Sampling and Sample Handling
Transcript of Sampling and Sample Handling
-
8/16/2019 Sampling and Sample Handling
1/15
-
8/16/2019 Sampling and Sample Handling
2/15
SAMPLING UNITS
The sampling process must ensure that the
items are representative of the bulk material.
The items chosen for analysis are called
sampling units or sampling increments.For example: Consider a population of 100
coins and our aim is to know the average
composition of lead in the collection of coins.
ur sample consists of five coins. !ach coin is
a sampling unit or sampling increment
-
8/16/2019 Sampling and Sample Handling
3/15
GROSS SAMPLE
" gross sample is a collection of
individual sampling units. #t must be
representative of the whole in
composition and in particle si$e
distribution.
-
8/16/2019 Sampling and Sample Handling
4/15
SIZE OF THE GROSS SAMPLE
Gross sample weight is determined by (1)
the uncertainty that can be tolerated
between the composition of the gross
sample and that of the whole, (2) the degreeof heterogeneity of the whole and (3) level of
particle size.
A well mixed solution of a gas or liquid is
heterogeneous only on the molecular scaleand the weight of the molecules themselves
governs the minimum weight of the gross
sample.
-
8/16/2019 Sampling and Sample Handling
5/15
A particulate solid such as an ore or
soil, represents the opposite
situation. In such materials,
individual pieces of solid differ from
each other in composition. In suchcases gross sample may weigh a gram
or more. The number of particles
required in a gross sample rangesfrom a few particles to 1012 particles.
-
8/16/2019 Sampling and Sample Handling
6/15
SAMPLING OF LIQUIDS AND GASES
Well mixed solutions of liquids and
gases require only a very small
sample because they arehomogeneous upto the molecular
level. Whenever possible The liquid or
gas to be analysed should be stirredwell before sampling to make sure
that the gross sample is homogeneous.
-
8/16/2019 Sampling and Sample Handling
7/15
Gases can be sampled in several
ways. In some cases, the sampling
bag is simply opened and filled
with the gas. In other cases, gas
can be trapped in a liquid or
adsorbed onto the surface of a
solid
-
8/16/2019 Sampling and Sample Handling
8/15
SAMPLING OF PARTICULATE SOLIDS
It is often difficult to obtain a random
sample from a bulky particulatematerial. Random sampling can be
done during the transferring of the
material.
-
8/16/2019 Sampling and Sample Handling
9/15
SAMPLING OF METALS AND ALLOYS
Samples of metals and alloys are
obtained by sawing, milling or drilling.
Solid material from the interior andexterior must be sampled.
Representative sample can be obtained
by sawing across the piece at random
intervals and collecting the sawdust asthe sample. Alternatively by drilling
the sample and collect the drillings.
-
8/16/2019 Sampling and Sample Handling
10/15
LABORATORY SAMPLE
For heterogeneous solids, the gross
sample may weigh from hundreds of
grams to kilograms or more.
Reduction of the gross sample to a
finely ground powder by crushing,
grinding, mixing and dividing. The
finely ground sample is calledlaboratory sample.
-
8/16/2019 Sampling and Sample Handling
11/15
Laboratory samples of solids often
contain water that is in equilibrium
with the atmosphere. The common
practice is to remove moisture from
solid samples prior to weighing.
Traditionally drying is accomplished
by heating the sample in an oven orvacuum oven and by storing in a
decicator
-
8/16/2019 Sampling and Sample Handling
12/15
FORM OF WATER IN SOLIDS
Essential water:
It is the water that is an integral part
of a solid chemical compound in astoichiometric amount.
Eg. BaCI2 2H2O, CaC2O4 2H2O
-
8/16/2019 Sampling and Sample Handling
13/15
Water of constitution is a second type
of essential water.KHSO4 (s) = K2S2O7(s) + H2O(g)
Ca(OH)2 (s)= CaO(s)+ H2O(g)
These compounds yieldstoichiometric amounts of water
when heated
-
8/16/2019 Sampling and Sample Handling
14/15
Nonessential water:
It is the water that is physically retainedby a solid.
It is not necessary for the
characterisation of the chemicalconstitution of the sample.
Adsorbed water is a type of nonessential
water that is retained on the surface
solids. Another type of nonessential
water is called sorbed water amounting
to about 20% of the total mass of the solid.
-
8/16/2019 Sampling and Sample Handling
15/15