practical flame photometry

7/28/2019 Practical Flame Photometry

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Title:To carry out the assay of sodium chloride or potassium chloridein a compound sodium chloride solution(Ringers solution) with

flame emission spectroscopy(Flame photometry).

(Direct calibration technique)Theory:If a solution containing a metallic salt(or some other metallic compounds) is aspirated or dispersed with an

atomiser into a flame, a vapour which contains atoms of the metal may be formed. Some of these gaseous metal

atoms(0.01-1.0)% may be raised to an energy level which sufficiently high to permit the emission of radiation

characteristic of the metal, e.g. the characteristic yellow colour (589.3nm) imparted to flames by compound of

sodium. This is the basis of flame photometry in which measurement of the intensity of light emitted at a

particular wavelength from atoms that are excited thermally in a flame. The resulting radiation passes through a

lens and an optical filter which permits only the radiation characteristic of the element under investigation to

pass through the photocell. The output from the photocell is measured on a suitable digital read-out system.

Let us consider transitions between two quantized energy levels,E toE1.Itcorresponds to the absorption of

radiant energy, and the amount of energy absorbed is calculatedby Bohrs equation

E=E1 -E = h = hc/

Hence, the transition fromE1 toEcorresponds to emission of radiant energy of frequency . Each element

possesses a definite, characteristic line spectrum, hence , there are different excitation states associated with

different elements. The consequent emission spectra involve not only transitions from excited states to the

ground state, e.g.E3 toE,E2 toE etc (indicated by the full lines) butE3 toE2,E3 toE1(indicated by the broken

lines). Hence, it follows that the emission spectrum of a given element may be quite complex. The sensitivity offlame photometry for different elements depends on E, i.e. the elements with the lowest Evalues are the

most easily excited in the flame and give a greater proportion of thermally excited atoms than those with higher

Evalues. In practice only the alkali and alkaline earth metals that have Eless than 3 ev, i.e. sodium,

potassium, calcium, lithium and barium produce a sufficient number of thermally excited atoms at the

temperature of an air-natural gas flame.

The following is a concise description of the sequence of events that normally occurs in flame photometry:


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Liquid sample containing Formation of the Evaporation of droplets resulting

element aspirated into a flame liquid droplets in the formation of residue

Wavelength and intensity of Formation of excited Decomposition of residue into

emitted radiation measured by atoms and emission of neutral atoms.

flame photometry radiation from the atoms

The instrumentation is briefly described as: air at a given pressure is passed into a nebulizer/atomizer and the

suction this produces draws a solution of the sample into the atomizer, where it joins the air stream as a fine

mist and passes into a mixing chamber where large droplets fall out and flow to waste through the drain tube

and fine mist mix with the fuel gas supplied. The mixture at a given pressure passes into the burner. Radiation

from the resulting flame passes through a lens, and finally through an optical filter which permits only the

radiation characteristic of the element under investigation to pass through the photocell. The output from the

photocell is measured on a suitable digital read-out system.

To convert the measured absorption values into the concentration of the substance being determined , it is

necessary to resort on two techniques:

a) Direct calibration, b) Standard addition.

a) Direct calibration:

This involves setting the full-scale and zero readings with the highest standard and blank solutions respectively

and spraying the other standard solutions (3-5 in number) containing intermediate concentration of the element.

A graph is constructed of emission vs. concentration and the concentration of the element in the sample is read

out from the graph, as the concentration corresponding to the emission of the sample. If calibration graphs show

negative curvature, particularly at higher concentration, linearity is improved by using more dilute solutions. If

the graph is linear over the concentration range in which the sample concentration occurs, a single-point


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standardization technique may be used. If two or more elements are to be determined , then mixed standard

solution should be prepared. At least two, or preferably three, separate absorption readings should be made with

each solution, and an average value taken.

Chemicals required:1)Sodium chloride(AR), 2)Potassium chloride(A.R.), 3)Ringers solution.

Apparatus required:1)Flame photometer 130(Systronic, 2)250ml/100ml(V.F),3)Burette(50ml),4)Beakers(100ml/250ml)

Procedure:A)For sodium chloride:

Preparation standard stock solution:

Weigh accurately 0.250g of sodium chloride(A.R.) and potassium chloride(A.R.) each, previously dried at

110C for 2 hours, transfer into a 250ml(V.F.) and dissolve in distilled water. Then dilute to the mark with the

distilled water(1000g/ml each sodium chloride and potassium chloride).

Preparation of standard series for sodium chloride:Pipet out 10ml of the stock solution and dilute to 100ml with water(100 g/ml or ppm). Dilute 2.5ml, 5.0ml,

7.5ml, 10.0ml and 12.5ml of this solution to 100ml with water to give 2.5g/ml, 5.0g/ml, 7.5g/ml, 10.0g/ml

and 12.5g/ml of sodium chloride solution.

Preparation of sample solution:

Pipet out 1.0 of the sample solution and dilute to 100ml with water(85 g/ml). Further dilute 5ml the sample

solution(85 g/ml) to 50ml with water(8.5 g/ml ,Approx.).


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Preparation of standard series for potassium chloride:

Pipet out 10ml of the stock solution and dilute to 100ml with water (100 g/ml or ppm). Dilute

5.0ml,10.0ml,15.0ml.20.0ml and25ml to 100 with water to give 5.0 g/ml,10.0g/ml,15.0g/ml.20.0g/ml

and25.0 g/ml of potassium chloride solution.

Preparation of sample solution:

Pipet out 5.0ml of the sample solution and dilute to 100ml with water(15 g/ml, Approx.).

Estimation:

1.Turn on the flame photometer with the help of instructor.

2.Aspirate std. mixed solution of highest concentration(12.5 g/ml Sodium chloride / 25g/ml potassium

chloride).

3.Set sensitivity control at LOW. If the readout is less than 100( the desired full scale), select higher sensitivity-

MD or HI.4.Turn SET F.S. COARSE in anticlockwise direction till 100(the desired full scale)is reached. SET F.S. FINE

may be adjusted for precise setting(ALWAYS ADJUST FULL SCALE WITH LOWEST POSSIBLE

SENSITIVITY).

5.Aspirate intermediate concentration of std.mixed solution for 30 seconds. Distilled water should be aspirated

for a minimum of 5 seconds between two solutions.

6)Finally, aspirate unknown sample solution(30sec.) and not down the readouts.

Observations:

For sodium chloride/potassium chloride:Concentration

(g/ml)

Readouts Readouts

(Mean)

1) ,2) ,3)

1) ,2) ,3)

1) ,2) ,3)

1) ,2) ,3)

1) ,2) ,3)

Sample 1) ,2) ,3)

Calculation:Each 100ml of Ringers solution contains, sodium chloride/potassium chloride=

Results:


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Conclusion:

Performed By Checked By

Name:

Roll No.:

practical flame photometry

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