DETERMINING THE PERCENTAGE BY WEIGHT
OF MANGANESE IN STEEL

Task

Prepare a calibration curve in order to define the content of manganese in a sample of low-alloy steel.

Steel

Steels are iron alloys, they contain iron, in addition to the most important alloying element carbon.

Steel can be divided in various ways:

  • following the procedure of extracting (oxygen converter “pig iron”, electric furnace “recycling of scrap”, Siemens-Martin furnace)
  • in composition:
    • Carbon steel (are steels where the main alloying element is carbon, its content is less than 1 % C)
    • Alloy steels are those steels where alloying elements are added to improve certain properties. The distinction is in the low-alloy steel (to 5 %) and high-alloy steel (above 5 %).
    • Special alloy steels are stainless steels, where adjacent to carbon and other alloying elements is a higher share of chromium, at least 11.5 % Cr.
  • in use:
    • Structural steels, which contain mainly up to 0.6 % of carbon which is the main alloying element in structural steels. We call them common structural steels (intended for the manufacture of general steel structures).
    • Noble structural steels, which contain, in addition to carbon, alloying elements such as Mn, Si, Cr, Ni, Mo, V (we use them to produce complex structures and machine parts).
    • Tool steels, which contain more than 0.6 % carbon and alloying elements such as Cr, W, V, Mo, Co (used to make tools).

Elements such as nitrogen, oxygen, phosphorous and sulphur are not considered as alloying elements but as the impurities, which result from the method used for obtaining the steel, the process of cooling and the use of technology.

The most commonly used alloying elements are Mn, Mo, Si, Al, Co, W, V, Ti, Ni,… and many others that we add to lower or nullify the effects of impurities. We divide them into elements that form carbides with carbon (Cr, Mn, W, Mo, Ti) or not (Ni).

Background

Dissolve steel shavings in a mixture of acids. Add sulfuric(VI), nitric(V) and phosphoric(V) acid and heat it. Iron and manganese are diluted with displacement of hydrogen.

2 Fe(s) + 6H3O+(aq) 3H2(g) + 2 Fe3+(aq) + 6 H2O(l)

Mn(s) + 2 H3O+(aq) H2(g) + Mn2+(aq) + 2 H2O(l)

Phosphoric(V) acid merges with yellow iron(III) ions in a colourless compound. The oxidation of manganese(II) ions to manganese(VII) ions with potassium iodate(VII) – KIO4(s) turns the solution purple.

2 Mn2+(aq) + 5 IO4(aq) + 9 H2O(l) 2 MnO4(aq) + 5 IO3(aq) + 6 H3O+(aq)

The manganese content in the sample is determined spectrophotometrically. 


Laboratory equipment and reagents

Reagents:

  • sulphuric(VI) acid , ψ(H2SO4/H2O)=1:3
  • phosphoric(V) acid, concentrated
  • nitric(VI) acid, ψ(HNO3/H2O)=1:1
  • potassium iodate(VII) p.a.
  • sodium nitrate(III), w(NaNO2)=0,02
  • potassium permanganate(VII) p.a.

Glassware and equipment:

  • beaker 150 mL,
  • watch glass Φ 80,
  • graduated cylinder 10 mL,
  • graduated pipette 10 mL,
  • graduated pipette 5 mL,
  • graduated cylinder 50 mL,
  • volumetric flask 50 mL,
  • laboratory tables,
  • analytical balance,
  • precision balance,
  • electric heating plate,
  • fuming hood,
  • spectrophotometer SpektraTM with appropriate equipment,
  • blisters,
  • 10 mL bottles with dispensing nipples.

Hazards


sulfuric(VI) acid
R: 35, S: 26-30-45


nitric(V) acid
R: 8-35, S: 23-26-36-45

phosphoric(V) acid
R: 34 S (1/2)-26-45 

KMnO4
R: 22, 8, 50/53 S:  60, 61

sodium nitrite
R: 8-25 S: 44



Procedure

Preparation of stock solution:

Weigh 0.7191 g of KMnO4, dissolve it in boiled and cooled distilled water and dilute to the mark in a 250 mL volumetric flask. Transfer 10 mL of the reconstituted solution into a 500 mL volumetric flask and dilute contents to the mark. The ready-made solution has a concentration of 20 µg/mL (Mn7+). Decant the prepared solution of manganese ions in to a 10 mL bottle.

Preparation of reagents:

  • Sulphuric(VI) acid, ψ(H2SO4/H2O)=1:3
    Use graduated cylinder to measure 30 mL of distilled water into a 50 mL beaker. While the water is cooling down add 10 mL of concentrated sulphuric(VI) acid.
  • Nitric(V) acid, ψ(HNO3/H2O)=1:1
    Measure 10 mL of water into the 50 mL beaker and add 10 mL of nitric(V) acid.
  • Sodium nitrate(III), w(NaNO2)=0.02
    On a precision balance weigh 1.00g of sodium nitrite(III) into a 100 mL beaker and add distilled water to 50.00 g.

Sample preparation:

All the measurements should be carried out in triplicate.

On an analytical balance weigh about 0.1 g of steel shavings into a 150 mL beaker (accurately record the weight into the chart), add 10 mL of sulphuric acid and some boiling chips. Cover the beaker with the watch glass and heat it on the electric heating plate (in the fume hood) until the steel is dissolved.

After dissolution take the beaker off the heating plate and add 2 mL of phosphoric acid.

When the solution cools down add 0.5 mL of nitric acid. Don't worry if there are any particles of graphite in the beaker, they do not interfere with the analysis.

Heat the solution until the appearance of white steam. Cool the solution down, add about 20 mL of distilled water and two laboratory tables of potassium iodate(VII).

Again heat the solution on the electric heating plate. If manganese is present in steel as an alloying element, the solutions turn purple.

Quantitatively transfer contents of the beaker into the flask (50 mL) and dilute it to the mark.

The resulting solution of the sample is suitable for spectrophotometric analysis. Pour it in a 10 mL bottle.

(In the other bottle prepare a few millilitres of prepared solution and add 5 drops of sodium nitrate(III) solution. The solution gets discoloured so you can use it for a blank when calibrating the instrument.)

Spectrophotometric measurements:

  • Prepare a blank and eight calibration solutions with dosage drops on a blister as shown in Table 1

Table 1:

standard N°

0 (blank)

1

2

3

4

5

6

7

8

Sample

drops of distilled water

10

9

8

7

6

5

4

3

2

0

drops of stock solution

0

1

2

3

4

5

6

7

8

0

drops of sample

-

-

-

-

-

-

-

-

-

10

T (%)

 

 

 

 

 

 

 

 

 

 

A

 

 

 

 

 

 

 

 

 

 

γ Mn7+
[µg/mL]

0

2

4

6

8

10

12

16

18

 

  • Transmittance of solutions is measured using the green LED at maximum intensity.
  • The instrument is calibrated by putting in a blank with the green LED at maximum intensity and setting the transmittance to 100 %.
  • Enter the measurement results in the chart.
  • Calculate the absorbance and enter it in the chart.
  • Use the program Excel to draw a calibration curve and write down the equation of the regression line.

Example of the calibration curve:

Read off the concentration of manganese ions in the sample solution, or calculate it using the equation.

Table 2:

N° of sample

m of sample [g]

A

γ(Mn7+)[µg/mL]

1

 

 

 

2

 

 

 

3

 

 

 



Calculation

Calculate the percentage by weight of manganese in steel.

w(Mn,%) =

g (Mn7+).V(sample,aq).100

m(sample)


Result

Table 3:  

description of the sample steel

w(Mn) in steel

 

 



Developed and prepared by: Romana Mele and Helena Prevc, ŠC SSKŠ Ljubljana