Nitric acid

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Nitric acid (HNO3), is a colorless oxidizing acid. It is the only pure acid that can dissolve copper and silver, making it an important acid for refiners. It is used both by itself when working with silver or mixed with hydrochloric acid forming aqua regia. Salts of a metal and nitric acid is called nitrates.

Contents

Dangers

Nitric acid is of course acidic and splashes could easily blind or burn any unprotected person. It is also a powerful oxidizer and could react with organic substances forming highly flammable products. When reacting with metals nitric oxides are formed. With oxygen from the air a red cloud could be formed. These gases are toxic and cancerogenic.

Usages

In refining there is no need for nitric acid of higher concentration than 70%. When used for dissolving silver a concentration of 30-35% is enough and often necessarily to dissolve the created silver nitrate. For aqua regia 70% nitric acid is usually used.

Silver cell electrolyte

Destruction or removal of nitric acid from a solution

When used for dissolving various metals as gold or platinum, traces of nitric acid could make the metal redissolve when we turn the dissolved metal back to metallic form. To stop the metal to dissolve again we need to remove any traces of nitric acid or nitrate salt. This is called denoxing.

Reaction with sulfamic acid

Nitric acid is destroyed by sulfamic acid.

Making nitric acid

(Main article : Making nitric acid )

Nitric acid can be hard to find for some refiners, but there are ways to make it in small amounts.

  • Mixing copper, hydrochloric acid and a nitrate salt and capture the gases in distilled water or hydrogen peroxide.
  • Mixing sulfuric acid with a nitrate salt and distill off the nitric acid formed.
  • Mixing sulfuric acid with a water solution of a nitrate salt that would create low solubility sulfates. Less sulfate salt is included if the solution is cooled so it is called the cold nitric recipe.

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