The determination of ferrous iron in mineral chromites and other chromite-bearing samples is limited by the difficult solubility of the chromite lattice. Reaction of the ferrous iron immediately upon release from the crystal seems to be the preferred way of measurement. Attempts to dissolve chromite by other methods depend on strong oxidizing agents that leave no ferrous iron to be measured. The V+4 – V+5 system is stable enough to yield good results when it is used to react with released ferrous iron.

The situation is complicated, however, by the lack of any standard materials with a certified ferrous iron value. National Bureau of Standards (NBS) 103a chrome refractory, for example, lists an FeO value that is total iron expressed as ferrous oxide. While this value should be fairly close, several relative percent of the ferrous iron are oxidized by atmospheric oxygen in grinding and storage.

With the limitations in mind, researchers at this Center have used results from this method as data for the calculation of smelter charges and of total reduction in a smelter run.

The method requires an overnight digestion, but is otherwise simple and rapid. Reproducibility has been usually within 2 relative percent.

Equipment

• 150-mL beaker and cover glass.
• 600-mL beaker.
• Stirring rod.
• Hotplate.
• Pipette (50-mL capacity).
• Burette.

Materials

• Vanadium-acid mixture.
• 0.1N Fe+² standard solution.
• Sodium diphenylamine sulfonate solution.

Procedure

1. Weigh a sample to contain 1.5 meq wt Fe+² or less into a 150-mL beaker.
2. Add just enough distilled water to wet and disperse the sample.
3. Pipette 50 mL of the vanadium-acid mix onto the sample while swirling the beaker to maintain dispersion. Pipette a blank for digestion with each batch of samples.
4. Cover and digest overnight on a hotplate at about 100° C.
5. Remove from the hotplate and cool to room temperature or below.
6. When the solution is cool, slowly pour it into a 600-mL beaker containing 150 to 200 mL of water, while stirring. Thoroughly rinse the small beaker into the large beaker, and dilute the solution to about 500 mL total volume.
7. Titrate with standard Fe+² solution using three to five drops of sodium diphenylamine sulfonate indicator, which changes from purple to green at the endpoint.

Titration equation:

V+5 + Fe+² → V+4 + Fe+³.

Calculation:

(mL blank titer – mL sample titer) x N Fe+²

= meq Fe+² in sample.

meq Fe+² x meq wt Fe/sample wt x 100 = pct Fe+²

If metallic iron is present, it must be corrected for by subtracting three times the amount of iron present (in milliequivalents) from the milliequivalents of V+5 consumed, as in the following equation:

(mL blank titer mL sample titer) x N Fe+²

– 3(meq Fe°) = meq Fe+².

The result is then used in the second equation above.

Procedure Notes

1. Complete solution is more reliably obtained when the sample is limited to 0.5 g or less. Fine grinding is essential to the method, and all samples should be ground to 100 mesh or finer.
2. It is necessary to give some attention to maintaining the sample dispersion. The samples have a tendency to form lumps when the acid is added, and this negates the beneficial effects of fine grinding. Lumps of sample stubbornly resist attack.
3. The acid mix will slowly attack glass. Thin or etched beakers should be avoided.
4. Take care, this is a very strong acid solution.
5. If metallic iron is present it will consume 3 meq of V+5 solution for each milliequivalent of iron present. A separate metallic iron analysis is done, and the results are used to correct the Fe+² results.