Choice of acid for dissolution of samples for trace metal analysis
Acid digestion on a hot plate (Image Courtesy : http://1.bp.blogspot.com/)
Trace metal analysis by AAS or ICP techniques requires introduction of sample into the system in the liquid form. Solids or powders need to be dissolved using acids so that the metals of interest are extracted in the liquid phase. The objectives of sample digestion are:
- Conversion of metals to soluble salts
- Removal of matrix interference
- Destruction of organic or biological matter in the sample
The desirable features of the digestion technique adopted should be:
- Complete matrix decomposition and extraction of analytes into solution with minimal loss
- Safety of operations
- Low consumption of acids
- Time-saving
- Reproducibility
Acids have been routinely used for digestion of samples. Open or closed digestions using single or combination of mineral acids are popular for wide range of applications covering:
Inorganic materials – minerals, ores, rocks and soils
Organics – pharmaceuticals, foods and beverages, toys, polymers and petrochemical products
Biological specimens – plants, meat, blood and urine
Acid digestions can be carried out at ambient temperature or by heating
Features of common acids
Mineral acids such as hydrochloric acid, nitric acid, sulphuric acid, hydrofluoric acid can be used alone or in combination. The characteristics of such acids are presented to help decide your choice for the application in hand.
Hydrochloric acid
Non-oxidizing acid with a boiling point of 84°C. Forms soluble chlorides with most elements except Hg, Ag, TI and dissolves salts of weaker acids such as carbonates, borates and phosphates. Oxides of Al, Be, Cr, Sb, Sn and Si show lower solubilities and are not digested with HCl
Nitric acid
Oxidizing acid with boiling point of 
. Frequently mixed with HCl, 
, HF or
. Forms soluble nitrates except with elements such as Au, Pt, Al, B, Cr, Tl and Zr
.
Caution! – compounds containing aromatic rings or OH groups such as fats form explosive mixtures due to formation of nitroglycerine and should not be digested with nitric acid
. Frequently mixed with HCl, , HF or. Forms soluble nitrates except with elements such as Au, Pt, Al, B, Cr, Tl and Zr.
Caution! – compounds containing aromatic rings or OH groups such as fats form explosive mixtures due to formation of nitroglycerine and should not be digested with nitric acid
Hydrofluoric acid
Non-oxidizing acid with a boiling point of 
. Useful for digestion of minerals, ores, soils, stones and plant materials. However, only inert material containers are used as HF attacks glass containers
. Useful for digestion of minerals, ores, soils, stones and plant materials. However, only inert material containers are used as HF attacks glass containersSulphuric acid
Non-oxidizing acid with boiling point of 
Useful for digestion of minerals, plastics,etc. Can be used as mixture with other acids.. However, cannot be used for dissolving salts of Ba, Pb, Sr as these for insoluble sulphates with sulphuric acid
Useful for digestion of minerals, plastics,etc. Can be used as mixture with other acids.. However, cannot be used for dissolving salts of Ba, Pb, Sr as these for insoluble sulphates with sulphuric acid
Aqua Regia (
in the ratio 3:1)
in the ratio 3:1)
Aqua regia is suitable for dissolving sulphides and noble metals. It should be prepared fresh before use
Perchloric acid
Perchloric acid is seldom used alone as it is strongly oxidizing and can lead to explosion hazards but more often it is used in combination with nitric acid. Perchloric acid digestions should not be allowed to proceed to dry stage.
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