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Introduction
Introduction
Carbon steel
Plain carbon steel contains a certain amount of carbon, silicon, sulphur, phosphorus and manganese. For special purpose, varying amount of other elements such as chromium, vanadium, molybdenum, tungsten, titanium, nickel, cobalt, zirconium and copper are added. The physical properties of steel depend highly on the content of these elements. For example, stainless steel has a high chromium content that forms an invisible layer on the steel to prevent corrosion and staining (Figure 1). Thus, the quantitative analysis of these elements is of great practical importance.
In this experiment, manganese (Mn) in carbon steel will be determined spectrophotometrically as the purple color permanganate ion, MnO4¯. This is commonly used and accurate method of determining the low concentrations of manganese in steel.
Figure 1: Powdered stainless steel with its products.
Analytical technique used for the determination of Mn
In this experiment, the steel sample is dissolved in nitric acid to give a solution of manganese (II) ions. The periodate ion, added as the potassium salt, KIO4, readily oxidizes manganese (II) to permanganate according to the following equation:
2Mn2+ + 5IO4-+ 3H2O --> 2MnO4- + 5IO3- + 6H+
Note: Dissolving a solid sample in an acid is called “digestion”. Digestion is one of the sample preparation techniques for the analysis of metal ions in solid samples. Metal ions that trapped in the solid matrix need to be taken out from the matrix for analysis as most of the analytical techniques only analysed samples in liquid form.
In this experiment, UV-Vis spectrophotometer will be used to determine the concentration of MnO4- which represents the Mn2+. UV-Vis spectrophotometric is a fast, simple and inexpensive method to determine the concentration of an analyte in solution. It can be used for relatively simple analysis, where the type of compound to be analyzed (‘analyte’) is known, to do a quantitative analysis to determine the concentration of the analytes. In UV-Vis, a beam with a wavelength varying between 180 and 1100 nm passes through a solution in a cuvette. The sample in the cuvette absorbs this UV (λ < 400 nm) or visible radiation (λ > 400 nm). The amount of light that is absorbed by the solution depends on the concentration, the path length of the light through the cuvette and how well the analyte the light absorbs at a certain wavelength. Through Beer-Lambert law, the concentration of analyte can be determined using UV-Vis spectrophotometer. According to Figure 3, I0 is always larger than I. The value of I is reduced with increasing concentration of solution (darker solution) and consequently increased the value of absorbance (A). According to Beer-Lambert law, the absorbance (A) is proportional to the concentration of analyte (c). Please refer Video 1 for further explanation of Beer-Lambert law.
For quantitative analysis, the calibration curve is used to determine the concentration of analyte in the sample. A calibration plot can be prepared by preparing a series of standard solution with a range of concentration and measures the absorbance (A) of each standard solution using UV-Vis spectrophotometer. To increase the accuracy of the measurement, the permanganate solution can be standardized first using a primary standard, sodium oxalate. The oxalate anion, C2O42- reduces permanganate to manganese (II) in acid solution at 60-70℃ according to the following equation:
2MnO4- + 5C2O42- + 16H+ --> 2Mn2+ + 10CO2 + 8H2O
Figure 2: UV/Vis Spectrophotometer.
Figure 3: Beer-Lambert law.
Methodology
Methodology
Standardization of Permanganate with Oxalate (Video 2)
An ~1.000 g Mn dm-3 solution will be supplied. Standardize this solution with oxalate solution as follows:
Weigh out accurately about 1.6 g of sodium oxalate and make up to 250 cm3 in a standard flask.
In a fume hood, acidify a 25 cm3 aliquot with 5 cm3 of 5 mol dm-3 sulfuric acid, warm the mixture to 60-70℃ and titrate with potassium permanganate until a faint pink coloration persists for at least 30 seconds.
Determination of the Calibration Curve
Accurately dilute the standard potassium permanganate solution and prepare a series of five standards which give an absorbance range between 0.1 to 0.9. Measure the absorbance of these five solutions using a spectrophotometer set at 525 nm. Use water as the reference solution.
Determination of manganese in steel
Safety note: This part of the experiment should be carried out in the fume hood
Accurately weigh out duplicate sample (approx. 0.2 g) of the steel sample into 150 cm3 beakers.
Cover the beaker with watch glass; add 30 cm3 of 1:1 nitric acid.
Warm to dissolve the alloy (add further nitric acid if necessary) and then heat to gentle boiling for a few minutes to expel oxides of nitrogen. (Video)
Cautiously add about 1 g ammonium peroxydisulphate and boil for 10-15 minutes. If the solution is pink or contains brown oxide of manganese (as a deposit of MnO2) add about 0.1 g sodium bisulphate and heat for 5 minutes.
Cool, rinse down the watch glass and transfer the solution quantitatively to a 100 cm3 volumetric flask and dilute to the mark with distilled water.
Pipette two 25 cm3 aliquots of the sample solution into small beakers and add 5 cm3 of phosphoric acid.
To one of the two aliquots add 0.5 g KIO4 and boil the solution for 5 minutes.
The second aliquot is not treated with periodate and will serve as the blank.
Cool to room temperature, transfer each aliquot quantitatively to a 50 cm3 volumetric flask and dilute to the mark with distilled water.
Measure the absorbance of the solution and the blank using distilled water as the reference solution.
Results
Results
Weight of Sodium Oxalate = 1.6025 g
Experiment was carried out in duplicate:
Weight of steel sample (A) = 0.2010 g
Weght of steel sample (B) = 0.2015 g
Table 1: Result of the standardization of permanganate solution.
Table 2: Results from the analysis of standard solution for calibration.
Table 3: Steel samples analysis.
Task
Task
Prepare a technical report based on the given data in the above section.
Calculate the concentration of permanganate solution based on Table 1 data. Present the concentration in the unit of g Mn dm-3 .
Prepare the calibration plot for Mn based on the data in Table 2.
Determine the concentration of Mn based on the data presented in Table 3. Then, calculate the percentage of manganese in steel.
Answer the following question:
Question:
Question:
After going through the experiment, explain what is calibration plot. What is the role of the calibration plot in the chemical analysis?
Explain in brief, what is colorimetric analysis.
Define spectrophotometry.
What is the purpose of measuring blank?
Describe in brief, what is sample preparation in analytical chemistry.
Additional videos
Additional videos
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