Chapter 2
Chemical 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.
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.
Desiccator
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.
Weighing bottles
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.
Filtering Crucible
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.
Decantation and transferring precipitate
Folding and seating a filter paper
Vacuum Filtration
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
Typical pipets
Automatic pipet
Burets and volumetric flask
Reading a buret
Using pipet
Titration
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
1.
2.
3.
4.
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
5. Never remove a page from the notebook
6. Do not overcrowd entries
7. Keep first few pages for table of contents
Lab note book
Safety