At this Center, results from this method have been used almost exclusively as data for the calculation of the total degree of reduction in a smelter batch, rather than as an indicator for the metallization of chromium. When a charge of chromite material is smelted or prereduced, the crystal structure is changed, and once the chromite spinel structure has been opened, the chromium becomes much more available to acid attack. Partially reduced chromium species and chromium in the form of silicates, carbonates, or other carbon compounds, as well as chromium metal, are then available to acid attack. (These species of chromium are often grouped under the name “metallic” chromium.) (It has been noted that a very finely ground sample of a mineral chromite concentrate will generally give up a few relative percent of its chromium to this method.)

Equipment

• 250-mL beaker.
• 400-mL beaker.
• Stirring rod.
• Hotplate.
• Filter paper.
• Funnel.
• Burette.

Materials

• Sulfuric acid, reagent grade, diluted 1:1 with distilled water.
• Hydrofluoric acid, 46 to 51 pct, reagent grade.
• Saturated manganese solution.
• Silver nitrate 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 250-mL beaker.
2. Add 80 to 90 mL of distilled water, 10 mL of 1:1 sulfuric acid, and 5 to 10 mL of hydrofluoric acid.
3. Place the beaker on a hotplate and bring to a gentle boil or just below.
4. Digest at a gentle boil, or just below, for 40 to 50 min, replacing lost water whenever the digestion approaches half the original volume.
5. Remove the solution from the hotplate, cool to about room temperature, and filter through a medium-speed qualitative paper (such as Schleicher and Schuel (S&S) 597) into a 400-mL beaker. Wash 5 to 10 times with distilled water.
6. Dilute the combined filtrate and washes up to 200 to 250 mL total volume.
7. Add one drop of saturated manganese solution, 2 or 3 mL of 2.5-pct silver nitrate solution, and 3 to 6 g of ammonium persulfate, rinse sides, and stir thoroughly.
8. Bring the solution to a boil, and boil several minutes to destroy the excess persulfate.
9. Remove the beaker from the hotplate, and immediately add 3 to 5 mL of concentrated hydrochloric acid to decompose the permanganate.
10. Stir thoroughly, rinse sides, and cool to room temperature or below.
11. When the solution is cool, add about 10 mL of concentrated phosphoric acid and titrate with a standard ferrous iron solution using three to five drops of sodium diphenylamine 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 acid-soluble Cr.

1 mL 0.1000N Fe+² = 1.733 mg Cr.

Procedure Notes

1. Sample size is usually roughly calculated to yield a convenient titration. Sample sizes from a few hundred milligrams for prereduced smelter charges up to several grams for slag are easily handled, though large samples of finely ground material sometimes have a tendency to bump.
2. The described acid mixture is proper for all normal determinations. Another small volume of hydrofluoric acid may be added if high silica or metal passivation is suspected. Do not add more sulfuric acid. A high sulfuric acid concentration can slow or prevent the oxidation of chromium from Cr+³ to Cr+6.
3. Gentle digestion is preferred over vigorous boiling because it requires much less attention and entails less danger of passivating any metallics present.
4. Very fine gangue particles will sometimes pass through (“slime” through) the paper, but they do no harm in the oxidation or titration steps, other than making the solution cloudy.
5. Aim at 150 to 200 mL solution volume at the start of the titration.
6. Saturated manganese solution is to indicate the completion of the chromium oxidation. All of the chromium will be oxidized before the manganese is converted to permanganate. The silver nitrate is a catalyst in the chromium oxidation. Crystalline ammonium persulfate is used to save the time and labor of preparing fresh solution.
7. When the oxidation is complete and the manganese turns red, it turns quickly. 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 persulfate and return the solution to the hotplate. Repeat these operations as necessary. Lost water may be replaced without any hazard to results.
8. 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.
   10. Be aware that a small amount of chlorine gas is evolved during cooling.
9. The phosphoric acid is used as a complexing agent for the ferric iron present. If significant amounts of titanium or aluminum 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 maintain constant normality of the solution. 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.