ANALYSIS OF CHROMIUM IN STEEL
E5
You are required to submit ONE report from the group 4, 5 and 6. A report includes results,
calculations and questions.
PURPOSE
To determine the level of chromium in steel by oxidation to dichromate and potentiometric titration
with iron (II).
PROCEDURE
Because the time allowed for this practical session does not allow completion of the entire analysis,
duplicate steel samples have been dissolved for you.
The hydrochloric acid solution referred to below is a specific concentrations for this analysis. Make
sure you use the one labelled Exercise 5.
The following steps are performed on each sample.
1.
2.
3.
4.
5.
6.
7.
Add 15 mL of silver nitrate solution, and 10 mL of ammonium peroxydisulfate solution.
Cover the beaker with a watchglass and boil the solution for 10 minutes. The purple colour of
permanganate should appear – if not, do not proceed, and inform your teacher.
Add 15 mL of HCl to the solution, and continue boiling for 3 minutes.
Add further HCl dropwise until the purple colour disappears. The solution will become cloudy
as a consequence of the chloride precipitating with the silver.
Continue boiling for 10 minutes, then cool in an icebath.
Set up apparatus suitable for the potentiometric titration.
Titrate with iron (II) solution.
CALCULATIONS
For each sample,
•
calculate the moles of iron (II) in the titration
•
divide this by 3 to determine the moles of Cr
•
calculate the mass of Cr
•
calculate the %w/w of Cr in the sample
Calculate the average %w/w.
Calculate the relative error of your average result.
QUESTIONS
1.
What function does the ammonium peroxydisulfate serve?
2.
Why is it necessary to remove the permanganate colour before the titration?
3.
What properties of steel does the addition of chromium improve?
4.
Compare your result for the %w/w Cr with the stated value. Suggest reasons for any difference.
5.
Name one other analytical technique that could be used to determine the level of alloying
elements in steel.
E5. ANALYSIS OF CHROMIUM IN STEEL
RESULTS SHEET
Steel sample code
Cr concentration in steel
Molarity Fe (II)
Mass (g)
Titre (mL)
Sample 1
Sample 2
Tester’s signature ________________________________
Date _______________
Supervisor’s signature ______________________________
Date ______________