Detection Target
Determination of manganese (and carbon, phosphorus, copper, nickel, vanadium, chromium, boron, sulfur) content in steel.
Overview
This solution conforms to the ISO 629 Steel and cast iron — Determination of manganese content — Spectrophotometric method. This International Standard specifies a spectrophotometric method for the determination of manganese in steel and cast iron. The method is applicable to products having manganese contents between 0.001 and 4 % (m/m).
Principle
A steel sample of known mass is dissolved in a mixed acid consisting of nitric acid, sulfuric acid, and phosphoric acid. The following chemical reactions occur:
Fe + 6HNO3 = Fe(NO3)3 + 3NO2↑ + 3H2O
Mn + 4HNO3 = Mn(NO3)2 + 2NO2↑ + 2H2O
Fe3+ + 2H3PO4 = [Fe(H2PO4)2]+ + 2H+
2Mn2+ + 5S2O82- + 8H2O = 2MnO4-+ 10SO42- + 16H+
The resulting MnO4- solution is measured for absorbance at a wavelength of 530 nm using a spectrophotometer, with a blank sample as the reference solution.
A series of standard MnO4- solutions with known concentrations are prepared and treated under the same conditions, and their absorbance values are measured using the spectrophotometer. A calibration curve (working curve) is plotted with absorbance (A) on the y-axis and standard solution concentration (c) on the x-axis.
By referring to this calibration curve, the concentration corresponding to the sample solution's absorbance can be determined, allowing the manganese content in the steel sample to be calculated.
Instruments and Reagents
1. Instrument: HKL-629 Spectrophotometric for Determination of manganese in steel, Analytical balance, Volumetric flask (50 ml), Pipette (10 ml), Graduated pipette (5 ml), Dropper, Rubber bulb (for pipetting), Alcohol lamp
2. Reagents: HNO3-H2SO4-H3PO4 Mixture (1:1:1 ratio), 1% AgNO3 (silver nitrate solution), KMnO4 standard solution (Mn 1 mg·ml-1), (NH4)2S2O8 (15%), Steel sample
Test Procedure
1. Preparation of Standard KMnO4 Solutions
Using a pipette, transfer 10 ml of the standard KMnO4 solution into a 100 ml volumetric flask, then dilute to the mark with deionized water. Seal the flask and mix well. Next, take six 50 ml volumetric flasks. Using a pipette (or graduated pipette), add the prepared standard KMnO4 solution, mixed acid, (NH4)2S2O8, and AgNO3 to each flask. Dilute to the mark with deionized water, seal the flasks, and mix thoroughly.
2. Steel Sample Preparation and Solution Preparation
Weigh 60–80 mg of the steel sample accurately using an analytical balance and transfer it into a 50 ml beaker. Add 17 ml of mixed acid and heat on a low-temperature hot plate in a fume hood to dissolve the sample.
Once brown NO2 gas ceases to evolve, add 10 ml of (NH4)2S2O8 and 3 ml of AgNO3 solution, and then continue heating until boiling. After boiling for 1 minute, remove from heat. Allow the solution to cool to room temperature, and then transfer it completely into a 50 ml volumetric flask. Dilute to the mark with deionized water, seal the flask, and mix well.
3. Absorbance Measurement
Set the spectrophotometer wavelength to 530 nm. Using a 0.5 cm cuvette, measure the absorbance of the five standard KMnO4 solutions and the steel sample solution.