Flame spectroscopy new

FLAME SPECTROSCOPY

FLAME SPECTROSCOPY

• also known as Flame Emission, Flame Photometry, Atomic Emission Spectroscopy

• a flame by its heat can raise atoms from lower energy to an excited state of higher energy

PRINCIPLE OF FES

• the solvent gets evaporated leaving behind the corresponding solid salt

• the solid salt undergoes vaporization and gets converted into its respective gaseous state

• the progressive dissociation of either a portion or all of the gaseous molecules gives rise to free neutral atoms or radicals

Instrumentation

Two types of Flame Photometers:

(a) Simple Flame Photometer, and

(b) Internal Standard Flame Photometer.

Simple Flame Photometer

A = Inlet for compressed Air,

B = Drain outlet (to maintain constant pressure head in the mixing Chamber),

C = Liquid sample (sucked into the Nebulizer),

D = Inlet for Fuel-Gas to the Laminar-Flow-Burner,

E = Nebulizer to atomize the liquid sample,

F = Mixing Chamber for Fuel Gas, Compressed Air, and Atomized Liquid Sample,

G = Burner,

H = Flame,

I = Convex lens,

K = Optical filter to transmit only a strong-line of the element, and

L = Amplifier to amplify the feeble electrical impulse and a built-in direct read-out device.

Internal Standard Flame Photometer

A = Inlet for compressed air,

B = Inlet for Acetylene (Fuel-Gas),

C = Liquid sample sucked in by an atomizer,

D = Flame,

E = Mirror,

F = An optical filter to allow the transmission of only one strong-line of the element,

G = Amplifier to amplify the weak electrical current,

H = A Null detector to record the intensity of the element under study and the internal-standard (Lithium),

I = A calibrated potentiometer,

J1 = Lines due to the ‘sample’

J2 = Lines due to the Internal Standard ‘Lithium’, and

K1 & K2 = Photocells to convert light-energy to electrical impulse.

APPLICATION OF FES

ASSAY OF SODIUM, POTASSIUM AND CALCIUM IN BLOOD SERUM AND WATER

(i) Standard potassium and sodium solutions, approximately 500 ppm : Weigh accurately 0.95 g

of dried KCl and 1.25 g of dried NaCl into separate 1-litre volumetric flasks. Dissolve in water and dilute to the mark.

(ii) Standard calcium solution, approximately 500 ppm : Weigh accurately 1.25 g of CaCO3, which has been dried at 110°C, into a 500-ml breaker. Add about 200 ml of DW and 10 ml of conc. HCl. Cover the breaker with a watch-glass during addition of acid to prevent loss of solution as CO2 is evolved. After the solution is complete, transfer it quantitatively into a 1-litre volumetric flask and dilute to the mark with DW.

APPLICATION OF FES

• (iii) Radiation buffer* for sodium determination : Prepare a saturated solution with reagent-grade CaCl2, KCl, MgCl2, in that order.

• (iv) Radiation buffer for potassium determination : Prepare a saturated solution with reagent-grade NaCl, CaCl2 and MgCl2, in that order.

• (v) Radiation buffer for calcium determination : Prepare a saturated solution with reagent-grade NaCl, KCl, MgCl2 in that order.

PROCEDURE

Procedure(a) Preparation of working curves : Transfer 5 ml of the appropriate radiation buffer to each series of 100-ml volumetric flasks. Add a volume of the standard solution which will cover a concentration ranging between 0 to 100 ppm. Dilute to 100 ml with DW and mix well. Measure the emission intensity of these samples by taking at least three readings for each. Between each set of measurements, aspirate DW through the burner. Correct the average values for background luminosity, and prepare a working curve from these data.

(b) Analysis of blood serum/water sample : Prepare aliquot portions of the sample as described in the above paragraph (a). If necessary, use a standard to calibrate the response of the spectrometer to the working curve. Then measure the emission intensity for the unknown. After correcting the data for background, determine the concentration by comparison with the working curve.

ASSAY OF BARIUM, POTASSIUM AND SODIUM IN CALCIUM ACETATE

• The technique of flame emission spectroscopy is used to determine the concentration of Ba, K, and Na ions by measuring the intensity of emission at a specific wavelength by the atomic vapour of the element generated from calcium acetate i.e., by introducing its solution into a flame.

For Emission Measurements

• Introduce water into the atomic vapour generator, adjust the instrument reading to zero, introduce the most concentrated solution into the generator and adjust the sensitivity to give a suitable reading ; again introduce water or the prescribed solution into the generator and when the reading is constant readjust, if necessary, to zero.

Boltzmann Equation

Ratio between the population of atoms at a higher state of energy level and the population of atoms at a higher energy level and the population at the ground state increases with temperature

EQUATION

the fraction of free atoms which are excited thermally

N1/N2 = (g1/g0) e-ΔE/kT

where

N1 = number of atoms in the excited state

N2 = number of ground state atoms

g1/g2 =degeneracy of energy levels

E = energy of excitation (=hv)k = the boltzmann's constantT = Temperature in Kelvin

SAMPLE PROBLEM

Let’s calculate the Boltzmann distribution for

the lowest state of a sodium atom at 298K,

2500K (a typical air/acetylene flame) and 10,000K

(a typical ICP ‘flame’)The )E for this transition is 3.371x10 J/atom

Assume the excited state has a degeneracy of 2,and the ground state is nondegenerate (degeneracies

should always be given in a problem

SOLUTION:

298 K

N1/N2 = (g1/g0) e-ΔE/kT

= 2/1 e –(3.371^-19)/(1.381^-23x298)

= 2/1 e -81.9

= 2 ( 2.6994913 ^ -36)

= 5.40 ^ -36 J

SOLUTION

2500 K

N1/N2 = (g1/g0) e-ΔE/kT

= 2/1 e –(3.371^-19)/(1.381^-23x2500)

= 2/1 e -9.76

= 2 ( 5.771462214^-5)

= 1.15 ^ -4 J

SOLUTION

10000 K

N1/N2 = (g1/g0) e-ΔE/kT

= 2/1 e –(3.371^-19)/(1.381^-23x10000)

= 2/1 e -2.44

= 2 ( 8.7160851 ^-2)

= 1.74 ^-1 J

BOHR’S EQUATION

E2-E1=hv

where, h = Planck's constant

v = Frequency of emitted light

E2-E1 = energy levels of excited and ground states

the expression (c) is the Bohr's equation which enables us to calculate– the wavelength of the emitted radiation which

is characteristic of the atoms of the particular element from which it was initially emitted

– wavelength of radiation given out from a flame is indicative of the elements that might be present in that flame

– intensity of radiation may quantify the exact amount of the elements present

PROBLEM.

1. Hydrogen has a red emission line at 656.3 nm, what is the energy and frequency of a photon of this light? Speed of light = 2.998 x ^8 m/s

E = hv = hc/λ

E = 6.626x ^ -34 J/s ( 2.998x ^8m/s)/ 656.3 nm (10^9m/1nm)

E= 3.027x10 -18 J