4) Chemicals, Apparatus, and Unit Operations of Analytical
Chemistry
Introduction
At the heart of analytical chemistry is a core set of operations and equipment that is necessary for laboratory
work in the discipline and that serves as the foundation for its growth and development.
In this section, the tools, techniques, and chemicals that are used by analytical chemists are introduced. The
development of these tools began over two centuries ago and continues today.
As the technology of analytical chemistry has improved with the advent of electronic analytical balances,
automated titrators, and computer-controlled instruments, the speed, convenience, accuracy and precision
of analytical methods have generally improved as well. For example, the determination of the mass of a
sample that required 5 to 10 minutes 40 years ago is now accomplished in a few seconds.
Computations that took 10 to 20 minutes using tables of logarithms may now be carried out almost
instantaneously with a computer spreadsheet. Our experience with such magnificent technological
innovations often elicits impatience with the sometimes tedious techniques of classical analytical chemistry.
It is this impatience that drives the quest to develop better methodologies. Indeed, basic methods have often
been modified in the interest of speed or convenience, without sacrificing accuracy or precision.
We must emphasize however, that many of the unit operations encountered in the analytical laboratory are
timeless. These tried and true operations have gradually evolved over the past two centuries.
Although attempt is made to explain why unit operations are carried out in the way that we describe, it is
often allowed to modify a procedure or skip a step here or there to save time and effort.
However as an Analytical Chemistry student you cautioned against modifying techniques and procedures
unless you have discussed your proposed modification with your instructor and have considered its
consequences carefully
➢ Such modifications may cause unanticipated results, including unacceptable levels of accuracy or
precision; in a worst-case scenario, a serious accident could result.
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 standards of good analytical practice and with levels of accuracy and
precision consistent with the limitations of the technique.
1) 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.
Classifying Chemicals
Reagent Grade
➢ Reagent-grade chemicals conform to the minimum standards set forth by the Reagent Chemical Committee
of the American Chemical Society (ACS), Reagent grade chemicals 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
1
,➢ Primary-standard reagents are reagents that have been carefully analyzed by the supplier, and the assay is
printed on the container label. There are several suppliers of primary-standard grade reagents across the
world. The National Institute of Standards and Technology (USA) is an excellent example of sources for
primary standards. This agency also provides reference standards, which are complex substances that have
been exhaustively analyzed.
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 liquid 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.
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 confidence. However, 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 possible, 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 bottle. The money saved by
returning excesses is seldom worth the risk of contaminating 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 chemical or reagent.
7. Observe local regulations concerning the disposal of surplus reagents and solutions.
2) 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 semi-permanent 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 (III) 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.
2
, ➢ Properly cleaned glassware will be coated with a uniform and unbroken film of water. It is seldom necessary
to dry the interior surface of glassware before use: drying is ordinarily a waste of time at best 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.
Evaporating Liquids
➢ It is frequently necessary to decrease the volume of a solution that contains a non- volatile solute. The figure
below illustrates how this is done.
Figure: Arrangement for the evaporation of a liquid.
➢ The ribbed cover glass permits vapors to escape and protects the remaining solution from accidental
contamination. Using glass hooks to provide space between the rim of the beaker and a conventional cover
glass is less satisfactory than using the ribbed cover glass shown.
➢ Evaporation is frequently difficult to control because of the tendency of some solutions to overheat locally.
The bumping that results can be sufficiently vigorous to cause partial loss of the solution. Careful and gentle
heating will minimize the danger of such loss. Where their use is permissible, glass beads will also minimize
bumping.
• Bumping is sudden, often violent boiling that tends to spatter solution out of its container.
➢ Some unwanted species can be eliminated during 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 operation must be performed in a hood).
✓ Urea is effective in removing nitrate ion and nitrogen oxides from acidic solutions. Ammonium
chloride is best removed by adding concentrated nitric acid and evaporating the solution to a small
volume. Ammonium ion is rapidly oxidized when it is heated; the solution is then evaporated to
dryness.
3
Chemistry
Introduction
At the heart of analytical chemistry is a core set of operations and equipment that is necessary for laboratory
work in the discipline and that serves as the foundation for its growth and development.
In this section, the tools, techniques, and chemicals that are used by analytical chemists are introduced. The
development of these tools began over two centuries ago and continues today.
As the technology of analytical chemistry has improved with the advent of electronic analytical balances,
automated titrators, and computer-controlled instruments, the speed, convenience, accuracy and precision
of analytical methods have generally improved as well. For example, the determination of the mass of a
sample that required 5 to 10 minutes 40 years ago is now accomplished in a few seconds.
Computations that took 10 to 20 minutes using tables of logarithms may now be carried out almost
instantaneously with a computer spreadsheet. Our experience with such magnificent technological
innovations often elicits impatience with the sometimes tedious techniques of classical analytical chemistry.
It is this impatience that drives the quest to develop better methodologies. Indeed, basic methods have often
been modified in the interest of speed or convenience, without sacrificing accuracy or precision.
We must emphasize however, that many of the unit operations encountered in the analytical laboratory are
timeless. These tried and true operations have gradually evolved over the past two centuries.
Although attempt is made to explain why unit operations are carried out in the way that we describe, it is
often allowed to modify a procedure or skip a step here or there to save time and effort.
However as an Analytical Chemistry student you cautioned against modifying techniques and procedures
unless you have discussed your proposed modification with your instructor and have considered its
consequences carefully
➢ Such modifications may cause unanticipated results, including unacceptable levels of accuracy or
precision; in a worst-case scenario, a serious accident could result.
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 standards of good analytical practice and with levels of accuracy and
precision consistent with the limitations of the technique.
1) 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.
Classifying Chemicals
Reagent Grade
➢ Reagent-grade chemicals conform to the minimum standards set forth by the Reagent Chemical Committee
of the American Chemical Society (ACS), Reagent grade chemicals 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
1
,➢ Primary-standard reagents are reagents that have been carefully analyzed by the supplier, and the assay is
printed on the container label. There are several suppliers of primary-standard grade reagents across the
world. The National Institute of Standards and Technology (USA) is an excellent example of sources for
primary standards. This agency also provides reference standards, which are complex substances that have
been exhaustively analyzed.
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 liquid 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.
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 confidence. However, 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 possible, 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 bottle. The money saved by
returning excesses is seldom worth the risk of contaminating 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 chemical or reagent.
7. Observe local regulations concerning the disposal of surplus reagents and solutions.
2) 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 semi-permanent 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 (III) 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.
2
, ➢ Properly cleaned glassware will be coated with a uniform and unbroken film of water. It is seldom necessary
to dry the interior surface of glassware before use: drying is ordinarily a waste of time at best 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.
Evaporating Liquids
➢ It is frequently necessary to decrease the volume of a solution that contains a non- volatile solute. The figure
below illustrates how this is done.
Figure: Arrangement for the evaporation of a liquid.
➢ The ribbed cover glass permits vapors to escape and protects the remaining solution from accidental
contamination. Using glass hooks to provide space between the rim of the beaker and a conventional cover
glass is less satisfactory than using the ribbed cover glass shown.
➢ Evaporation is frequently difficult to control because of the tendency of some solutions to overheat locally.
The bumping that results can be sufficiently vigorous to cause partial loss of the solution. Careful and gentle
heating will minimize the danger of such loss. Where their use is permissible, glass beads will also minimize
bumping.
• Bumping is sudden, often violent boiling that tends to spatter solution out of its container.
➢ Some unwanted species can be eliminated during 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 operation must be performed in a hood).
✓ Urea is effective in removing nitrate ion and nitrogen oxides from acidic solutions. Ammonium
chloride is best removed by adding concentrated nitric acid and evaporating the solution to a small
volume. Ammonium ion is rapidly oxidized when it is heated; the solution is then evaporated to
dryness.
3
