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CHEMICALS,
APPARATUS, AND
UNIT OPERATIONS OF
ANALYTICAL
CHEMISTRY - 3
Reference:
Skoog, D.A, West, D.M., Holler, F.J., Crouch, S.R. 2014. Fundamentals
of Analytical Chemistry, 9th ed., Brookes/Cole Cengage Learning
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Filtration and Ignition of Solids
Simple crucibles
• Serve as containers
• Porcelain, Al2O3, silica, and Pt crucibles
• maintain a constant mass; used to convert a precipitate
into a suitable weighing form;
• solid is collected on filter paper;
• filter and contents transferred to a weighed crucible;
paper ignited.
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Filtration and Ignition of Solids
Simple crucibles
• Ni, Fe, Ag, and Au crucibles are as containers for hightemp. fusion of samples that are not soluble in aqueous
reagents
Filtering Crucibles
• Serve not only as containers but also as filters
• Vacuum is used to hasten filtration
• Tight seal between crucible and filtering flask is
accomplished with several types of rubber adaptors
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Filtration and
Ignition of Solids
• Collection of a
precipitate with a
filtering crucible is
frequently less
time consuming
than with paper.
Fig. 1. Adaptors for filtering crucibles.
(Source: Skoog et al. 2014, p. 28_
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Filtration and
Ignition of
Solids
• Sintered glass or fritted
glass crucibles
• Marked fine (f), medium
(m) and coarse (c)
• Made of quartz
• Temp limit is ~200oC; same
for crucibles w/ unglazed
porcelain or Al2O3 frits
• Now costly as quartz
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Filtration and Ignition of Solids
• Gooch Crucibles
• Has perforated bottom
that supports a fibrous mat
• Asbestos was used before
as matting fiber
• Glass mats have replaced
asbestos, can tolerate
temp >500oC, less
hygroscopic than asbestos.
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Filtration and Ignition of Solids
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Filter Paper
• An important filtering medium
• Ashless filter paper – from cellulose fibers that have been
treated with HCl and HF (to remove metallic impurities
and silica) and then neutralized with NH3
Note: residual NH3 may affect the analysis result for Nitrogen
by the Kjeldahl method.
• All filter papers tend to pick up moisture from the
atmosphere
• It is necessary to destroy the paper by ignition if the
precipitate that is collected on it is to be weighed
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Filter Paper
• Typically 9- or 11-cm circles of ashless paper leave a
residue that weighs less than 0.1 mg.
• Coarse-porosity ashless paper is most effective for filtering
gelatinous precipitates such as hydrous iron (III) oxide
Note: Problem of clogging the pores can be minimized by
mixing a dispersion of ashless filter paper with the
precipitate prior to filtration.
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Source: Skoog et al, 2014,p. 29
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Filtration and Ignition of Solids
• Heating Equipment
1. Drying oven – capable of maintaining a constant
temperature to within 1oC
• 110oC is satisfactory drying temperature
• Efficiency greatly increased by forced circulation of air;
passage of pre-dried air through an oven under partial
vacuum represents additional improvement
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Filtration and Ignition of Solids
2. Microwave Lab ovens
• Greatly shorten drying cycles, e.g., slurry samples
• Shorten heating time (12-16 hrs in conventional oven
requires only 5-6 minutes in the microwave oven)
3. Heat Lamp
• Dry precipitate collected on ashless paper and to char a
paper as well
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Filtration and Ignition of Solids
4. Burners
• Sources of intense heat
• Types: Meker provides the highest temperature, followed
by Tirrill and Bunsen burners
• Heavy Duty Electric (Muffle) Furnace
• Capable of maintaining controlled temps. Of 1100oC or
higher
• Long-handled tongs and heat-resistant gloves are needed
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Filtering and Igniting Precipitates
Preparation of Crucibles
• Crucible – used to convert a precipitate to a form suitable
for weighing; must maintain a constant mass throughout
drying or ignition
• Crucible is cleaned thoroughly and subjected to the same
regimen of heating and cooling as that required for the
precipitate.
• Heating-, cooling- and weighing-process is repeated until
constant mass has been achieved, i.e., until consecutive
weighings differ by ±0.3 mg
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Filtering and Igniting Precipitates
Filtering and Washing Precipitates
• Decantation – as much as supernatant liquid as possible is
passed through the filter while the precipitated solid is
kept essentially undisturbed in the beaker (precipitating
vessel) where it was formed
Speed the overall filtration rate by delaying the time at
which the pores of the filter become clogged with
precipitate
When flow ceases, the drop of liquid at the end of the
pouring spout is collected with the stirring rod and
returned to the beaker
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Filtering and Igniting Precipitates
Wash liquid is next added to the beaker and thoroughly
mixed with the precipitate
Solid is allowed to settle, and then this liquid is also
decanted through the filter.
Most washing should be carried out before the bulk of the
solid is transferred; results
This technique results in a more thoroughly washed
precipitate and a more rapid filtration.
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Filtering and Igniting Precipitates
The bulk of the precipitate is moved from beaker to filter
by directed streams of wash liquid where a stirring rod
provides direction for the flow of material to the filtering
medium
Last traces of precipitate that cling to the inside of the
beaker are dislodged with a rubber policeman, a small
section of rubber tubing that has been crimped on one
end and fitted onto the end of a stirring rod
Last traces of the gelatinous precipitates (e.g., hydrous
oxides) can be wiped with ashless filter and ignited
together with the precipitate.
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Filtering and Igniting Precipitates
Filters are never filled to more than ¾ of capacity to
prevent the possible loss of precipitate through creeping
(spreading of precipitate over a wetted surface against
the force of gravity)
Addition of a small amount of nonionic detergent (e.g.,
Triton X-100) to the supernatant liquid or wash liquid can
help minimize creeping.
Do not permit a gelatinous precipitate to dry until it has
been washed completely dry; further addition of wash
liquid (to the dry precipitate) simply pass through these
cracks and accomplish little or no washing.
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Filtering and Igniting Precipitates
a) Washing by
decantation
b) Transferring the
precipitate
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Filtering and Igniting Precipitates
Preparation of Filter Paper
a) Filter paper is folded exactly in half and crease it firmly
b) Fold the paper a second time
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Filtering and Igniting Precipitates
c. Tear off one of the corners on a line parallel to the
second fold.
d. Open the untorn half of the folded paper to form a
cone.
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Filtering and Igniting Precipitates
e. Seat the cone firmly into the funnel
f. Moisten the paper slightly and gently pat the paper into
place.
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Filtering and Igniting Precipitates
Transferring Paper and Ppt to Crucible
• Filter and contents must be transferred from the funnel to
a crucible that has been brought to constant mass
• Flatten the cone along its upper edge; corners are
flattened inward; the edge is then folded over; finally,
paper and contents are cased into the crucible, w/ bulk
of ppt is neat the bottom
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Filtering and
Igniting
Precipitates
Transferring a filter paper
and precipitate from a
funnel to a crucible
a. Pull the triple-thick
portion of the cone to
the opposite side of
the funnel.
b. Remove the filter cone
from the funnel, and
flatten the come along
its upper edge
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Filtering and
Igniting
Precipitates
c. Fold the corners
inward.
d. Fold the top edge of
the cone toward the
tip and enclose the
precipitate in the
paper
e. Gently ease the folded
paper and its contents
into the crucible.
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Filtering and Igniting Precipitates
Ashing Filter Papers
• Using heat lamp
Crucible is placed on a clean, nonreactive surface (e.g.,
such as wire screen covered with Al foil)
Lamp is then positioned about 1cm above the rim of the
crucible and turned on
The process is accelerated by the addition of not more
than a drop of concentrated NH4NO3 solution
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Filtering and Igniting Precipitates
Burner is used
Produces much higher temperatures than heat lamp
Mechanical loss of precipitate may occur if moisture is
expelled too rapidly in the initial stages of heating or
paper burns into flames
Partial reduction of some precipitates can occur through
reaction with the hot carbon of the charring paper
Reduction can be minimized by positioning the crucible in
a tilted position to allow for the easy access of air with the
cover be kept handy to extinguish any flame.
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Filtering and Igniting Precipitates
Ashing Filter Papers
• Heating should begin with a small flame
• Char the paper using moderate heat, with the crucible
slightly tilted, covering partially the mouth of the crucible
with the cover.
• The amount of smoke given off indicates the intensity of
heating that can be tolerated.
• A significant increase in smoke indicates that the paper is
about to flash, and that heating should be temporarily
discontinued.
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Filtering and Igniting Precipitates
Incorrect DRAFT COPY DO NOT DISTRIBUTE
correct
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Filtering and Igniting Precipitates
• Any flame should be immediately extinguished with a
crucible cover.
• The cover may be discolored from the condensation of
carbonaceous products.
• These products must be ultimately be removed from the
cover by ignition to confirm the absence of entrained
particles of precipitate.
• When no further smoking can be detected, heating is
increased to eliminate the residual carbon.
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Filtering and Igniting Precipitates
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Filtering and Igniting Precipitates
• Strong heating can then be undertaken.
• This sequence precedes the final ignition of a precipitate
in a muffle furnace
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Filtering and Igniting Ppts
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Filtering and Igniting
Precipitates
• Using Filtering Crucibles
• A vacuum filtration train is
used when a filtering crucible
can be used instead of paper.
• The trap isolates the filter flask
from the source of vacuum
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Rules for manipulating heated objects
1. Practice unfamiliar manipulations before putting them to
use.
2. Never place a heated object on the benchtop. Instead,
place it on a wire gauze or a heat-resistant ceramic
plate.
3. Allow a crucible that has been subjected to the full
flame of a burner or a muffle furnace to cool
momentarily before transferring it to the desiccator.
4. Keep the tongs and forceps used to handle heated
objects scrupulously clean. Do not allow the tips to touch
the benchtop.
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Assignment
1. Learn how to use Chem Draw application.
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