There are two main areas in analytical chemistry. The first one is instrumental analysis, where specific apparatus perform measurements with the help of computers and software for processing and data analysis. The second is the more classical approach commonly referred to as wet chemistry.
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The term “wet chemistry” refers to chemistry done generally in the liquid phase. Since it does not involve advanced instrumentation and is mainly based on a series of basic techniques, wet chemistry is sometimes considered an old-fashion version of the chemical sciences. Nevertheless, it is very effective and widely used for several purposes.
Wet chemistry involves activities such as measuring, mixing, and weighing chemicals, as well as measuring conductivity, density, pH, temperature, and other properties of liquids. It also includes procedures such as precipitation, extraction, and distillation.
Wet chemistry is often used to provide qualitative information. Qualitative analysis can indicate the presence of a specific chemical in an unknown solution. These methods use specific unique reactions that take place with the chemical under examination. For instance, it is possible to detect proteins in urines by adding a strong acid to a sample in a test tube. Since strong acids cause protein denaturation, the appearance of a cloudy ring is indicative of the presence of proteins in the sample (Heller's test).
Quantitative analysis via wet chemistry
Quantitative techniques to determine the exact amount of a substance are also available in wet chemistry. Such techniques can occur via gravimetric methods (by measuring the weight) and volumetric analysis (i.e. titrations).
In titrations, a reagent with a known concentration is added to a solution with a substance of unknown concentration. The amount of unknown substance is proportional to the volume of reagent needed to induce a change.
Titration often uses indicators (i.e., phenolphthalein, methyl orange, bromothymol blue) that change color depending on the pH of the solution. The point where the color change occurs is called the endpoint and is used to determine the concentration of the substance under investigation.
Gravimetric analysis consists of measuring the weight or concentration of a solid that has formed following either precipitation from a solution or dissolution in a liquid. In the first case, a reagent is added until the formation of the precipitate ceases. The precipitate is then dried and weighed to determine the amount (and concentration) in the liquid. When instead a substance is dissolved, the liquid can be filtered and then evaporated. The resulting solid is dried and weighed to determine its concentration.
In addition, colorimetry is another common wet chemistry technique that is based on a change in color upon reaction. Both qualitative and quantitative information can be obtained, with the latter requiring sensory equipment that can measure the changes in wavelengths of specific colors.
Some applications
Several colorimetric assays have been developed for the determination of codeine – an opioid analgesic derived from morphine. Some of these tests include the Marquis and Mecke reagents, which give a violet color in the presence of codeine, and the nitric acid test, which produces an orange color that slowly changes to yellow, and that can differentiate codeine from other opiates.
Citrate-stabilized gold nanoparticles have been used in a rapid and selective colorimetric method for the detection of codeine sulfate. The nanoparticles react with codeine sulfate to produce a green color. The amount of codeine sulfate can be determined by measuring the change in absorbance intensity from 520 to 582 nm.
Wet chemistry methods are often used for the determination of lignin content in wood. Lignin is a polysaccharide and is one of the key components of wood, particularly important in the formation of cell walls.
One of the methods involves the depolymerization of the polysaccharides via a two-step treatment with sulfuric acid (72% and 3% respectively). Lignin is quantified by measuring the remaining solid (Klason lignin) and the acid-soluble lignin. Although effective and widely used, the method is time-consuming and labor-intensive.
Many methods for the analysis of food composition are based on wet chemistry. The determination of moisture content is one example. The analysis is typically performed with oven-drying methods, where the loss of weight is used to calculate the moisture content of the sample. The methods are simple and the time required may vary from a few minutes to over 24 h.
Other methods involve a distillation process to measure the amount of water removed from a food sample by evaporation and are sometimes preferred because they cause less thermal decomposition of some foods.
Lipid determination (in the form of fatty acids and their derivatives) is an important type of analysis for food labeling and quality control. The total lipid content is commonly determined by organic solvent extraction methods, with multiple alternatives available depending on the type of food product.
Wet chemistry also has application in environmental chemistry, where methods are used to investigate the state of the environment, such as the tests for measuring the Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD).
Conclusions
While limitations exist, such as extensive preparation time and intensive labor, wet chemistry offers many advantages for chemical analysis. Since instruments are not necessary and determinations can be both quantitative and qualitative, many wet chemistry methods are preferred to other instrumental approaches.
Some methods, such as colorimetric tests, are very useful for screening samples in large quantities, especially in resource-limited settings. With simple and well-defined protocols, wet chemistry finds application in several areas, from environmental studies to food analysis.
Sources:
- Pratiwi, R., Noviana, E., Fauziati, R., Carrao, D. B., Gandhi, F. A., Majid, M. A. & Saputri, F. A. (2021). A Review of Analytical Methods for Codeine Determination. Molecules, 26.10.3390/molecules26040800
- Lourenço, A., Gominho, J., Marques, A. V. & Pereira, H. (2013). Comparison of Py-GC/FID and Wet Chemistry Analysis for Lignin Determination in Wood and Pulps from Eucalyptus globulus. 2013, 8, 14
- Otles, S. & Ozyurt, V. H. 2015. Classical Wet Chemistry Methods. In: CHEUNG, P. C. K. (ed.) Handbook of Food Chemistry. Berlin, Heidelberg: Springer Berlin Heidelberg.
Further Reading