Sunday, 11 March 2018

Measurement Units Commonly Used in Analytical Work

Measurement Units Commonly Used in Analytical Work

As an analytical scientist your main concern would have been to report your findings in terms of what is present in your sample and in what amount. Most instruments convert input in other units to concentration or other required units. You might be inclined to think that it is sufficient to have a knowledge of concentration units and their interconversions. An understanding of different units is bound to give you a better understanding on how the systems function to give you the desired results.

Flow chart Measurement Units Commonly Used in Analytical Work
All analytical measurements are based on the three basic fundamental units of length, mass and time. The results are expressed in quantities derived from these fundamental units. Let us now look into the different properties in analytical work and their quantification units.

Length

The SI unit for length is meter which in absolute terms is the length traversed by light in vacuum during a time interval of 1/229792458 of a second. The old prototype of the meter is still preserved at the International Bureau of Weights and Measures. The meter is too long a unit for expressing molecular/ atomic dimensions and wavelengths of electromagnetic radiation. These are expressed in subunits of the meter.
1 μm= 10^ ^-6 m
1nm=10^-^9 m
1 cm^ -^1 =1/wavelength(nm)

Mass

The kilogram is equal to the mass of international prototype of kilogram. A gram is 1/1000 of the weight of the kilogram. However, in most analytical work even gram is considered to be too large quantity and masses expressed in fractions of the gram.
1 Kg = 1000 g
1 mg = 10 ^-^3g = 10^ ^-6 Kg
1 µg = 10^-^6 g = 10^-^9 Kg
1pg=10 ^-^9 g = 10^-^1^2 Kg

Time

A second was originally defined as 1/86400 fraction of the mean solar day. The definition has since been revised and the accepted definition defines a second as the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of Cs – 133 atom.
Time measurements are important for reactions in kinetic studies, chromatographic separations, radioactive measurements and fluorescence studies. The units range several seconds to sub-fractions of a second.
1ms = 10^-^ 3 s
1 µs = 10^-^6s
1 ps = 10^-^9s

Volume

Accurate measurement of volumes is necessary in titrimetry, dilutions and quantitative reactions.Litre is too large a unit so fractions commonly used in analytical work are :
1ml = 10^-^3 L
1μl = 10^-^6L

Concentration

Concentration expresses the amount of substance present in the sample. It is the most important unit for expression of your results.
Moles – amount of substance that contains as many elementary entities such as atoms, molecules, ions as there are in 0.012 to Kg of C – 12.
Molarity (M) – number of gram moles of salute in 1 L of solution i.e number of modular weight expressed in grams.
Molality(m) – not in common use. It is the number of moles of solute dissolved in 1 kg of solvent.
Normality(N) – molarity xn, where n is the number of protons exchanged in a reaction.
ppm or ppb – concentration expressed in parts per million or parts per billion.
1 ppm = mg solute/liter=1μg/ml
1ppb= 1μg solute/ litter =1 nanogram solute/ml
Percentage by weight is weight of salute needed to give the desired concentration.
Percentage by volume is volume of solute required to give the desired concentration.

Pressure

Pressure measurements are necessary in measurements involving gases and liquids and when expressing barometric pressure readings. It is defined as the ratio of force to area over which the force is applied. SI unit is Pascal which is 1 N/metre^2.
1 bar = 10^5 N/m^2
1 Torr= 1 mm Hg = 133.3 Pa.
1 atmosphere= 101.3 K Pa = 760 mm Hg = 14.70 psi

Temperature

Precise temperature control is necessary in reaction studies, volumetric measurements and chromatographic separations.
Temperature is the degree of hotness or coldness of a body. It is expressed in degrees Kelvin (0K) where Kelvin is a fraction 1/273.16 of the thermodynamic temperature of triple point of water. 0K is also referred to as 0 C. Pure water has freezing point of 0°C and boiling point of 1000 C at mean sea level.

Energy measurements

All chemical reactions involve changes in energy mainly as heat energy and light energy. Even at spectroscopic level transitions involve energy changes which lead to identification and quantification of elements and molecule speices.

Heat

Heat should not be confused with temperature. The SI unit of heat is Joule whereas in most thermal analysis calorie or millicalorie is in common use.
1 cal is equal to amount of heat required to raise temperature of 1 g of water by 1°C.

Light

Light energy is necessary for initiation of certain photochemical reaction processes. The common unit is Candela which is the illumination in a given direction of a light source that emits monochromatic radiation of frequency 540 X10^1^2 Hz and radiant intensity of 1/ 683 watt per steradian.

Electrical units

All analytical instruments give response in electrical units which gets converted to desired units such as pH, temperature, humidity, concentration, pressure, resistance, conductance, etc. The electrical units are based on measurements of current and voltage.
1 ampere is equal to current maintained in straight parallel conductors of infinite length of negligible circular cross-section and placed 1 m apart in a vacuum that would produce a force between them of 2X10^-^7 per metre of length.
1ma = 10^-^3amp
1μa = 10^-^6 amp
One volt is potential difference between two parallel, infinite planes spaced 1 m apart that create an electric field of 1 N per coulomb. It is also the potential difference between two points of a conducting wire when an electric current of one ampere dissipates 1 watt of power between.
1mV= 10^-^3 V
1 µV = 10^-^6V
1 ohm is resistance offered to flow of current of 1 ampere between two points at a potential difference of one volt.
1mho is unit used for conductance measurements and is reciprocal of ohm.

Absorbance

Absorbance plays a vital role in spectroscopic studies such as UV -Vis, Infrared, Atomic Absorption spectroscopy,etc.
A = - log_1_0 I/I_0
Where I is intensity of transmitted light of a particular wavelength after passing through the sample and I0 is the intensity before reaching the sample.  Absorbance being a ratio does not have units but conventionally it is expressed in terms of absorbance unit AU.
An attempt has been made to introduce to you the units and their importance in analytical work. As you gain proficiency in inter-conversions you will start appreciating what you are doing and feel greater involvement in your work.
Please do leave your comments and suggestions.
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