CHAPTER 18 CHEMICALS AND APPARATUS: PUTTING THE TOOLS TO WORK Introduction to Analytical Chemistry

CHAPTER 18 CHEMICALS AND APPARATUS:

PUTTING THE TOOLS TO WORK

Introduction toIntroduction toAnalytical ChemistryAnalytical Chemistry

Copyright © 2011 Cengage Learning18-2

18A-1 Classifying Chemicals

Reagent Grade Reagent-grade chemicals conform to the minimum

standards set forth by the Reagent Chemical Committee of the American Chemical Society.



Primary-Standard Grade National Institute of Standards and Technology is an

excellent source for primary standards. This agency also provides reference standards, which are complex substances that have been exhaustively analyzed.



Special-Purpose Reagent Chemicals Included among these are solvents for spectrophotometry

and high-performance liquid chromatography.


18D-1 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.

A typical microanalytical balance has a capacity of 1 to 3 g and a precision of ± 0.001 mg.


Figure 18-2

Figure 18-2 Electronic analytical balance. (From R. M. Schoonover, Anal. Chem., 1982, 54, 973A. Published 1982 American Chemical Society.)


18D-3 The Single-Pan Mechanical Analytical Balance

Figure 18-4 is a diagram of a typical single-pan mechanical balance.


18D-5 Sources of Error in Weighing

Correction for Buoyancy A buoyancy error will affect data if the density of the object

being weighed differs significantly from that of the standard weights.


Figure 18-5

Figure 18-5 Effect of buoyancy on weighing data (density of weights 8 g·cm3). Plot of relative error as a function of the density of the object weighted.



Temperature Effects Attempts to weigh an object whose temperature is different

from that of its surroundings will result in a significant error. Errors due to a difference in temperature have two sources.

First, convection currents within the balance case exert a buoyant effect on the pan and object.

Second, warm air trapped in a closed container weighs less than the same volume at a lower temperature.



Other Sources of Error A porcelain or glass object will occasionally acquire a static

charge that is sufficient to cause a balance to perform erratically.


Figure 18-7

Figure 18-7 Typical weighing bottles.


Figure 18-8

Figure 18-8 Components of a typical desiccator. The base contains a chemical drying agent, which is usually covered with a wire screen and a porcelain plate with holes to accommodate weighing bottles or crucibles.


18E-3 Manipulating Weighing Bottles

Figure 18-9 depicts the arrangement recommended for drying a sample. The weighing bottle is contained in a labeled beaker with a ribbed cover glass.


Figure 18-9

Figure 18-9 Arrangement for the dryingof samples.


Example 18-2

A 40.00-mL sample is taken from an aqueous solution at 5°C. What volume does it occupy at 20°C?


Figure 18-17

Figure 18-17 Typical pipets: (a) volumetric pipet, (b) Mohr pipet, (c) serological pipet, (d) Eppendorf micropipet, (e) Ostwald–Folin pipet, (f ) lambda pipet.


Figure 18-18

Figure 18-18 A handheld, batteryoperated, computer-controlled, motorized pipet. Courtesy of Rainin Instrument Co.Woburn, MA.


18H Calibrating Volumetric Ware

Volumetric glassware is calibrated by measuring the mass of a liquid (usually distilled water) of known density and temperature that is contained in the volumetric ware. In carrying out a calibration, a buoyancy correction must be made (Section 18D-4) since the density of water is quite different from that of the weights.


18I The Laboratory Notebook

The book should be permanently bound with consecutively numbered pages.


18I-1 Maintaining the Laboratory Notebook

1. Record all data and observations directly into the notebook in ink.

2. Supply each entry or series of entries with a heading or label.

3. Date each page of the notebook as it is used.4. Never attempt to erase or obliterate an incorrect

entry. Instead, cross it out5. Never remove a page from the notebook.


18J Using Spreadsheets in Analytical Chemistry

The spreadsheet is one of the most useful tools that has resulted from the personal computer revolution. Spreadsheets are used for record keeping, mathematical calculations, statistical analysis, curve fitting, data plotting, financial analysis, database management, and a variety of other tasks.


18K Safety in The Laboratory

Strict adherence to the rules will go far toward preventing (or minimizing the effect of ) accidents.


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CHAPTER 18 CHEMICALS AND APPARATUS: PUTTING THE TOOLS TO WORK Introduction to Analytical Chemistry