S10. DETERMINATION OF SODIUM AND POTASSIUM IN BEER BY FLAME
PHOTOMETRY.
PURPOSE
To analyse samples of beer for their sodium and potassium content, using a flame photometer.
INTRODUCTION
Flame photometry is a technique that uses the measurement of emitted light from a sample in
a flame as a measure of how much analyte is present. This is the reverse of what happens in
AAS, where the amount of light absorbed is measured.
Flame photometry needs no lamps. It only needs town gas and a supply of compressed air,
plus a simple power point. Its disadvantage is that it can be used for only a very few metals.
The internal standard method may be of use in overcoming variations in flame conditions.
The internal standard chosen should have properties similar to the element being analysed but
should not be present in the sample. When graphing the results using the internal standard the
ratio of the analyte to the internal standard is plotted against the concentration of the analyte
PREWORK
Calculate the concentration of an intermediate standard which would be used to prepare the
following standard series and how it could be prepared from a stock solution of 1000mg/L .
The formula C1V1= C2V2 will be of use for this calculation
1. 100 mL of calibration standards containing 1, 5, 10 and 20 mg/L Na using dilutions of 1,
5, 10 and 20 mL of the intermediate solution.
2. 100 mL of calibration standards containing 5, 10, 20 and 50 mg/L K using dilutions of
2.5, 5, 10 and 25 mL of the intermediate solution.
PROCEDURE
I. Sample and standard preparation
1. Prepare the Na and K intermediate standards as per Prework 1 and 2
2. Obtain about 50 mL of beer. De-gas this by pouring from one beaker to another a number
of times and then place the beaker in an ultrasonic bath for 5 minutes
3. Prepare 100 mL of Na calibration standards as per Prework 1, and label them Flasks A, B,
C and D.
4. Prepare 100 mL of each of the following MIXED sodium/potassium/ lithium standards.
Aliquots of each metal are pipetted into ONE flask! Use the intermediate sodium and
potassium standards and the 1000mg/L lithium standards
Solution
Blank
Flask 1
Flask 2
Flask 3
Flask 4
Volumes (mL)
Na intermediate
0
1
5
10
20
K intermediate
0
2.5
5
10
25
1000 mg/L
5
5
5
5
5
5. Pipette 5 mL of the 1000mg/L Li stock solution and 10 mL of degassed beer into a 100
mL volumetric flask, and make up to the mark with de-ionised water.
General instrument set-up for flame photometry
• Turn on the compressed air and then the gas. Light the flame using the ignition switch on
the instrument.
• While aspirating water through the capillary tube, adjust the gas/fuel mixture so that sharp
blue cones are formed at the burner holes.
• Insert the correct filter for the element you are reading
• Aspirate the most concentrated standard, and set the instrument to give a convenient
reading by selection of the appropriate range (100 is a useful number but it does not
matter where you start). Wipe the capillary with a tissue then rinse with water before
placing in another solution.
• Aspirate the blank and set the zero with the zero or blank control. Recheck the top
standard, then the blank again until the readings are steady.
• Read and record all solutions, rinsing well with water and the next solution between each
reading.
• Aspirate water to rinse out the instrument before turning if off.
• Turn off the gas first followed by the air.
II. Direct intensity method for determination of sodium
6. The teacher will demonstrate the method of set up for the flame photometer available in
your laboratory. Use the blank to set the zero value and the 20 mg/L Na standard to set the
100 (or an appropriate) emission value.
7. Aspirate standards A-D and the diluted sample and record the sodium emission value.
Between each sample, ensure that you aspirate some deionised water to flush the system.
III. Internal standard method for determination of Na and K
8. Set the zero with the blank
9. Using flask 4, set the sodium emission to 100 (or an appropriate value)
10. Aspirate each standard and sample, reading the emission intensities for Na and Li by
moving the filter selector. Do not adjust the sensitivity dials for lithium, simple record the
number.
11. Repeat steps 8-10 for K (and also Li) in place of Na
IV. Evaluation of matrix effects on emission intensity
12. Record the emission intensities for sodium of the following solutions, which will be
provided: 10 mg/L Na, 10 mg/L Na + 10 mg/L K; 10 mg/L Na + 100 mg/L K. Use the 10
mg/L sodium solution to set the instrument to 100.
CALCULATIONS
Graphing emission intensities against concentration will often result in curves not straight line
plots. To overcome this problem it may be necessary to plot the log of the intensity against the
concentration. If this does not ‘straighten’ the curve you may need to consult your teacher.
1. For the direct intensity set of results, plot the emission intensity against the concentration
of Na in the standards.
2. If the line is not linear, you will need to plot the log of the intensity against concentration.
3. Read the Na value for the sample from the graph.
4. Correct the Na value by the dilution factor.
5. For the internal standard method calculate the following ratios:
6. Plot the ratio against the concentration
7. If the line is not linear, you will need to plot the log of the ratio against concentration
8. Read the values for Na and K of the appropriate graph
9. Correct the value for the dilution factor.
DISCUSSION
• compare the Na results to each other
• evaluate which method (direct intensity or internal standard) should be more accurate
• comment on preparing a mixed standard with both Na and K as compared to separate
standards
• comment on the reason for the different intensity values for the sodium solution in Part IV
QUESTIONS
1. Why are flame photometers only built for analysis of Na, K, Li and occasionally Ca?
2. Flame photometers are relatively inexpensive because they do not contain a
monochromator to separate the radiation into its different wavelengths. How does the
photometer distinguish between the light emitted by Na, K and Li (ie what does the switch
between the three elements do)?
3. Explain why flame temperature variations cause problems in flame emission instruments,
but not in AAS.
S10. RESULTS SHEET
Date of analysis
II. Direct intensity for determination of sodium
Conc Na
Emission
intensity
log emission
intensity
0
1
5
10
20
Beer
III. Internal standard method for determination of Na and K
Na Conc
Na Int
Li Int
Na Int/ Li Int
K Int
Li Int
K Int/ Li Int
0
1
5
10
20
Beer
K Conc
0
5
10
20
50
Beer
IV. Evaluation of matrix effects on emission intensity
Solution
Na intensity
10 mg/L Na
10 mg/L Na + 10 mg/L K
10 mg/L Na + 100 mg/L K
Have you?
Date
Signature
Completed the instrument log
Completed the sample register
Completed the standard register
Teachers signature