UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL ENGINEERING
BULUMA MARK EUGINE

F18/1494/2011

GROUP 4

EXPERIMENT 3: FLAME PHOTOMETRY.
THE OBJECTIVE OF THE EXPERIMENT.
1. To determine presence and concentration of sodium ions in drinking water.
2. To determine the presence and concentration of potassium ions in an unknown fertilizer sample. THEORY BEHIND THE EXPERIMENT.
The major cation of the extracellular fluid is sodium. The typical daily diet contains 130-280 mmol (8-15 g) sodium chloride. The body requirement is for 1-2 mmol per day, so the excess is excreted by the kidneys in the urine.
Hyponatraemia (lowered plasma [Na+]) and hypernatraemia (raised plasma [Na+]) are associated with a variety of diseases and illnesses and the accurate measurement of [Na +] in body fluids is an important diagnostic aid.
Potassium is the major cation found intracellularly. The average cell has 140 mM K+ inside but only about 10 mM Na+. K+ slowly diffuses out of cells so a membrane pump (the Na +/K+ATPase) continually transports K+ into cells against a concentration gradient. The human body requires about 50-150 mmol/day.
Hypokalaemia (lowered plasma [K+]), hyperkalaemia (increased plasma [K+]) and hyperkaluria (increased urinary excretion of K+) are again indicative of a variety of conditions and the clinical measurement of [K+] is also of great importance.
INTRODUCTION TO THE EXPERIMENT
Flame photometry is a traditional and simple method for determining sodium and potassium concentration in biological fluids. It relies on the principle that an alkali metal salt drawn into a non-luminous flame will ionise, absorb energy from the flame and then emit light of a characteristic wavelength as the excited atoms decay to an unexcited ground state. The intensity of emission is proportional to the concentration of the element in the solution. Quantitative analysis of these species is performed by measuring the flame emission of solutions containing the metal salts. Solutions are aspirated into the flame. The hot flame evaporates the solvent, atomizes the metal, and excites a valence electron to an upper state(higher orbitals). Light is emitted (emission of a photon of radiation) at characteristic wavelengths for each metal as the

electron returns to the ground state. Optical filters are used to select the emission wavelength monitored for the analyte species. A photocell detects the emitted light and converts it to a voltage, which can be recorded. Since Na+ and K+ emit light of different wavelengths, by using appropriate coloured filters (monochromator), the emission due to Na+ and K+ (and hence their concentrations) can be specifically measured in the same sample.
The following are some advantages of using flame photometry:
1. It is a fast and sensitive analytical method.
2. Because of the very narrow and characteristic emission lines from the gas-phase atoms in the flame plasma, the method is relatively free of interferences from other elements.
3. Typical precision and high accuracy for analysis of dilute aqueous solutions.
4. The method is suitable for many metallic elements, especially for those metals which are easily excited to higher energy levels at flame temperature -- Na, K, Ca, Rb, Cs, Cu, Ba.
The following are some disadvantages of flame photometry:
1. They respond linearly to ion concentrations over a rather narrow concentration range, so suitable dilutions usually have to be prepared.
2. It employs relatively expensive machines.
3. Many different experimental variables affect the intensity of light emitted from the flame.
Therefore, careful and frequent calibration is necessary for good results. This makes the method a bit complex and tedious.
The following are uses and applications of flame photometry:
Potash and fertilizer industry
Highly accurate determination of potassium and sodium concentrations
Soil and environmental analysis
Laboratory measurements for determination of alkali and alkaline earth elements.
Drinking water treatment
Measurement of calcium and sodium concentrations in drinking water.
Glass industry
Measurement of sodium concentrations in glass.
Clinical applications
Electrolyte determinations in blood and urin in areas without laboratory automation.

Used to measure the element lithium in serum or plasma in order to determine the correct dosage of lithium carbonate, a drug used to treat certain mental disturbances, such as manic-depressive illness (bipolar disorder).
APPARATUS USED
Reagent bottles, 2ml pipettes, A flame photometer, Volumetric flasks.
REAGEANTS USED.
1000 ppm (mgl-1) Na stock solution (from NaCl), 1000 ppm K+ stock solution from KCl and an unknown sample of fertilizer. Distilled water.
PROCEDURE OF THE EXPERIMENT.
A. SODIUM IN DRINKING WATER.
1. From the prepared 1000ppm (mgl-1) Na+ stock solution (from NaCl) we made 0, 5, 10, 15 and 20ppm working standard solutions.
2. These solutions were then aspirated into the flame photometer and the galvanometer reading at each reading (abscienssae) against the concentration, (ordinates) noted and recorded in table 1 (Sodium in Drinking water) on the result sheet attached onto this report. 3. A calibration curve for Na was prepared as indicated on graph 1 attached to this report.
4. A sample of ordinary drinking tap water was sprayed into the flame of the flame photometer. The galvanometer deflection was noted, and then the concentration of sodium in ordinary drinking tap water was evaluated from the calibration curve.
B. POTASSIUM FERTILISER SAMPLE.
I.

STANDARD K+ SOLUTION.

From 1000ppm K+ stock solution, ( made by dissolving 0.47 gm of A.R KCl in deionized water and diluting 250ml in a Volumetric flask). We prepared a stock solution of 100ppm K+. From this solution, we prepared 1,2,4,8 and 10ppm solution for calibration of the photometer.
II.

PREPARATION OF UNKNOWN SAMPLE.

14g of finely ground sample of fertilizer was accurately weighed then dissolved in deionized water in a 250ml volumetric flask. 25ml of this solution was dissolved in deionized water in a
250ml volumetric solution.
The solution was sprayed into the flame of the flame photometer. The emission was compared to the calibration curve of potassium.

ANALYSIS OF THE RESULTS.
From the graphs,
Concentration of sodium in drinking water
= 10.15*5
=52.75ppm
Concentration of potassium in the fertilizer
=5.5*2.5
=13.75ppm
The straight line graphs indicate that the quantity of emission is directly propoetional to concentration. The following are possible sources of errors that could have caused deviation in the results obtained from:
1. Contamination of samples during preparation.
2. Wrong handling of the photometer.
CONCLUSION.
The experiment was a huge success since the objectives of the experiment were all met.
The concentration of sodium in drinking water was evaluated to be 52.75ppm.
The concentration of potassium in the fertilizer was evaluated to be 13.75ppm.
QUESTIONS.
1. Why is it difficult to determine group II elements than group I elements using flame photometry?  It is because group II elements have the two valence electrons in the s-orbitals thus requires more energy to excite the to a higher state.
2. What precautions should one observe when using a flame photometer?
 Ensure that the photometer drain is leading into a sink and that the instrument is connected to gas, air and electricity supplies. Ensure the mains supply gas tap is off.

 Turn the "Sensitivity" and instrument "Gas" controls control fully counterclockwise
(towards you).
 Insert the sodium optical filter.
 Switch on the instrument and unclamp the galvanometer by turning counterclockwise.
 Open the mica window, turn on the mains gas supply, light the gas and close the window.
(CAUTION: DO NOT LEAN OVER THE INTRUMENT OR YOU WILL SET
YOUR HAIR ALIGHT)
 Turn on the air supply control and adjust the air pressure to 10 lb/in 2. Leave for 1-2 minutes to stabilize.
 Place a beaker of distilled water into position at the left hand side of the instrument and insert the narrow draw tube into it to allow water to pass through the photometer.
(NOTE: once set up, the photometer must have water running through it at all times when a salt solution is not being measured. The rate of uptake is fast, so make sure there is always enough water in the beaker).
 Adjust the gas control to give a flame with a large central blue cone then, with water passing through the instrument, slowly close the gas control until ten separate blue cones just form.
 Set the galvanometer to zero using the "Set zero" control.
 Replace the distilled water with the 5 mM NaCl standard and adjust the "Sensitivity" control till the galvanometer reads 100.
 Quickly but carefully, replace the 5 mM NaCl standard with standards of decreasing concentration from 4 mM to 0.25 mM and note the readings in the Table below.
 Run water through the instrument again for 1-2 min then place the draw tube into a beaker containing the 1 in 50 diluted rehydration sachet solution and note the galvanometer reading.
 Run water through the instrument again and replace the sodium with the potassium filter.
 Repeat the above procedure with the KCl standards, setting to 100 with 2.0 mM KCl, then reading the others in reverse order. Then read the 1 in 50 diluted rehydration sachet solution.  Finally, run water through the instrument until the flame appears free of colour again.
 When the instrument is no longer required, switch off in the following sequence:
i.
ii. iii. iv.
v.

Turn off the gas control and the mains gas supply
Wait for the flame to die out.
Turn off the air supply.
Switch off the electricity
Clamp the galvanometer.

CAUTION Although the flame is quite small, it has so high a temperature that contact of the flesh with even the outer edge of the flame will instantly produce a thirddegree burn. Except for lighting the flame, the hands should be kept completely out of the housing whenever the flame is burning, even if it has been turned very low between analyses

REFERENCES
Sawyer, Heineman, Beebe, Chemistry Experiments for Instrumental Methods, Wiley,
New York, 1984.
D. C. Harris Quantitative Chemical Analysis 4th Ed., W. H. Freeman and Company, New
York 1995 Chapter 21
D. A. Skoog - D. M. West - F. J. Holler: Fudamentals of Analytical
Chemistry (Saunders College Publishing, Fort Worth, US 1992.)
Publishers, Boca
Raton, US 1994.)