The total chromium content in mineral chromite and ferrochrome slags can range from a fraction of a percent in the slags to greater than 30 pct in the ore concentrates. Because of this variation and because of the difficulty in dissolving chromite-containing samples, the method of choice at this Center has been fusion with sodium peroxide, followed by persulfate oxidation and titration with ferrous iron.

The wet method is sufficiently accurate and reproducible for high-quality results on a routine basis. The samples may be determined very rapidly without a sacrifice in quality. Batches of 20 determinations have been completed in 6 h by one analyst without assistance.

Equipment

• Zirconium crucible (approximately 30-mL capacity seems most convenient).
• Meker or similar gas-air mix burner.
• 400-mL beaker and cover.
• Glass stirring rod.
• Hotplate.
• Rubber policeman.
• Burette.

Materials

• Sodium peroxide, reagent grade, granular, 20 mesh or finer.
• Sulfuric acid, reagent grade, diluted 1:1 with distilled water.
• Saturated manganese solution.
• Silver nitrate, reagent grade, 2.5-pct solution.
• Ammonium persulfate, reagent grade, crystal.
• Hydrochloric acid, reagent grade, concentrated.
• Phosphoric acid, reagent grade, concentrated.
• Ferrous iron solution, standardized.
• Sodium diphenylamine sulfonate solution.

Procedure

1. Weigh the sample into a zirconium crucible.
2. Add 4 to 12 g of sodium peroxide, and stir until the peroxide and sample are thoroughly mixed.
3. Fuse over a burner until all sample particles are dissolved. Swirl and inspect occasionally to keep unattacked particles dispersed.
4. When the fusion is complete, allow the crucible and melt to cool.
5. When they are cool, tap gently to free the solidified melt from the bottom of the crucible.
6. Place the solidified melt and the emptied crucible in a 400-mL beaker, add 150 to 200 mL of cold distilled water, and cover quickly.
7. When leaching activity has subsided, slide the cover aside slightly and slowly add about 25 mL of 1:1 sulfuric acid solution. Reclose the cover and wait for the completion of any further leaching action.
8. Remove and rinse the cover glass, and rinse down the beaker sides.
9. Place a glass stirring rod in the beaker and stir the contents thoroughly.
10. Remove and thoroughly rinse the crucible. Inspect the inside of the crucible and police it if necessary.
11. To the solution in the beaker, add one drop of a saturated manganese solution, 2 or 3 mL of a 2.5-pct solution of silver nitrate, and 3 to 6 g of ammonium persulfate. Stir thoroughly.
12. Bring the solution to a boil, and boil for several minutes to decompose the excess persulfate.
13. Remove the beaker from the hotplate, and immediately add 5 mL concentrated hydrochloric acid to decompose the permanganate.
14. Stir and cool the solution to room temperature or below.
15. When the solution is cool, add about 10 mL of concentrated phosphoric acid, and titrate with a standard ferrous iron solution using two to five drops of sodium di-phenylamine sulfonate solution as the indicator, which changes from purple to green at the endpoint.

Titration equation:

Cr+6 + 3Fe +² → Cr+³ + 3Fe+³.

Calculation:

mL titer x N Fe+² x eq wt Cr/sample wt x 1,000 x 100 = pct total Cr.

1 mL 0.1000N Fe+² = 1.733 mg Cr.

Procedure Notes

1. Sample size is usually roughly calculated to yield a titration of about 20 to 50 mL (as a convenience). However, a minimum of 0.2 g is used when adequate sample is available. A maximum of about 1 g is used to keep the melt volume to a manageable level.
2. Increase the sodium peroxide to match sample size using an approximate ratio of 20 parts of peroxide to 1 part of sample. Again, consideration must be given to melt volume at high sample weights. Thorough mixing cannot be overstressed. Sample particles in the melt can aggregate and stubbornly resist attack.
3. The burner should be capable of bringing the crucible to red heat on the bottom. Mixing remains important. If particles are allowed to aggregate, fusing time may have to be extended, which increases the attack on the crucible itself, shortening its useful life, increasing the tendency of the cooled melt to stick to the crucible, and adding a significant concentration of zirconium to the analyte solution.
4. If the melt is to be leached quickly, the crucible can be set out on any heat-resistant material to cool. If the melt has to be set aside for a while, it should be set on a hotplate on low heat to keep it from absorbing atmospheric water (which makes the melt stick to the crucible).
5. If gentle tapping does not free the melt, try moderately hard tapping. If the melt still sticks, lay the crucible on its side in the bottom of the beaker.
6. The leaching reaction can range from very slow to very vigorous. A high aluminum concentration tends to slow the reaction. A slow leach reaction may become a very fast leach reaction when sulfuric acid is added, so care must be exercised. Once the leach reaction has finished, the addition of sulfuric acid is generally accompanied by moderate effervescence unless a significant amount of carbonate is present.
7. Rinse down any visible precipitate particles and police out any residue in the crucible. Stirring should produce a clear solution unless hydrolyzed zirconium or aluminum is present. Do not add more sulfuric acid unless iron or chromium hydroxide precipitate is present. Excess sulfuric acid may slow or prevent the oxidation of chromium.
8. The manganese is to indicate the completion of the chromium oxidation. All of the chromium will be oxidized before the manganese begins to be converted to permanganate. If permanganate is the salt added, it may not all reduce and may leave a permanganate pink. Proceed regardless. The permanganate color after the decomposition of the excess persulfate is the important condition, not the pink in the intermediate stages. The silver nitrate is a catalyst in the oxidation reaction. Crystalline ammonium persulfate is used to save the time and labor of preparing fresh solution.
9. When the oxidation is complete and the manganese turns red, it turns quickly, in 1 or 2 s. As the excess persulfate is destroyed, a mild effervescence can be seen in the solution. If the solution does not turn red after boiling for a few minutes, remove it from the hotplate and inspect it for precipitate that looks like silver chloride. If found, add more silver nitrate and ammonium persulfate, and return the solution to the hotplate. If no such precipitate is found, just add more ammonium persulfate, and return the solution to the hotplate. Repeat the operations as necessary. Lost water may be replaced without any hazard to results.
10. The chloride in the hydrochloric acid will be oxidized to chlorine gas in the process of reducing the permanganate. It will also precipitate the silver. The chloride will not react with the dichromate produced.
11. Be aware that a small amount of chlorine gas is evolved during cooling.
12. The phosphoric acid is added to complex the ferric iron. If significant amounts of aluminum or titanium are present, hydrofluoric acid may be substituted for the phosphoric acid. The ferrous iron solution is prepared from ferrous ammonium sulfate in 12-L batches. A piece of purified aluminum sheet kept in the carboy helps to keep the normality of the solution constant. Sodium diphenylamine sulfonate is used because it is easily soluble in distilled water and thus preparation is simplified. Any of the diphenylamine indicators will work, giving a sharp endpoint by changing from purple to green.