Choice of acid for dissolution of samples for trace metal analysis

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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 \(122^OC\). Frequently mixed with HCl, \(H_2O_2\), HF or\( H_2SO_4\). 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 \(108^OC\). Useful for digestion of minerals, ores, soils, stones and plant materials. However, only inert material containers are used as HF attacks glass containers

Sulphuric acid

Non-oxidizing acid with boiling point of \(304^OC\) 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 (\(HCl:HNO_3\) 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.

Caution! – never add perchloric acid to an organic matrix

Acid digestions should always be carried out inside fume hoods to protect yourself and others from corrosive fumes. The benefits of microwave digestion over open acid digestions have been discussed earlier. A video showing the microwave digestion process will be presented shortly.

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