FLAME PHOTOMETRY

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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.

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