Acid Digestions of Organic Samples

Nitric and Perchloric Acid Digestions

Nitric acid is rarely used alone. It is best used in combination with sulfuric and/or perchloric acids for organic sample digestion. For samples that are not highly aromatic and/or contain a high -OH functionality, I prefer to use nitric acid followed by perchloric acid. The only element that may be lost from a nitric/perchloric digestion is Hg. Care should be exercised and the literature consulted before attempting to use nitric acid in combination with other acids for organic sample digestions.

CAUTION: The use of perchloric acid should only be attempted by those individuals well-versed in the safe use of this reagent. Consult "Perchloric Acid and Perchlorates"¹ for safety guidelines.

The following are some key rules that I recommend when using nitric/perchloric acid digestions:

1. Organic matrices should always be pre-treated with nitric acid (see exceptions above).
2. Perchloric acid should never be used alone.
3. Perchloric acid digestions should never be allowed to go to dryness.
4. Hot perchloric acid should never be added to an organic matrix.
5. Sample sizes should never exceed 1 gram (dry weight for biologicals).
6. Perchloric acid fumes should be not be allowed to ‘go free' unless a perchloric acid hood is used.
7. Unknown organic matrices should be analyzed by molecular spectroscopy to determine primary structure before attempting the use of either nitric or perchloric acid.

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