Spectrophotometric Determination of Manganese in Steel
Manganese (Mn) is a material that is proven to increase the hardenability of steel, and it also decreases the cooling rate during the hardening, this increases how hard the steel gets which is much more efficient. The purpose of this experiment was to determine the percent weight of manganese in an unknown steel sample. The absorbance data from the diluted samples and the absorbance measurement for the unknown sample were both found using the spectrophotometer. The percent weight of manganese was determined by using both the standard steel sample and an unknown steel sample provided by the instructor via the spectrophotometry method. This method is used to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through the sample solution. An illustration of this is shown in Figure 1. The absorbance of 0.176 was used to determine the percent weight by using the molarity of MnO4^- solution that was in the unknown steel sample. It was determined by dividing the grams of manganese by the total mass of the sample, which yielded 0.20%.
Introduction: Principles and Theory
Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. A spectrophotometer is used to measure the intensity of light that is absorbed in a part of a spectrum. Chemist, especially Analytical chemists uses the method of spectrophotometry to measure how much light a chemical substance is reflecting over a range of wavelengths. By using Beer’s Law, the concentration of a chemical species in a sample can be found. Beer’s Law relates to the attenuation of light to the properties of the material through which the light is traveling and it also states that the absorbance is directly proportional to the concentration.
Three chemical reactions are used in this experiment. These includes:
?2Mn?^(2+) (aq)+?5S?_2 O_8^(2-) (aq)+?8H?_2 O(l)??2MnO?_4^- (aq)+10SO_4^(2-) (aq)+?16H?^+ (aq)
?2Mn?^(2+) (aq)+?5IO?_4^- (aq)+?3H?_2 0(l)??2MnO?_4^- (aq)+?5IO?_3^- (aq)+6H^+ (aq
3Mn(aq)+?2NO?_3^- (aq)+?8H?^+ (aq)??2Mn?_2^+ (aq)+2NO(g)+?4H?_2 O(aq)
The purpose of these reactions is so that the oxidation state of manganese is changed. This was used to determine the absorbance of the solutions for the percent weight of manganese.
Figure 1: Diagram of the use of a spectrophotometer. A sample solution was inserted in the spectrophotometer and since a lamp provides the source of light, a beam of light will be shined on it. The solution is rotated, and the monochromator splits the light into wavelength only allowing specific wavelength light to reach the exit slit. The light then interacts with the sample and the detector measures the transmittance and absorbance of the sample.
About 0.60 to 0.65g (round to 4th decimal place) of standard steel sample was weighed and about 0.6 to 0.7g (round to 4th decimal place) of an unknown steel sample was also weighed. A standard solution was then prepared along with the unknown solution by heating and diluting each solution. The solutions were prepared by dissolving both samples in 50 mL of 5N nitric acid. The solutions were boiled for about 1 to 2 minutes to removed oxides of nitrogen. Any black materials in the solution after boil can be ignored when determining the spectrophotometric manganese. About 1g of ammonium persulfate was then added and it was gently boiled again for about 15 mins making sure not to evaporate solution to dryness. If a precipitate of manganese forms or if a purple color forms, add a few drops of sodium sulfite then boil for a few minutes but if there is no formation of a precipitate then continue onto the next step. The solutions were then diluted with deionized water to about 100 mL. About 0.5g of potassium periodate and 10 mL of 85% phosphoric acid were added to the solutions. The solutions were boiled again for about 3 mins to oxidize the manganese to the permanganate ion keeping in mind not to evaporate to dryness. The solutions were cooled and diluted with deionized water to 250.0 mL in a volumetric flask. The stock solution has now been made. It was then used to prepare 4 dilution standards. The stock solution was prepared using the measurements in Table 1. The measurement in Table 2 were used to prepare the 4 dilution standards. After this, the dilutions were transferred into a cuvette and was inserted into the spectrophotometer. The Shimadzu mini-UV instruments were used to obtain a spectrum between 400-700 nm. After the spectrum was acquired, the unknown sample was inserted to determine the absorbance value.
Table 1: Preparation of Unknown and Standard Steel Sample
Standard Steel Sample Unknown Steel Sample
Weights (g) 0.6228 0.6063
Ammonium Persulfate 1.0066 g 1.0042 g
Potassium Periodate 0.5081 g 0.5039 g
Nitric Acid 50.00 mL 50.00 mL
18 M omega-cm Deionized Water 50.00 mL 50.00 mL
85% Phosphoric Acid 10.00 mL 10.00 mL
Table 2: Diluted Standard Solution
Diluted Standards Molarity (M) Volume (mL) Absorbance Deionized Water (mL) Dilution Ratio
Standard 1 0.00023167 M 10.00 mL 0.549 0.00 mL 1
Standard 2 0.00017375 M 30.00 mL 0.433 10.00 mL .75
Standard 3 0.00011584 M 10.00 mL 0.248 10.00 mL .50
Standard 4 0.00005792 M 10.00 mL 0.092 30.00 mL .25
Figure 2: Graph of known/standard sample dilutions.Absorbance vs.Concentrations.Linear Trend
0.6228g standard steel sample ×0.00511Mn con.×(1mole÷54.949g Mn)=(0.000057917mol Mn÷0.250L)=0.00023167 M known
Equation 2: y=mx+b
0.179(unknown 1 absorbance) =2686.6x-0.0585
X = 0.00008840 moles/liter unknown concentration
0.0000221moles X (54.93g Mn/1 mole Mn) = 0.0012362g Mn
Equation 3: % Weight Mn = (g Mn)/ (g Mn) X 100 = (0.0012362g Mn)/ (0.6063g unknown sample) X 100
% Weight Mn = 0.20%
Discussion and Conclusion
Throughout this experiment, the absorbance and concentration of the 4 standard solutions were compared to the absorbance of the unknown sample to find its concentration. By using the equation from the graph (Figure 2), I was able to calculate the concentration of the unknown sample to be 0.00008840 mol. A systematic error that was made was not boiling the solution long enough to properly boil. Another systematic error was not using the right mL of solution when making the diluted solutions. This error caused the percent weight of manganese to be inaccurate. The perfect weight of manganese in the unknown solution was found to be 0.20% which was outside the range of the expected range of 0.05 – 0.15%. This affected how concentrated the diluted standard solution turned out. A random error that was made was not wiping down the cuvette before placing them in the spectrophotometer. This error affected the absorbance of the solutions. Biggest advantage during this lab was how efficient and fast the spectrophotometer was. It was able to produce an absorbance measurement in a matter of second if the correct parameters are set. Some ways to improve the lab techniques is to have a lot of extra flask to prepare solutions and to have more heaters so the heating up of solutions can go a little faster.
Harris, “Quantitative Chemical Analysis”, 8th ed. P. 395-402