Sampling and Sample Handling

Embed Size (px)

Citation preview

  • 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