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Title:To carry out the assay of total sodium or potassium in terms of mmol/l in an ORS with flame emission spectroscopy

(Flame photometry).(Standard addition technique)

Theory:If a solution containing a metallic salt(or some other metallic compound) 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 atom that are excited thermally in a flame. The radiation from the resulting filter 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◦ to E1, corresponds to the absorption of radiant energy, and the amount of energy absorbed by Bohr’s equationΔE = E1 - E◦ = hʋ = hc/λHence, the transition from E1 to E◦ corresponds to emission of radiant of frequency ʋ. 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 to E◦, E2 to E◦ etc (indicated by the broken line). Hence, it follows that the emission spectrum of a given element may be quite complex.The sensitivity of flame photometry for different elements depends on ΔE, i.e. the elements with the lowest ΔE values are the most easily excited in the flame and give a greater proportion of thermally excited atoms than those with higher ΔE values. In practice only the alkali and alkaline earth metals that have ΔE less 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:

Liquid sample containing → Formation of the → Evaporation of droplets resulting

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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 atoms

The instrumentation is briefly described as: air at a given pressure is passed into an 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.

b)Standard addition:This method is useful for determining very low concentrations of the elements present in the sample and for overcoming anionic or physical interferences from certain substances in the sample that affect the efficiency of the emission of the element. This method involves preparing a number(usually 4-6) of equal dilutions of the sample, each containing an added known concentration of the element. Ideally, the concentrations of added element should be approximately 0, 0.5x, x, 2x and 4x, where x is the concentration of the element in the sample. The full-scale reading and zero are set with the most concentrated solution and solvent respectively, then other solutions are sprayed. The data are plotted on a graph in which the abscissa is the concentration of added element. Extrapolation of the graph to an extended abscissa gives the concentration of element in the sample.

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

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 0.635g of sodium chloride(A.R.) and 0.477 g of potassium chloride(A.R.), previously dried at 110◦C for 2 hours, transfer into different 250ml(V.F.) and dissolve in distilled water. Then dilute to the mark with distilled water(1000μg/ml each sodium and potassium ). Pipet out 5ml of each stock solution and dilute to 50ml with water (100 μg/ml or ppm) in two different V.F.

Preparation of sample solution for total sodium :Dissolve the content of a sachet of ORS in a litre of water(1700 μg/ml. sodium,Approx.). Dilute 2.9ml of the sample solution to 50ml with water(100 μg/ml sodium). Transfer 5.0ml/5.0ml of the sample solutions into five different 100ml V.F.s ( on diluting to 100ml these solutions give about 5.0. μg/ml sodium). Except in one volumetric flask containing 5.0ml of the sample solution, in second flask add 2.5ml, in third flask 5ml, in fourth flask 10ml and in fifth flask 20ml diluted stock solution of sodium(100 μg/ml). Then dilute all the flasks to 100ml with water.

Preparation of sample solution for potassium:Dissolve the content of a sachet of ORS in a litre of water(787 μg/ml. potassium,Approx.). Pipet out 6.4ml of the sample solution and dilute to 50ml with water(100 μg/ml, Approx.). Transfer 5.0ml/5.0ml of the diluted sample solution into five different 100ml V.F.s(On diluting to 100ml , these solution give 5 μg/ml. potassium). Except in one volumetric flask containing 5.0ml of the sample solution, in second flask add 2.5ml, in third flask 5ml, in fourth flask 10ml and in fifth flask 20ml diluted stock solution of potassium (100 μg/ml). Then dilute all flasks to 100ml with water.

Operation/Estimation:1.Turn on the flame photometer with the help of instructor.

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2.Aspirate std. mixed solution of highest concentration. 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.Then, aspirate sample solution(30sec.) and note down the readouts. Each sample solution aspiration should be followed by solvent or blank aspiration for 5 seconds.

Observations:

For sodium chloride/potassium chloride: Concentration

(μg/ml)Readouts Readouts

(Mean)0 1) ,2) ,3)

2.5 1) ,2) ,3)5 1) ,2) ,3)10 1) ,2) ,3)20 1) ,2) ,3)

Calculation:The molar concentration of sodium(75mmol/l, claimed) /potassium(20mmol/l,claimed) =

Results:

Conclusion: