that have been treated with hydrochloric and hydrofluoric acids to remove metallic impurities and silica, ammonia is then used to neutralize the acids. It is necessary to destroy the paper by ignition if the precipitate collected on it is to be weighed.
Heating Equipment Many precipitate can be weighed directly after being brought to constant mass in a low temperature drying oven. Such an oven is electrically heated and capable of maintaining a constant temperature to within 1oC. The maximum attainable temperature ranges from 140 to 260oC, depending on make and model, for many precipitate 110oC is a satisfactory drying temperature. Microwave laboratory ovens are currently appearing on the market. Where applicable, these greatly shorten drying cycles. Muffle furnace (a heavy duty electric furnace) is capable of maintaining controlled temperatures of 1100oC or higher. Long handled tongs and heat resistance gloves are needed for protection.
Volume Measurement • Pipets • Burets • Volumetric flask • Measuring cylinder
Calibrating Glassware Volumetric glassware is calibrated by measuring the mass of a liquid (water) of known density and temperature that is contained in the volumetric ware.
Laboratory Notebook • Record all data and observations • Supply each entry with a heading • Date each page of the notebook • Never attempt to erase an incorrect entry, cross it out with single horizontal line • Never remove a page from the notebook • Do not overcrowd entries • Keep first few pages for table of contents
