Question

Why concentrated nitric acid is add in Limit test for iron?

Answer 1

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• Nitric acid is used because all nitrate salts are soluble in water. If we use H2SO4 or HCl they will produce sulphate or chloride salts. Some metal sulphates and chlorides are not very soluble in water.

• The nitrogen dioxide is absorbed in water to form nitric acid. The resulting acid-in-water solution (about 50–70 percent by weight acid) can be dehydrated by distillation with sulfuric acid. Nitric acid decomposes into water, nitrogen dioxide, and oxygen, forming a brownish yellow solution.

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Answer 2

Hey there

HNO3 is added to samples for two purposes. One, at the time of sample collection-to reduce pH <2. Below pH 2, precipitation, adsorption to container wall and microbial degradation are minimized. Though any acid will serve the purpose, HNO3 is preferred because of its oxidizing nature. Adding HNO3 converts metal ions into their nitrate salts, which are highly soluble.

Secondly, sample digestion is required before AAS. The purpose is to destroy the matrix, which otherwise interfere during atomization. Also digestion converts all form of metal into a single oxidation state. Though different acids i.e. HNO3, H2SO4, HCLO4 i different combinations are used, HNO3 suits very well for water and waste water samples. The choice of acid mixture depends on the organic matter content and turbidity of the sample. Water having turbidity <1 NTU can be analyzed without digestion. Go through Standard Methods of APHA for details.

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