Photometric Determination of Manganese in Steel
(Comparison of solution photometry and atomic absorption spectrometry)
The purpose of this experiment is to:
o improve proficiency with quantitative handling of materials and laboratory measurements, particularly where several chemical conversion steps and multiple dilutions are required).
o provide an introduction to the fundamental concepts of absorption spectrophotometry (Fig. 1), comparing the capabilities of two different techniques [solution photometry and flame photometry (atomic absorption)] for determining the same substance.
o provide one (of three) proficiency test(s) designed to certify each student as qualified to perform instrument and reagent validation operations required by Good Laboratory Practice guidelines, and to conduct more complex analytical studies.
o introduce the concepts of standard addition determinations (Fig. 2) and calibrations constructed by linear regression (atomic absorption). These methods will be used in later analytical studies (B.2, B.3).
Figure 1. Spectrophotometric experiment.
Figure 2. Standard addition procedure.
Synopsis of the Analytical Problem
The manganese content of a steel sample is to be determined by two different techniques. Both require the dissolution of the sample. The solution photometry method requires that the Mn be oxidized to the MnO4- ion (purple color), so that the visible color intensity of the solution can be measured. The atomic absorption method only requires that all the manganese be dissolved, but not necessarily in the same oxidation state. Thus, the procedure involves first dissolving all the manganese in a measured steel sample, and filtering out any undissolved solids. This solution can then be used directly for determination by atomic absorption. A portion of this solution must then be treated with an oxidizing agent to convert all of the Mn ions to the MnO4- state, so that this latter portion can be used for solution photometry. Because a calibration curve for solution photometry is affected by other substances in the steel sample (the "matrix"), it is more convenient to use a standard addition measurement procedure. For the atomic absorption method, however, either a calibration curve or standard addition may be used. For this study you will use both methods for the atomic absorption determination and compare the values obtained. Comparison of results from both photometric techniques is also desired.
Significance of the Manganese Determination
Materials with specific characteristics are crucial for the fabrication of modern structures, microelectronic devices, and other durable goods. The properties of these materials are typically dependent on the concentrations of minor components. Various steel alloys have differing properties dependent on the relative and absolute content of certain minor amounts of various metals, such as manganese. The analytical determinations of these minor components are crucial to providing a reproducible product with predictable properties.
This analysis is based upon the color imparted to a solution by the permanganate ion. A known weight of steel is dissolved in dilute nitric acid. Any carbon in the sample is eliminated by treatment with potassium peroxodisulfate:
2S2O82- + C + 2H2O ® CO2 + 4SO42- + 4H+
Conversion of Mn2+ to MnO4- is undesirable at this point and does not ordinarily occur. If the need arises, ammonium hydrogen sulfite is added to reduce any permanganate that does form:
5HSO3- + 2MnO4- + H+ ® 2Mn2+ + 5SO42- + 3H2O
Each solution is then diluted to a known volume, and three aliquots are withdrawn. [NOTE: the remaining solution is saved for the subsequent atomic absorption determination.] The first aliquot is the sample itself. A precisely known amount of manganese(II) (the standard addition) is introduced to the second. The third serves as the blank. Phosphoric acid is added to each aliquot to eliminate iron(III) as a source of interference (a colorless complex is produced). Potassium periodate is then added to the first two aliquots; upon heating, oxidation to permanganate occurs:
2Mn2+ + 5IO4- + 3H2O ® 2MnO4- + 5IO3- + 6H+
Each solution is diluted to a known volume, following which the absorbance due to permanganate is measured at 525 nm. The data are then used to calculate the percentage of manganese in the sample.
Glassware needed from stockroom:
Volumetric flasks (G.S.): (5) 50 mL; (2) 250 mL; (1) 100 mL
Pipets: (1) 10 mL; (1) 25 mL
(8) 13x100 test tubes, dust- and fingerprint-free
(3) 150 mL beakers
A. Preliminary Steps
(NOTE: Best results are obtained when solution photometry measurements are made on the same day that sample solutions are prepared.)
A.1 Clean steel samples, if so directed, with acetone; air dry, and then oven dry at 110oC for about 5 minutes. Permit samples to cool in a desiccator.
A.2 Weigh duplicate (NOTE 1) samples (to the nearest 0.1 mg) into individual 250 mL beakers. Record the weights and label these samples 1 and 2. Dissolve each in 50 mL of 3M HNO3; cover, heat (in the hood) near the boiling point until dissolved, or for about one hour if there are undissolved particles. Replace the HNO3 as it evaporates.
A.3 After the solutions have cooled, slowly introduce about 1 g (NOTE 2) of ammonium peroxodisulfate (K2S2O8 may be used instead); boil gently for 10 minutes to remove the excess oxidant. If a pink tinge (or traces of a brown precipitate) is observed, cool, add 6 drops of 45% ammonium hydrogen sulfite and boil for an additional 5 minutes.
A.4 Permit the solutions to cool to room temperature; then transfer quantitatively with filtering to individual, labeled, 250 mL volumetric flasks. (Use #42 filter paper; or #41 paper if gelatinous solids are observed). Wash beaker and precipitate thoroughly with hot 0.05M HNO3. Dilute to volume and mix well. These solutions will be used in steps C and D. NOTE: Transfer about 100 mL of solution from each flask into two clean, dry erlenmeyer flasks; stopper and label these flasks as corresponding samples 1 and 2; save these for later analysis by atomic absorption.
B. Preparation of Solution for Standard Addition
Dilute a 10.00 mL aliquot of the standard manganese solution to 100 mL in a volumetric flask. Each mL of this diluted solution will contain 0.100 mg of Mn and will increase the absorbance of 50 mL by approximately 0.09 units.
C. Selection of "Matched" Test Tubes
It is necessary to find, by experiment, two test tubes which absorb similarly. (One tube will contain the blank solution, the other will contain the sample solutions in turn.) Clean eight 13 x 100 mm test tubes and fill with water to about 3 cm depth. (See instructions for operation of the Bausch and Lomb Spectronic 20 Spectrophotometer, Appendix 3.) Adjust the dark current and set the wavelength at 525 nm. It is standard procedure to wipe the exterior of the tubes with lint-free tissue prior to measurement. Establish a reproducible method for placement of tubes in the cell compartment. Insert a tube and adjust the light control until the absorbance measures about 0.300. Read and record the absorbance to three figures. Now insert a different tube and, without changing any controls, read and record absorbance. Continue until all eight tubes have been measured, then select the best matched pair.
D. Preparation and Measurement of Samples
D.1 To obtain an estimate concerning a proper volume for a standard addition, pipet 25 mL portions of sample 1 to two small beakers, add about 5 mL (graduate cylinder) of 85% phosphoric acid to each, and no more than 0.4 g of potassium periodate, KIO4, to one of the beakers; the solution in the second beaker will serve as the blank and should contain no periodate. Generate permanganate by heating (hood) until the KIO4 dissolves; swirl frequently. (The KIO4 dissolves very slowly; time will be saved if it is ground to a fine powder in a mortar prior to addition.) Quantitatively transfer sample and blank to 50 mL volumetric flasks (NOTE 3), dilute to volume and mix. Rinse, then fill the matched tubes with the blank and the permanganate solution. Measure the absorbance of the permanganate solutions against the blank (see operating instructions). Calculate a suitable volume of standard solution to be added to the samples. (Realize that the sum of the absorbance of the sample plus standard should not be more than 0.7, and that each mL of standard increases the absorbance about 0.09.)
D.2 Now transfer an additional 25 mL aliquot from sample 1 and two 25 mL aliquots from sample 2 to beakers. Add the calculated volume of standard solution (pipet or buret) to the beaker containing sample 1 and to one of the beakers containing sample 2. Add 5 mL of 85% phosphoric acid and 0.4 g of KIO4 to the beakers and heat as before to generate permanganate. Cool, transfer to 50 mL volumetric flasks, and measure the absorbance. (Remember to rinse the test tube containing the sample solution with several small rinses of the solution before filling the tube for measurement.) It may be wise to re-measure the solution prepared in D.1 at the same time these solutions are measured. It is best to measure absorbance of solutions on the same date that permanganate is generated.
See subsequent Spectrometry Report Section.
(1) The lab instructor will indicate an appropriate sample size to weigh out. If the individual steel samples (1 and 2) weigh within 50 mg of one another, a single blank can be used for both.
(2) The amounts of potassium peroxodisulfate, and potassium periodate need not be known with great accuracy. Use a top-loading balance for these weighings.
(3) The final dilution must be performed with some care, since it involves the transfer of approximately 30 mL of solution with a maximum permissible volume of 50 mL. Use a small funnel or a stirring rod to transfer the bulk of the solution; then rinse the flask with several small portions of distilled water.