Site Loader

University of the Witwatersrand
School Of Chemistry
Analytical Chemistry II Laboratory Report

Name: Ngwako
Surname: Mphela
Student number: 1423394
Demonstrator: Dr M.Humphries
Date performed: 20 September 2018
Due Date: 27 September 2018
Experiment (A7): Analysis of Manganese in steel

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!

order now

To determine the mass percentage of manganese in steel
This paper report on the experiment whose objective was to determine the mass
percentage of manganese in steel. Manganese is a minor constituent that is found in
many steels. Numerous techniques can be employed to determine the quantitative
amount of a metal that is present in an alloy. E.g. Calorimetric determination or
spectroscopic techniques, based on the different oxidation state of metal. In this
experiment, visible spectroscopy and the standard addition method are used.
Spectroscopy is one of the most powerful analytical techniques in modern science.
Before the advent of spectrophotometric techniques, chemists interested in
determining the amount of a particular substance present in a sample had to analyse
the sample via a series of chemical reactions specific to that species and then carefully
weigh the products. This process was extremely time consuming, prone to error, and
generally impractical for measuring trace amounts. Today, most routine assaying is
done quickly and efficiently by means of spectroscopy. Spectroscopy works by
correlating the concentration of a species in solution to the amount of light it absorbs.
In this experiment we will determine the quantity of manganese using visible
absorption spectroscopy. Because the wavelengths of light we use are in the visible
portion of the electromagnetic spectrum, our solutions will all be coloured. However,
this technique can also be used in other regions of the spectrum where the
wavelength is not visible to our eyes, but can be measured using a photo-detector. In
a solution containing a coloured compound, the intensity of the colour can be used to
measure the concentration of the compound; the more intense the colour, the higher
the concentration. The spectrophotometer measures how much light is absorbed at a
given wavelength. The light absorption at that wavelength is expressed as a numerical
value that can be related directly to the concentration of coloured compound in the
Potassium periodate is effective at oxidizing lower oxidation states of manganese to
MnO4-, which exhibits a maximum absorbance at 525 nm. Reaction :
2Mn2+ + 5IO4- + 3H2O ? 2MnO4 – + 51O3- + 6H+
A reagent blank, consisting of the unoxidized sample, is used to compensate for other
constituents in the sample that absorb at this wavelength. Ferric ion also absorbs at
the measurement wavelength, but its interference is avoided by adding phosphoric
acid, which reacts with the iron, converting it to a nonabsorbing complex ion.

7X 100ml volumetric flask, 6X 250ml beaker, spatula, pipette, hot plate, measurement
5X vial, spectrophotometer, cuvette.
1000ppm Manganese solution, Nitric acid, 6M, Ammonium persulphate, Sodium
bisulphate, phosphoric acid, potassium periodate and distilled water.

The standardised manganese solution diluted to give a working standard of 100ppm
Mn2+. 1.0g of steel sample was accurately weighed out into a 250ml beaker, and was
then dissolved in 50ml of 6M nitric acid. The solution was boiled gently for ten
minutes until all the steel had dissolved. 1.0g of ammonium persulphate was carefully
added and the resulting mixture was heated for further 10 minutes. The solution was
cooled and then made up to 100mL in a volumetric flask. 5mL of the dissolved steel
solution was pipetted into 6 labelled beakers and the solutions were treated as
Solution name Dissolved
85% H3PO4
Mn standard
KIO4 (g)
5.0 5.0 0.0 0.0
Blank 5.0 5.0 0.0 0.4
Standard 1 5.0 5.0 1.0 0.4
Standard 5.0 5.0 2.0 0.4
Standard 3 5.0 5.0 5.0 0.4
Standard 4 5.0 5.0 10 0.4

Each solution was boiled for 5 minutes, cooled, quantitatively transferred to a 100ml
volumetric flask and then made up to the mark with distilled water. The
spectrophotometer was set to measure at a wavelength of 525nm. The
spectrophotometer was zeroed using the “instrument zero” solution, and then in turn
the absorbance of various solutions were measured. A graph of absorbance against
concentration was plotted and the concentration of Mn in the dissolved steel was
Table of results:
Solution name Mn

Instrument zero 0.0 0.0
Blank 0.0 0.1544
Standard 1 1.0 0.1794
Standard 2 2.0 0.1846
Standard 3 5.0 0.2474
Standard 4 10 0.5407
Standard deviation 4.037 0.1784

Plot of the results

Figure 1: Plot of absorbance against concentration of Mn, from the data obtained
during the experiment.
The above graph was plotted using the actual obtained data. But since the correlation
coefficient is less than 0.95, I disregarded one point in order to obtain a more reliable
calibration curve as shown below.

Figure 2: Plot of absorbance against concentration of Mn, obtained by eliminating an one point from the
The actual plot(figure 1)
y = 0.0382x + 0.1239
R² = 0.9292
Plot of asorbance vs concentration of Mn
y = 0.0401x + 0.1346
R² = 0.9867
Plot of absorbance vs concentration of Mn

To get the concentration of Mn from the calibration curve, we must solve for absolute
x using the equation of the straight line(line of the best fit):
Therefore concentration of Manganese = 3.357±????????
Mass of manganese = ????????????????
Mass Percentage of Mn: ????????
?????×???? = 1.679%
Manganese increases hardenability and tensile strength of steel, but to a lesser extent
than carbon. It is also able to decrease the critical cooling rate during hardening, thus
increasing the steels hardenability much more efficient than any other alloying
elements. Manganese also tends to increase the rate of carbon penetration during
carburizing and acts as a mild deoxidizing agent. However when too high carbon and
too high manganese accompany each other, embrittlement sets in. Manganese is
capable to form Manganese Sulphide (MnS) with sulphur, which is beneficial to
machining. At the same time, it counters the brittleness from sulphur and is
beneficial to the surface finish of carbon steel.
Manganese in steel was determined upon dissolution as Mn(VII) after oxidation from
the Mn(II). During the dissolution of the sample, the steel was dissolved with nitric
acid to form Fe2+ and Mn2+. Nitrogen oxide gasses were also formed. These were
eliminated by boiling the solution. Addition of ammonium persulphate also helped in
eliminating the carbon/ other organic matter that was present in the solution. The
oxidation of manganese(II) ions to manganese(VII) ions with potassium iodate(VII) –
KIO4(s) turned the solution purple.
2 Mn2+(aq) + 5 IO4–(aq) + 9 H2O(l) ? 2 MnO4–(aq) + 5 IO3–(aq) + 6 H3O+(aq)
During the reaction it was noticed heating of the solution produced a colour
change of light green to pink. This occurred since the phosphoric acid reducing
Fe(III)to an iron phosphate complex releasing the pink colour of the permanganate.
In general, the manganese content of most steels is quite low, less than 1%, but that is
not the case in our experimental results. From our results, the manganese content was
determined to be 1.679% which greater that the expected. This may be due to some
errors that occurred during experimentation. E.g. Measurement errors and random

errors(fluctuations in measured quantities). Another contributing factor may be that
the cuvette was not well rinsed during the absorbance measurement since time was
against up, therefore the measurements were done in a hurry process.

The experiment was successfully performed and the objective was accomplished.
However, the results did not turn out as expected. The obtained manganese happened
to be greater than 1% which is not likely to happen. Hence a conclusion can be made
that there are some errors that occurred the experiment and thus affected the results.
The results can be improved in the future by avoiding/minimizing errors. E.g. Being
accurate when taking the measurements as well as working with “speed and accuracy”
so as to avoid being forced to work under pressure at the end of the experiment

Post Author: admin


I'm Irma!

Would you like to get a custom essay? How about receiving a customized one?

Check it out