QUANTITATIVE ANALYSIS IN CHEMISTRY: MEASUREMENTS, PREPARATION - CHEMISTRY - 2025

The quantitative analysis in chemistry, as in any discipline, consists essentially of determining amounts of a specific substance. These analyzes answer the question 'how much is there?' of a substance in different samples; of soil, seas, rivers, soft drinks, liquids, plant extracts, animals, solids, crystals, among many more.

The question 'how much is there?' It has been formulated since man conceived the interest of, for example, extracting minerals and precious stones, oils, spices, with the firm intention of commercializing them. Today money continues to be one of the main reasons a substance or analyte is quantified.

Hundreds of test tubes: glassware used daily in sample preparations for quantitative analysis. Source: Pexels.

One mineral may be richer in gold than another. To find out, it is necessary to determine the centesimal composition of the two minerals, and the one with the highest percentage of gold will be a more attractive source to extract the coveted metal. The same thing happens with foreign or radioactive metals.

The techniques to quantify and on which quantitative analyzes are based are very diverse and varied. Each implies a separate specialization, as well as its deep theoretical bases. However, the point where they all converge is always to answer the same question already mentioned; question that talks about quality, purity, performance, reliability, etc.

Measurements

To quantify any substance or material, it is essential to be able to measure any of its physical or chemical properties. The property selected will depend on the substance and the technique used. A useful clue to recognizing that an analysis technique can quantify is that it has the suffix -metry at the end of its name.

For example, the two classic measurement techniques in analytical chemistry are gravimetry (mass measurement), and volumetry (volume measurement).

They are considered classic because in principle they do not need too sophisticated instruments or the use of electromagnetic radiation; but analytical balances, mortars, crucibles and glassware.

Gravimetric

In gravimetry, it almost always seeks to obtain a precipitate after a series of methodological steps, to which the mass is determined.

For example, one technique to quantify chloride ions in a sample is to precipitate them as silver chloride, AgCl; a milky white precipitate which can be weighed.

Another simple example is to determine the moisture content of a body, material, or solid.

To do this, the mass of the solid is first weighed before introducing it to an oven at approximately 100ºC, long enough for the water to vaporize. Afterwards, it is weighed again and the difference between the final mass and the initial mass is equal to the mass of water that has been vaporized; that is, its moisture content,% H ₂ O.

If this analysis were performed on watermelons, it would not be surprising if their% H ₂ O was too high (~ 95%); while for the so-called nuts, it is expected that they contain little water (% H ₂ O <10%), a characteristic to which that name is attributed.

Volumetric

On the other hand, volumetry works with volumes from which, after applying titrations, the concentration of the dissolved species of interest is extracted. For example, an analyte whose color is sensitive to a specific reaction can be determined by colorimetric titrations.

Likewise, the acid number (AI) of the oils (edible or not) can be determined by acid-base titrations, using a solution of a standardized strong base (usually KOH or NaOH). With AI values, in addition to other parameters, different types of oils can be characterized and classified according to their sources and other variables.

These analytical measurements always give a result accompanied by a physical unit (and its experimental errors). What is the concentration? How pure is the sample? Does the given amount represent a health risk? What was the yield of the reaction?

These and more questions are answered after measurements and data processing.

Preparation of standards or standards

"With the same rod that you measure your standards, you will measure your samples." And this hypothetical rod will have divisions and subdivisions, each with different magnitudes of the property of the analyte that correlates with its concentration. Said magnitudes or values ​​are finally compared with those obtained when measuring the property of the analyte.

For this, a calibration curve must always be constructed from a selection of standards or standards whose concentrations are previously known.

And how to know them in advance? Because they are independent variables: the analyst decides, depending on the type of sample or analysis, how much of the pattern will weigh.

Sweet mushrooms

A hypothetical example could be framed in the study of the content of sugars or total carbohydrates of numerous families of mushrooms. The standard, made up of sugars previously detected thanks to qualitative analysis of the mushrooms, should ideally perfectly mimic the organic matrix of the samples.

Then, prepared, the patterns react causing a color change. If its intensity is measured by UV-vis spectroscopy, it can be compared with the intensities of the colors emitted by the sugars in the samples; and thus, through a mathematical clearance, determine the content of total sugars.

Once this is done, a calibration curve can be constructed from the samples, so that other mushrooms (from the same region or country) can have their sugars determined directly without preparing other standards.

Collection and treatment

In quantitative analyzes there are many variables that must be rigorously treated depending on the type of study. Often times, it is not enough to just go around collecting samples left and right; Where is it collected? Is it significant? What quantities? What will be the pre-treatment and other procedures?

In the example of mushrooms, it is necessary to know which families the sugars will be determined from, in which plantations or natural origin they will be collected, at what time of year, the orographic characteristics, etc. Once the mushrooms are collected (oils, grains, inks, meteorites, biological substances), what to do with them before quantitative analysis?

Almost always, a quantitative analysis is preceded by a qualitative analysis; identify what compounds the samples have, especially if you are working with them for the first time.

Some treatments can be purely physical: such as the grinding of a vegetable mass, or the acid digestion of a mineral. Others, on the other hand, are chemical: an esterification reaction, acidic or basic hydrolysis, substitution, amination, etc., to thus produce a species capable of being more easily quantified by the selected technique.

Examples

Some everyday examples of quantitative analysis in chemistry will be mentioned to finish:

-Determination of the alcoholic degree of beers, wines, and craft beverages.

-From the urine of a patient it can be known if there is an increase or decrease in the concentration of one or more components, which is related to the detection of a disease. Likewise, if a drug is excreted in the urine, it can be determined how much of the drug was "assimilated" by the body.

-Determination of the centesimal composition of mineral samples, terrestrial or extraterrestrial.

-Given some crude samples, the H / C ratio is determined to compare the degrees of aromaticity of crude oils from different sources. Heavy crude oils are characterized by having an H / C of less than 1; the lighter it is, the more H / C will have a value above 1.

-Determination of the nutritional composition of food and edible products.

-Studies of the stability of drugs as part of the relevant quality analyzes for their commercialization and storage.

-Monitoring of the degree of contamination caused by substances in water samples from rivers, streams, lagoons or seas. Likewise, the gaseous emanations from factories are determined their composition to prevent them from dislodging large amounts of gases that are harmful to the environment.

References

1. Daniel C. Harris. (2007). Quantitative Chemical Analysis. (Seventh edition). WH Freeman and Company.
2. The Editors of Encyclopaedia Britannica. (January 31, 2018). Quantitative chemical analysis. Encyclopædia Britannica. Recovered from: britannica.com
3. Chapter 1, Introduction to Quantitative Chemical Analysis. Recovered from: 5.csudh.edu
4. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Understanding Quantitative Analysis in Chemistry. Recovered from: thoughtco.com
5. Materials Evaluation and Engineering. (2019). Quantitative Chemical Analysis. Recovered from: mee-inc.com
6. Xin Wen. (2008). Quantitative Analysis (CHEM 201).. Recovered from: calstatela.edu