At times it may be necessary or desirable to dissolve a ferrochrome sample in acid rather than fusing it in sodium peroxide. Acid digestion methods are slow, but require little in the way of attention. Two alternative methods are described here. The A method is the basic method to use for most ferrochrome samples. The B method is for very stubborn samples.

Method A

Equipment Materials

• 250-mL beaker and cover.
• 400-mL beaker and cover.
• Hotplate.
• Funnel (optional).
• Filter paper (optional).
• Fume hood.

Materials

• Sulfuric acid, reagent grade, 5-pct solution.
• Hydrofluoric acid, reagent grade, concentrated.

Procedure

1. Weigh the sample into a 250-mL beaker.
2. Add 100 mL of 5-pct sulfuric acid solution and 1 to 2 mL of concentrated hydrofluoric acid.
3. Place the beaker on a low- to medium-heat hotplate, and cover.
4. When the sample has dissolved, remove the cover and take the solution to fumes of sulfuric acid; allow it to fume for a few minutes.
5. Remove the beaker from the heat, cool, and dilute with distilled water. Warm if necessary to redissolve the salts.
   6a. If the sample is for a total chromium determination, transfer the solution to a 400-mL beaker and go to step 11 of the total chromium method.
   6b. If the sample is for total iron determination, transfer the solution to a 400-mL beaker, add 3 to 5 g of ammonium chloride, and go to step 10 of the total iron method.
   6c. If the sample is for manganese determination, cool to room temperature or below and go to step 13 of the manganese method.

Procedure Notes

2. The acid digestion must be started gently. If the initial acid concentration is too high, the chromium in the ferrochrome will be passivated or become very unreactive. The low initial sulfuric acid concentration and the addition of the small amount of hydrofluoric, acid are to avoid this passivation.
3. The solution will slowly lose water; do not allow the total volume to drop below about 25 mL until all of the sample has dissolved.
4. Do not take the solution to copious fumes or allow it to fume for more than a few minutes. Excess fuming can form refractory chromium salts that are as difficult to dissolve as chromite itself.
6. The solution may contain carbon or precipitated silica. To remove them, if desired, filter the solution through a medium- to slow-speed qualitative paper and wash thoroughly.

Method B

Equipment

• 250-mL beaker and cover.
• 400-mL beaker and cover.
• Hotplate,
• Funnel (optional).
• Filter paper (optional).
• Perchloric acid fume hood.

Materials

• Perchloric acid, reagent grade, concentrated.
• Nitric acid, reagent grade, concentrated.

Procedure

1. Weigh the sample into a 250-mL beaker.
2. Add enough water to just cover the sample and 25 mL of perchloric acid.
3. If the sample is known to have a high carbon content, add 20 mL of concentrated nitric acid.
4. Cover the beaker and set on a medium- to high-heat hotplate
5. Bring to fumes of perchloric acid, and fume, covered, until all metal particles are dissolved.
6. See step 6 of method A

Procedure Notes

3. Nitric acid is necessary only when carbon is present in percent amounts. The nitric acid partially oxidizes the carbon so that when the perchloric acid begins to react with it the carbon does not react explosively. Carbon that has not been predigested with nitric acid will often react explosively when the perchloric acid reaches fuming temperature.
4-6. The sample is attacked by fuming perchloric acid to produce chromic acid. The process is slow, and the cover is necessary to provide some reflux. Chromic acid crystals are usually produced, and it may be necessary to cool the beaker and dissolve the crystals with water to see if the last particles of sample have dissolved. Silica is precipitated by the fuming perchloric acid and may be visible as light-colored, low-density particles in the solution. Carbon is oxidized to carbon dioxide by the fuming acid, but occasionally graphite will survive the treatment. If graphite is present, it will usually be found floating on the surface of the solution and occasionally as dark-colored, low-density particles on the bottom. Unattacked sample will be the only heavy or high-density particles in the beaker. If no heavy particles can be seen, then the solution is considered complete. If the sample is to be used for a manganese determination, the chromium should be reduced to the +3 state. (See step 11 of the manganese method.) If removal of the perchloric acid is desired, this can be done by adding 20 mL of 1:1 sulfuric acid and taking it to fumes.