N10. ANALYSIS OF MANGANESE IN STEEL
PREWORK
What special requirements are there for the disposal of arsenic solutions?
PROCEDURE
1. Weigh out accurately about 1g of the steel sample into a 250 mL conical flask in
duplicate. Record the code number of the sample
2. Repeat with a standard steel of known manganese content
3. Add, in this order 15 mL purified water, 3 mL conc sulfuric acid, 4 mL conc
phosphoric acid and 8 mL conc nitric acid
4. Heat until the steel dissolves. Boil until no more brown gas is evolved
5. Remove from the heat, and add 50 mL purified water, 10 mL 0.1M silver nitrate
and 2.5 g ammonium persulfate
6. Boil for 30 seconds
7. Cool rapidly to below room temperature, add 75 mL cold purified water and 5mL
0.2M sodium chloride.
8. Titrate immediately with 0.006M arsenic (III) oxide solution to a clear yellow
endpoint (apricot).
CALCULATIONS
1. write a balanced equation for the oxidation of arsenic (III) oxide to arsenic (V)
oxide
2. Combine this with the conventional permanganate half-equation to give the
overall reaction equation
To determine the molarity of the arsenite titrant:
3. Calculate the mass of manganese in each standard steel sample
4. Calculate the moles of permanganate formed in the analysis.
5. Using the reaction ratio, calculate the moles of arsenic (III) oxide used in the
titration
6. Calculate the molarity of the arsenic solution
7. Average the results
To determine the %w/w of Mn in the sample:
8. Calculate the moles of arsenic (III) oxide used in the titration
9. Calculate the moles of permanganate that reacted
10. Calculate the mass of Mn in the sample
11. Calculate the %w/w of Mn in the sample
12. Average the results
DISCUSSION
•
Explain why the standardisation of the arsenite solution is better done with a
standard steel, rather than standardised potassium permanganate
•
Explain the purpose of the following reagents in the analysis:
mixed acids
- ammonium persulfate
- silver nitrate
- sodium chloride
QUESTIONS
1. What purpose does manganese serve as an alloying element in steel?
2. If the iron content in iron ore was to be determined by titration with standardised
permanganate, what sample treatment would be required?
3. Why can’t hydrochloric acid be used in analyses involving permanganate?
REFERENCES
Vogel’s Textbook of Quantitative Analysis