FLAME PHOTOMETRY
Background
Flame photometry is an atomic emission method for the
routine detection of metal salts, principally Na, K, Li, Ca, and Ba.
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. Light is emitted
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. Comparison of emission intensities of
unknowns to either that of standard solutions, or to those of an internal
standard, allows quantitative analysis of the analyte metal in the sample
solution.
Flame photometry is a simple, relatively inexpensive, high
sample throughput method used for clinical, biological, and environmental
analysis. The low temperature of the natural gas and air flame, compared to
other excitation methods such as arcs, sparks, and rare gas plasmas, limit the
method to easily ionized metals. Since the temperature isn't high enough to
excite transition metals, the method is selective toward detection of alkali and
alkali earth metals. On the other hand, the low temperatures renders this method
susceptible to certain disadvantages, most of them related to interference and
the stability (or lack thereof) of the flame and aspiration conditions. Fuel and
oxidant flow rates and purity, aspiration rates, solution viscosity,
concomitants in the samples, etc affect these. It is therefore very important to
measure the emission of the standard and unknown solutions under conditions that
are as nearly identical as possible.
This experiment will serve as an introduction to sodium
analysis by flame emission photometry and will demonstrate the effects of
cleanliness and solution viscosity on the observed emission intensity readings.
The instrument is calibrated with a series of standard solutions
that cover the range of concentrations expected of the samples. Standard
calibrations are commonly used in instrumental analysis. They are useful when
sample concentrations may vary by several orders of magnitude and when the value
of the analyte must be known with a high degree of accuracy. This experiment
does not produce hazardous waste.
Procedure
Consult your Teaching Assistant for operating instructions
for the Buck PFP-7 Flame Photometer. Allow a sufficient warm-up period. Be sure
to aspirate deionized-distilled water between samples to clean out the sample
tube and aspirator. Sodium is ubiquitous. It is imperative that you use
scrupulously cleaned glassware to obtain good results.
Standard Preparations
Prepare sodium chloride standard solutions by volumetric
dilution of the stock solution. The following approximate concentrations
should be made: 5, 10, 25, 50, 75, and 100 mg/mL
as Na. Be sure to use clean methods. Use ultra-pure deionized-distilled water to
clean your glassware and for dilution of the 1000 mg/mL
standard. Prepare these standards in scrupulously clean volumetric glassware and
transfer the solutions to plastic bottles. Glass often is made from high sodium
glass. Allowing extremely high or low pH solutions to stand in glass could alter
the sodium concentrations in solution. Prepare 25 mg/mL
Na solutions in other solvents, 10% Ethanol, 50% Ethanol, 50% Glycerin. Standard
solutions may be pre-prepared by the laboratory instructor or may be made up as
a class or group project.
Unknown Preparation
Obtain a sodium unknown from your instructor in a
scrupulously clean 50 mL volumetric flask. Dilute to the mark with distilled
water.
Instrument Calibration
Set the readout to zero using distilled water as a blank.
Set the peak reading according to the instrument instructions using the most
concentrated sodium solution (100 mg/mL).
Measure the emission intensity of each of the remaining sodium standard
solutions, and of the sodium unknown solution. Check for accuracy and
repeatability by measuring the standards several times. Be sure to aspirate
deionized distilled water between measurements.
Cleanliness Check
Dip two fingers into a clean beaker containing about 20 mL
of distilled water. Measure and record the sodium emission intensity. Measure
and record the sodium emission intensity of tap water. Be sure to aspirate
deionized distilled water between measurements.
Viscosity Variations
Measure and record the sodium emission intensity of each
of the 25 mg/mL Na
solutions in various solvents. Be sure to aspirate deionized distilled water
between measurements.
Reproducibility Check
Remeasure the emission intensity of two or three of the
standard solutions. If a significant change has occurred, READJUST the zero and
peak readings, and RE-MEASURE the emission intensity of the standards and the
unknown.
Results
Plot a working curve from the data obtained in the
Instrument Calibration step. Calculate and report the sodium concentration of
the unknown, "fingered" solution, and tap water in units of
mg/ml.
Plot intensity reading as a function of viscosity for the
data of Viscosity Variation step.
Turn in the two graphs and the sodium concentrations of
the unknown, "fingered" solution, and tap water. Grades will be based 80% on the
reported unknown concentration and 20% on graphs. Tap water and "fingered"
solution data is for our future reference.
Reference
- 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
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