Manganese Determination by Atomic Absorption Spectrometry

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Procedures

 

This determination is based on a flame photometry method utilizing atomic absorption (AA). The liquid sample containing manganese ions is aspirated into a very well-defined and shaped flame. The ions are reduced in the flame to atoms. Light from a manganese lamp passes through the flame and is absorbed specifically by the manganese atoms in the flame. With a constant aspiration rate (flow), and consistent formation of sample aerosol droplets in the flame, the concentration of Mn atoms in the flame is proportional to the concentration in the liquid sample. Absorption of the light from the manganese lamp by the Mn atoms in the flame follows Beer's Law. The instrument obtains the flame absorbance of a sample by comparing the transmittance (P) with that of a blank solution (Po).

 

A = log[Po/P] = abC(unk)

 

Two methods are used to obtain the Mn concentration. One is the standard addition approach used with solution photometry. The other involves the construction of a calibration curve, using a linear regression analysis fit to absorbance data obtained for five standard solutions bracketing the range of absorbance for the unknown sample. [Procedures for each are in Appendices 4 and 5.]

 

 

Preparation of Solutions

 

 

Dilutions. Results of the solution photometry experiment should provide an idea of the Manganese concentration of the saved solutions 1 and 2 (refer to procedure A.4 in solution photometry section). Convert the concentration to ppm Mn. Keep in mind that the linear concentration range for absorbance measurements of Mn by AA is about 0.2 to 5 ppm. (The useful range is different for different metals.) Thus, the saved samples should be diluted (if necessary) by a dilution factor needed to obtain solutions of about 2 to 3 ppm. [NOTE: Record all dilution factors, so that subsequent measurements can be converted back to the original Mn content of the steel sample.)

 

 

Filtering. The saved samples 1 and 2 should have been filtered. However, if they were not filtered, or if any particles are observed floating or on the vessel bottom, re-filter the solution (see A.4 of solution photometry procedure). (The presence of particulate matter in AA samples can plug up the aspiration line or the burner.)

 

The blank solution used for the AA measurements can be simply deionized water.

 

 

Standards. For the calibration curve method, prepare five standard solutions in five 100 mL volumetric flasks, each solution being 1.0, 2.0, 3.0, 4.0, and 5.0 ppm, respectively. These solutions can be prepared by starting with the standard manganese stock solution (1.00 mg/mL); dilute the stock solution to 0.100 mg/mL (pipet 10.0 mL into a 100 mL volumetric flask and dilute to volume); then prepare the 5 calibration standards by pipeting 1.0, 2.0, 3.0, 4.0, and 5.0 mL, respectively, of the 0.100 mg/mL solution into each of 5 separate 100 mL volumetric flasks, and diluting each to volume. Carefully label each of the flasks.

 

For the standard addition method, three equal aliquots of each of the saved sample solutions should be added to each of three separate volumetric flasks (six flasks, three for each sample). The aliquots and flask size should be chosen to produce about 2 ppm Mn after dilution. Before dilution, standard additions should be added (by pipet or buret) into two flasks for each sample. The standard amounts added should be chosen so that the additions, after dilution, equal exactly 1.0 ppm and 2.0 ppm, respectively. Thus, six flasks with the compositions illustrated below should be prepared:

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Figure2. Solutions prepared for standard addition AA procedure.