CH 104: DETERMINATION OF MANGANESE IN STEEL THE MANUFACTURE OF IRON Iron is made from ore (usually hematite, Fe2O3) and carbon monoxide (CO) in a huge reactor called a blast furnace. A charge of iron ore, coke (coal cake, C), and limestone (CaCO3) are added at the top of the furnace, and a blast of hot air is sent up from the bottom. The coke burns at 1,500° C to 2,000° C and makes CO. 2C(s) + O2(g) → 2CO(g) As the charge falls, the CO(g) raises and causes the Fe2O3 to be reduced to Fe. Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g) The CaCO3 removes the impurities as slag. CaCO3(s) → CaO(s) + CO2(g) CaO(s) + SiO2(s) → CaSiO3(l) Lime + Sand → Slag

THE MANUFACTURE OF IRON The iron made by a blast furnace is called pig or cast iron. It is brittle and has approximately 4% C and smaller amounts of other impurities such as Mn, P, S, and Si.

THE MANUFACTURE OF IRON A modern blast furnace produces nearly 10,000,000 kg (11,000 tons) of pig iron per day. This pig iron is used to manufacture steel.

THE MANUFACTURE OF STEEL Approximately 800 million tons of steel is produced each year. That is, over 250 pounds of steel is made for each person on Earth each year. Steel is refined Fe that has less than 1.7% C. This process has 3 requirements. First, the approximately 4% C in pig iron is lowered to less than 1.7% C. Second, the Mn, P, S, and Si impurities from pig iron are removed as slag. Third, alloying elements, such as Cr, Mn, Mo, Ni, V, and W, are added to give the steel its desired properties.

THE MANUFACTURE OF STEEL Molten pig iron from the blast furnace is typically poured into a basic oxygen furnace and made into steel. Some of the C in pig iron is oxidized with O2(g) and removed as CO(g) and CO2(g). The inorganic impurities in pig iron are oxidized with O2(g), reacted with “basic oxides”, and removed as slag. P4(l) + 5O2(g) → P4O10(l) 6CaO(s) + P4O10(l) → 2Ca3(PO4)2(l) Basic Oxide + Acidic Oxide → Slag

THE MANUFACTURE OF STEEL Finally, alloying elements, such as Cr, Mn, Mo, Ni, V, and W, are added to give the steel its desired properties. For example, Cr, Mo, and Ni give corrosion resistance to stainless steels. Manganese (Mn) makes steel easier to deform at high temperatures. It is added to help the rolling and forging steps of steel production. In today’s experiment you will measure the Mn content of steel. You will be graded on the accuracy of your result.

Mn(s) + 4HNO3(aq) → Mn2+(aq) + 2NO3–(aq) + 2NO2(g) + 2H2O(l) SAMPLE PREPARATION Tare a 250 mL Erlenmeyer flask. Accurately weigh 0.3 g of steel to 3 significant digits into this flask. For example, your sample might weigh 0.306 g. Record this mass in your data sheet. Perform this step in a FUME HOOD. CAREFULLY add 50 mL of 6 M (dilute) nitric acid (HNO3). If necessary, carefully heat the reaction mixture on a hot plate until all the steel is dissolved. The purpose of this step is to dissolve the manganese. Mn(s) + 4HNO3(aq) → Mn2+(aq) + 2NO3–(aq) + 2NO2(g) + 2H2O(l)

SAMPLE PREPARATION Perform this step in a FUME HOOD. CAREFULLY add 1 g of ammonium peroxydisulfate ((NH4)2S2O8) and boil gently for 10 minutes. The purpose of this step is to remove any color from carbon-containing compounds. 2S2O82–(aq) + C(aq) + 2H2O(l) → CO2(g) + 4SO42–(aq) + 4H+(aq) Perform this step in a FUME HOOD. CAREFULLY add 0.1 g of sodium hydrogen sulfite (NaHSO3) and heat for another 5 minutes. The purpose of this step is to remove any color from permanganate ion (MnO4–). 5HSO3–(aq) + 2MnO4–(aq) + H+(aq) → 2Mn2+(aq) + 5SO42–(aq) + 3H2O(l) Perform this step in at your bench. You are done using the fume hood. Cool and dilute the solution to exactly 100 mL in a volumetric flask. Shake well to mix.

5IO4– + 2Mn2+ + 3H2O → 2MnO4– + 5IO3– + 6H+ SAMPLE PREPARATION Pipet three 25 mL aliquots of the sample into small beakers. Treat these aliquots by adding the following reagents: Boil aliquots 1 and 2 gently for 5 minutes and cool. Dilute each of the 3 aliquots to 100 mL in volumetric flasks. The nearly colorless manganese(II) ion (Mn2+) is oxidized to the purple permanganate ion (MnO4–). 5IO4– + 2Mn2+ + 3H2O → 2MnO4– + 5IO3– + 6H+ You will measure the concentrations of MnO4– in these 3 aliquots with a spectrophotometer at 525 nanometers (nm). Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g

SAMPLE ANALYSIS BY STANDARD ADDITION The standard addition method is used eliminate or reduce interference from the sample matrix. It has 3 steps. First, the signal from a sample is measured. Second, the signal from a mixture of this sample and a known amount of standard is measured. Third, these data are used to calculate the concentration of analyte in the sample.

SAMPLE ANALYSIS BY STANDARD ADDITION Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g Dilute each of the 3 aliquots to 100 mL in volumetric flasks. What is the purpose of Aliquot 1? All the Mn is from the sample. All this Mn is oxidized to MnO4–. Aliquot 1 is used to measure the concentration of Mn in the sample. What is the purpose of Aliquot 2? All the Mn is from the sample and the standard. All this Mn is oxidized to MnO4–. Aliquot 2 is used to measure the total concentration of Mn in the sample and in the standard. What is the purpose of Aliquot 3? All the Mn is from the sample. None of this Mn is oxidized to MnO4–. Aliquot 3 is used to set the spectrophotometer to 0 absorbance (100% transmittance). This accounts for any color from the sample not from MnO4–. Therefore, aliquot 3 is a blank and it is used to remove interferences.

SAMPLE ANALYSIS BY STANDARD ADDITION Applying Beer’s Law to this experiment: A = abc A= absorbance a = the absorptivity constant for MnO4– b = the path length of the sample cell c = the concentration of MnO4– in the sample cell Since a (absorptivity) and b (path length) are constant for a given wavelength and a given sample cell, A (absorbance) is directly proportional to c (concentration):

SAMPLE ANALYSIS BY STANDARD ADDITION Furthermore, absorbances are additive: Atotal = A1 + A2 + A3 + … The absorbance of aliquot 1 (Aaliquot1) is caused by the Mn from the sample being oxidized to MnO4–. Therefore, the absorbance of aliquot 1 (Aaliquot1) equals the absorbance of the sample (Asample). A1 = Aaliquot1 = Asample The absorbance of aliquot 2 is caused by the Mn from the sample and standard being oxidized to MnO4–. Therefore, the absorbance of aliquot 2 (Aaliquot2) equals the absorbance of the sample (Asample) plus the absorbance of the standard (Astandard). A2 = Aaliquot2 = Asample + Astandard

SAMPLE ANALYSIS BY STANDARD ADDITION Solving for the concentration of Mn in the sample cell: cstandard is the concentration of Mn in the sample cell. It is NOT the concentration of Mn in the bottle of “Standard Mn”. csample is the concentration of Mn in the sample cell. It is NOT the concentration of Mn in steel.

SAMPLE ANALYSIS BY STANDARD ADDITION A 0.306 g sample of steel is oxidized, dissolved, diluted to 100.0 mL, and prepared as follows. Afterwards, each of the 3 aliquots were diluted to 100.0 mL. The Standard Mn solution contained 102.6 mg of Mn / L. Aaliquot1 equaled 0.152. Aaliquot2 equaled 0.376. What is cstandard? Again, cstandard is the concentration of Mn in the sample cell. It is NOT 102.6 mg of Mn / L. Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g

SAMPLE ANALYSIS BY STANDARD ADDITION A 0.306 g sample of steel is oxidized, dissolved, diluted to 100.0 mL, and prepared as follows. Afterwards, each of the 3 aliquots were diluted to 100.0 mL. The Standard Mn solution contained 102.6 mg of Mn / L. Aaliquot1 equaled 0.152. Aaliquot2 equaled 0.376. What is csample? Again, csample is the concentration of Mn in the sample cell. It is NOT the concentration of Mn in steel. Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g

SAMPLE ANALYSIS BY STANDARD ADDITION A 0.306 g sample of steel is oxidized, dissolved, diluted to 100.0 mL, and prepared as follows. Afterwards, each of the 3 aliquots were diluted to 100.0 mL. The Standard Mn solution contained 102.6 mg of Mn / L. Aaliquot1 equaled 0.152. Aaliquot2 equaled 0.376. How many grams of Mn are in this steel sample? Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g

SAMPLE ANALYSIS BY STANDARD ADDITION A 0.306 g sample of steel is oxidized, dissolved, diluted to 100.0 mL, and prepared as follows. Afterwards, each of the 3 aliquots were diluted to 100.0 mL. The Standard Mn solution contained 102.6 mg of Mn / L. Aaliquot1 equaled 0.152. Aaliquot2 equaled 0.376. What is the % Mn in this steel? Aliquot Concentrated H3PO4 Standard Mn KIO4 1 2 3 3 to 5 mL 0.00 mL 5.00 mL 0.4 g 0.0 g

SAFETY Give at least 1 safety concern for the following procedure. Using acids (HNO3 and H3PO4), oxidizing agents (HNO3, (NH4)2S2O8, KIO4, and KMnO4), and reducing agents (Mn2+ and NaHSO3). These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water. Your laboratory manual has an extensive list of safety procedures. Read and understand this section. Ask your instructor if you ever have any questions about safety.

SOURCES Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company. McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. Merriam-Webster, Inc. 1987. Webster’s 9th New Collegiate Dictionary. Springfield, MA: Merriam-Webster, Inc. Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company. San José State University. 2007. Photometric Determination of Manganese in Steel. Available: http://www.sjsu.edu/faculty/chem55/55phot.htm [accessed 22 February 2007]. Specialty Steel Industry of North America. 2006. SSINA: Stainless Steel: About. Available: http://www.ssina.com/index2.html [accessed 12 October 2006]. Tro, NJ. 2008. Chemistry, A Molecular Approach. Upper Saddle River, NJ: Prentice Hall.