Preparing Samples
for Analysis
Chapter 36
The particle size of laboratory samples is often reduced prior to analysis by crushing and
grinding operations. The techniques used in the laboratory are similar to those used in largescale operations, such as the V-mixer/grinder used in a uranium plant shown in the photo.
A V-mixer for laboratory use is described in Section 36A. In addition, this chapter considers
several other methods of preparing samples for analysis, including various pulverizing and
mixing methods. The chapter also considers the forms that moisture takes in solid samples
and the methods of drying these samples.
I
Carl Iwasaki/Time Life Pictures/Getty Images
n Section 8B, we considered the statistics of sampling and sample handling. In this chapter,
we discuss some of the details of preparing laboratory samples. In addition, the influence of
moisture on samples and the determination of water in samples are explored.
36A Preparing Laboratory Samples
In Section 8B-4, we presented the statistical considerations for reducing the particle
size of the gross sample so as to obtain a laboratory sample. In the following section,
some of the specific techniques used are described.
Crushing and grinding the
sample often change its
composition.
36A-1 Crushing and Grinding Samples
❯
A certain amount of crushing and grinding is usually required to decrease the particle
size of solid samples. Because these operations tend to alter the composition of the
sample, the particle size should be reduced no more than is required for homogeneity
(see Section 8B-4) and ready attack by reagents.
Several factors can cause significant changes in sample composition as a result of
grinding. The heat inevitably generated can cause losses of volatile components. In
addition, grinding increases the surface area of the solid and thus increases its susceptibility to reaction with the atmosphere. For example, it has been observed that the
iron(II) content of a rock may be decreased by as much as 40% during grinding—
apparently a direct result of the iron being oxidized to the 13 state.
Frequently, the water content of a sample is altered substantially during grinding.
Increases are observed as a consequence of the increased specific surface area that
accompanies a decrease in particle size (page 287). The increased surface area leads
to greater amounts of adsorbed water. For example, the water content of a piece of
porcelain changed from 0 to 0.6% when it was ground to a fine powder.
In contrast, decreases in the water content of hydrates often take place during
grinding as a result of localized frictional heating. For example, the water content of
36A Preparing Laboratory Samples 971
gypsum (CaSO4 · 2H2O) decreased from about 21 to 5% when the compound was
ground to a fine powder.
Differences in hardness of the component can also introduce errors during crushing
and grinding. Softer materials are ground to fine particles more rapidly than are hard
ones and may be lost as dust as the grinding proceeds. In addition, flying fragments
tend to contain a higher fraction of the harder components.
Intermittent screening often increases the efficiency of grinding. Screening involves
shaking the ground sample on a wire or cloth sieve that will pass particles of a desired
size. The residual particles are then returned for further grinding; the operation is
repeated until the entire sample passes through the screen. The hardest materials,
which often differ in composition from the bulk of the sample, are last to be reduced
in particle size and are thus last through the screen. Therefore, grinding must be
continued until every particle has been passed if the screened sample is to have the
same composition as it had before grinding and screening.
A serious contamination error can arise during grinding and crushing due to
mechanical wear and abrasion of the grinding surfaces. Even though these surfaces
are fabricated from hardened steel, agate, or boron carbide, contamination of the
sample is nevertheless occasionally encountered. The problem is particularly acute
in analyses for minor constituents.
and grinding must
❮ Crushing
be continued until the entire
sample passes through a screen
of the desired mesh size.
abrasion of the
❮ Mechanical
surfaces of the grinding device
can contaminate the sample.
Courtesy of Spex SamplePrep, Inc.
Tools for Reducing Size
Several different tools can be used for reducing the particle size of solids, including jaw
crushers and disk pulverizers for large samples containing large lumps, ball mills for
medium-sized samples and particles, and various types of mortars for small amounts
of material.
The ball mill is a useful device for grinding solids that are not too hard. It consists
of a porcelain crock with a capacity of perhaps two liters that can be sealed and rotated
mechanically. The container is charged with approximately equal volumes of the sample and flint or porcelain balls with a diameter of 20 to 50 mm. Grinding and crushing
occur as the balls tumble in the rotating container. A finely ground and well-mixed
powder can be produced in this way.
A commercial laboratory mixer/mill is shown in Figure 36-1 along with several
stainless steel mixing vials. The unit has motion along three axes for vigorous grinding of
samples. Two or three vials can be accommodated simultaneously. The Plattner diamond
mortar, shown in Figure 36-2, is used for crushing hard, brittle materials. It is constructed
of hardened tool steel and consists of a base plate, a removable collar, and a pestle. The
sample is placed on the base plate inside the collar. The pestle is then fitted into place and
struck several blows with a hammer, therefore reducing the solid to a fine powder that is
collected on glazed paper after the apparatus has been disassembled.
Figure 36-1 A commercial mixer/
mill for pulverizing and blending
samples. The unit in the back can hold
two 0.75sor three 0.5s sample vials.
Plastic vials or the stainless steel vials in
front can be used along with plastic or
stainless steel mixing balls.
972 Chapter 36
Preparing Samples for Analysis
36A-2 Mixing Solid Samples
Figure 36-2 A Plattner diamond
mortar.
Finely ground materials may
segregate after standing for a
long time.
❯
It is essential that solid materials be thoroughly mixed to ensure random distribution
of the components in the analytical samples. A common method for mixing powders involves rolling the sample on a sheet of glazed paper. A pile of the substance is
placed in the center and mixed by lifting one corner of the paper enough to roll the
particles of the sample to the opposite corner. This operation is repeated many times,
with the four corners of the sheet being lifted alternately.
Effective mixing of solids is also accomplished by rotating the sample for some
time in a ball mill or a twin-shell V-blender. The latter consists of two connected cylinders that form a V-shaped container for the sample. As the blender is rotated, the
sample is split and recombined with each rotation, leading to highly efficient mixing.
It is worthwhile noting that long-standing, finely ground homogeneous materials
may segregate on the basis of particle size and density. For example, analyses of
layers of a set of student unknowns that had not been used for several years revealed a
regular variation in the analyte concentration from top to bottom of the container.
Apparently, segregation occurred as a consequence of vibrations and of density differences in the sample components.
36B Moisture in Samples
Laboratory samples of solids often contain water that is in equilibrium with the atmosphere. As a consequence, unless special precautions are taken, the composition
of the sample depends on the relative humidity and ambient temperature at the time
it is analyzed. To cope with this variability in composition, it is common practice to
remove moisture from solid samples prior to weighing or, if this removal is not possible, to bring the water content to some reproducible level that can be duplicated
later if necessary. Traditionally, drying was accomplished by heating the sample in a
conventional oven or a vacuum oven or by storing in a desiccator at a fixed humidity.
The drying processes were continued until the material had become constant in
mass. These drying treatments were time consuming, often requiring several hours
or even several days. In order to speed up sample drying, microwave ovens or infrared lamps are currently being used for sample preparation.1 Several companies now
offer equipment for this type of sample treatment (see Section 37C).
An alternative to drying samples before beginning an analysis involves determining
the water content at the time the samples are weighed for analysis so that the results can
be corrected to a dry basis. In any event, many analyses are preceded by some sort of
preliminary treatment designed to take into account the presence of water.
36B-1 Forms of Water in Solids
Water can be essential or nonessential water in solids.
Essential water is the water that is
an integral part of a solid chemical
compound in a stoichiometric
amount in a stable solid hydrate,
such as BaCl2 · 2H2O.
Essential Water
Essential water forms an integral part of the molecular or crystalline structure of a
compound in its solid state. Therefore, the water of crystallization in a stable solid
hydrate (for example, CaC2O4 · 2H2O and BaCl2 · 2H2O) qualifies as a type of
­essential water. Water of constitution is a second type of essential water and is found
1
For a comparison of the reproducibility of these various methods of drying, see E. S. Berry,
Anal. Chem., 1988, 60, 742, DOI: 10.1021/ac00159a003.
Unless otherwise noted, all content on this page is © Cengage Learning.
36B Moisture in Samples 973
in compounds that yield stoichiometric amounts of water when heated or otherwise
decomposed. Examples of this type of water are found in potassium hydrogen sulfate
and calcium hydroxide, which when heated come to equilibrium with the moisture in
the atmosphere, as shown by the reactions
Water of constitution is water that
is formed when a pure solid is
decomposed by heat or other
chemical treatment.
2KHSO4(s) 8 K2S2O7(s) 1 H2O(g)
Ca(OH)2(s) 8 CaO(s) 1 H2O(g)
Nonessential Water
Nonessential water is retained by the solid as a consequence of physical forces. It
is not necessary for characterization of the chemical constitution of the sample and,
therefore, does not occur in any sort of stoichiometric proportion.
Adsorbed water is a type of nonessential water that is retained on the surface of
solids. The amount adsorbed is dependent on humidity, temperature, and the specific surface area of the solid. Adsorption of water occurs to some degree on all solids.
A second type of nonessential water is called sorbed water and is encountered
with many colloidal substances, such as starch, protein, charcoal, zeolite minerals, and
silica gel. In contrast to adsorbed water, the quantity of sorbed water is often large,
­amounting to as much as 20% or more of the total mass of the solid. Solids containing even this amount of water may appear to be perfectly dry powders. Sorbed water
is held as a condensed phase in the interstices or capillaries of the colloidal solid. The
quantity contained in the solid is greatly dependent on temperature and humidity.
A third type of nonessential moisture is occluded water, liquid water entrapped
in microscopic pockets spaced irregularly throughout solid crystals. Such cavities often occur in minerals and rocks (and in gravimetric precipitates).
Nonessential water is the water
that is physically retained by a solid.
Adsorbed water resides on the surface
of the particles of a material.
Sorbed water is contained within the
interstices of the molecular structure
of a colloidal compound.
Occluded water is trapped in random
microscopic pockets of solids, particularly
minerals and rocks.
36B-2 Temperature and Humidity Effects on the Water
Content of Solids
In general, the concentration of water in a solid tends to decrease with increasing
temperature and decreasing humidity. The magnitude of these effects and the rate
at which they manifest themselves differ considerably according to the manner in
which the water is retained.
Compounds Containing Essential Water
The chemical composition of a compound containing essential water is dependent
on temperature and relative humidity. For example, anhydrous barium chloride
tends to take up atmospheric moisture to give one of two stable hydrates, depending
on temperature and relative humidity. The equilibria involved are
BaCl2(s) 1 H2O(g) 8 BaCl2 · H2O(s)
Relative humidity is the ratio of the
vapor pressure of water in the atmosphere to its vapor pressure in air that
is saturated with moisture. At 25°C,
the partial pressure of water in saturated air is 23.76 torr. Thus, when air
contains water at a partial pressure
of 6 torr, the relative humidity is
6.00
5 0.253 (or the percent relative
23.76
humidity is 25.3%).
BaCl2 · H2O(s) 1 H2O(g) 8 BaCl2 · 2H2O(s)
At room temperature and at a relative humidity between 25% and 90%, BaCl2 · 2H2O
is the stable species. Since the relative humidity in most laboratories is well within
these limits, the essential water content of the dihydrate is ordinarily independent of
atmospheric conditions. Exposure of either BaCl2 or BaCl2 · H2O to these conditions
causes compositional changes that ultimately lead to formation of the dihydrate. On
a very dry winter day (relative humidity , 25%), however, the situation changes; the
dihydrate becomes unstable with respect to the atmosphere, and a molecule of water
is lost to form the new stable species BaCl2 · H2O. At relative humidities less than
essential water content
❮ The
of a compound depends on
the temperature and relative
humidity of its surroundings.
974 Chapter 36
Preparing Samples for Analysis
g H2O retained/g solid
Sorption
Adsorption
Partial pressure H2O
Figure 36-3 Typical adsorption and
sorption isotherms.
about 8%, both hydrates lose water, and the anhydrous compound is the stable species. Therefore, we can see that the composition of a sample containing essential
water depends greatly on the relative humidity of its environment.
Many hydrated compounds can be converted to the anhydrous condition by oven
drying at 100°C to 120°C for an hour or two. Such treatment often precedes an
analysis of samples containing hydrated compounds.
Compounds Containing Adsorbed Water
Figure 36-3 shows an adsorption isotherm in which the mass of the water adsorbed
on a typical solid is plotted against the partial pressure of water in the surrounding
atmosphere. The diagram indicates that the extent of adsorption is particularly sensitive to changes in water-vapor pressure at low partial pressures.
The amount of water adsorbed on a solid decreases as the temperature of the
solid increases and generally approaches zero when the solid is heated above 100°C.
Adsorption or desorption of moisture usually occurs rapidly, with equilibrium often
being reached after 5 or 10 min. The speed of the process is often observable during
the weighing of finely divided anhydrous solids, where a continuous increase in mass
will occur unless the solid is contained in a tightly stoppered vessel.
Compounds Containing Sorbed Water
The quantity of moisture sorbed by a colloidal solid varies tremendously with
atmospheric conditions, as shown in Figure 36-4. In contrast to the behavior
of adsorbed water, however, the sorption process may require days or even weeks
to attain equilibrium, particularly at room temperature. Also, the amounts of water
retained by the two processes are often quite different from each other. Typically,
adsorbed moisture amounts to a few tenths of a percent of the mass of the solid,
whereas sorbed water can amount to 10 to 20%.
The amount of water sorbed in a solid also decreases as the solid is heated. Complete removal of this type of moisture at 100°C is by no means a certainty, however,
as indicated by the drying curves for an organic compound shown in Figure 36-4.
After this material was dried for about 70 min at 105°C, its mass apparently became
constant. Note, however, that additional moisture was removed by further increasing
the temperature. Even at 230°C, dehydration was probably not complete.
Compounds Containing Occluded Water
Occluded water is not in equilibrium with the atmosphere and, therefore, is insensitive to changes in humidity. Heating a solid containing occluded water may cause
2.0
230 C
184 C
Figure 36-4 Removal of sorbed
water from an organic compound
at various temperatures. (Reprinted
(adapted) with permission from
C. O. Willits, Anal. Chem., 1951, 23,
1058, DOI: 10.1021/ac60056a003.
Copyright 1951 American Chemical
Society.)
Water lost, %
1.6
130 C
1.2
105 C
0.8
0.4
0
0
40
80
120
160
Heating time, min
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36C Determining Water in Samples 975
a gradual diffusion of the moisture to the surface, where it evaporates. Frequently,
heating is accompanied by decrepitation in which the crystals of the solid are
suddenly shattered by the steam pressure created from moisture contained in the
internal cavities.
36B-3 Drying the Analytical Sample
How we deal with moisture in solid samples depends on the information desired.
When the composition of the material on an as-received basis is needed, the principal
concern is that the moisture content not be altered as a result of grinding or other
preliminary treatment and storage. If such changes are unavoidable or probable, it is
often advantageous to determine the mass loss on drying by some reproducible procedure (say, heating to constant mass at 105°C) immediately after the sample is received.
Then, when the time arrives for the analysis to be performed, the sample is again dried
at this temperature so that the data can be corrected back to the original basis.
We have already noted that the moisture content of some substances is substantially changed by variations in humidity and temperature. Colloidal materials containing large amounts of sorbed moisture are particularly susceptible to the effects of
these variables. For example, the moisture content of a potato starch has been found
to vary from 10 to 21% as a result of an increase in relative humidity from 20 to 70%.
With substances of this sort, comparable analytical data from one laboratory to another or even within the same laboratory can be achieved only by carefully specifying
a procedure for taking the moisture content into consideration. For example, samples
are frequently dried to constant mass at 105°C or at some other specified temperature. Analyses are then performed and results reported on this dry basis. While such
a procedure may not render the solid completely free of water, it usually lowers the
moisture content to a reproducible level.
36C Determining Water in Samples
Often, the only sure way to obtain a result on a dry basis is to determine the moisture
in a set of samples taken concurrently with the samples that are to be analyzed. There
are several methods of determining water in solid samples. The simplest involves determining the mass loss after the sample has been heated at 100°C to 110°C (or some
other specified temperature) until the mass of the dried sample becomes constant.
Unfortunately, this simple procedure is not at all specific for water, and large positive
systematic errors occur in samples that yield volatile decomposition products (other
than water) when they are heated. This method can also yield negative errors when
applied to samples containing sorbed moisture (for example, see Figure 36-4). Modern thermal analysis methods, such as thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetry, are also widely used in studying
the loss of water and various decomposition reactions in solid samples.2
Several highly selective methods have been developed for the determination of
water in solid and liquid samples. One of these, the Karl Fischer method, is described in Section 20C-5. Several others are described in monographs devoted to
water determination.3
2
See, D. A. Skoog, F. J. Holler, and S. R. Crouch, Principles of Instrumental Analysis, 6th ed., Ch. 31,
Belmont, CA: Brooks/Cole, 2007.
3
J. J. Mitchell, Jr., and D. M. Smith, Aquametry, 2nd ed., Vols. 1–3, New York: Wiley, 1977–1980;
E. Scholz, Karl Fischer Titration: Determination of Water, Berlin: Springer, 1984.
Decrepitation is a process in which
a crystalline material containing
occluded water suddenly explodes
during heating because of a buildup in
internal pressure resulting from steam
formation.