The volatilization is the process of converting a chemical a liquid or solid to a gaseous state or vapor state. Other terms used to describe the same process are vaporization, distillation, and sublimation.
One substance can often be separated from another by volatilization and can then be recovered by vapor condensation.
The substance can be more rapidly volatilized by either heating it to increase its vapor pressure or by removing the vapor using an inert gas stream or a vacuum pump.
Heating procedures include volatilization of water, mercury, or arsenic trichloride to separate these substances from interfering elements.
Chemical reactions are sometimes used to produce volatile products as in the release of carbon dioxide from carbonates, ammonia in the Kjeldahl method for nitrogen determination, and sulfur dioxide in the determination of sulfur in steel.
The volatilization methods are generally characterized by great simplicity and ease of operation, except when high temperatures or highly corrosion-resistant materials are needed (Louis Gordon, 2014).
Vapor pressure volatilization
Knowing that the boiling temperature of water is 100 ° C, have you ever wondered why rainwater evaporates?
Is it 100 ° C? If so, why don't I get warm? Have you ever wondered what gives the characteristic aroma to alcohol, vinegar, wood or plastic? (Vapor Pressure, SF)
Responsible for all this is a property known as vapor pressure, which is the pressure exerted by a vapor in equilibrium with the solid or liquid phase of the same substance.
Also, the partial pressure of the substance in the atmosphere on the solid or liquid (Anne Marie Helmenstine, 2014).
Vapor pressure is a measure of the tendency of a material to change to the gaseous or vapor state, that is, a measure of the volatility of substances.
As the vapor pressure increases, the greater the capacity of the liquid or solid to evaporate, thus being more volatile.
Vapor pressure will increase with temperature. The temperature at which the vapor pressure on the surface of a liquid equals the pressure exerted by the environment is called the boiling point of the liquid (Encyclopædia Britannica, 2017).
The vapor pressure will depend on the solute dissolved in solution (it is a colligative property). On the surface of the solution (air-gas interface) the most superficial molecules tend to evaporate, exchanging between phases and generating a vapor pressure.
The presence of solute reduces the number of solvent molecules at the interface, reducing the vapor pressure.
Figure 1: decrease in vapor pressure when having a dissolved solute.
The change in vapor pressure can be calculated with Raoult's Law for non-volatile solutes which is given by:
Where X2 is the mole fraction of the solvent. If we multiply both sides of the equation by P ° then it remains:
Substituting (1) in (3) is:
(4)
This is the variation in vapor pressure when a solute dissolves (Jim Clark, 2017).
Gravimetric analysis
Gravimetric analysis is a class of laboratory techniques used to determine the mass or concentration of a substance by measuring a change in mass.
The chemical we are trying to quantify is sometimes called the analyte. We could use gravimetric analysis to answer questions such as:
- What is the concentration of the analyte in a solution?
- How pure is our sample? The sample here could be a solid or in solution.
There are two common types of gravimetric analysis. Both involve changing the phase of the analyte to separate it from the rest of a mixture, resulting in a change in mass.
One of these methods is precipitation gravimetry, but the one that really interests us is volatilization gravimetry.
Volatilization gravimetry is based on thermally or chemically decomposing the sample and measuring the resulting change in its mass.
Alternatively, we can catch and weigh a volatile decomposition product. Because the release of a volatile species is an essential part of these methods, we collectively classify them as volatilization gravimetric analysis methods (Harvey, 2016).
Gravimetric analysis problems are simply stoichiometry problems with a few extra steps.
To perform any stoichiometric calculation, we need the coefficients of the balanced chemical equation.
For example, if a sample contains barium chloride dihydrate (BaCl 2 H 2 O) impurities, the amount of impurities can be obtained by heating the sample to evaporate the water.
The difference in mass between the original sample and the heated sample will give us, in grams, the amount of water contained in the barium chloride.
With a simple stoichiometric calculation, the amount of impurities in the sample will be obtained (Khan, 2009).
Fractional distillation
Fractional distillation is a process by which the components of a liquid mixture are separated into different parts (called fractions) according to their different boiling points.
The difference in volatilities of the compounds in the mixture plays a fundamental role in their separation.
Fractional distillation is used to purify chemicals and also to separate mixtures to obtain their components. It is used as a laboratory technique and in industry, where the process is of great commercial importance.
Vapors from a boiling solution are passed through a tall column, called a fractionation column.
The column is packed with plastic or glass beads to improve separation by providing more surface area for condensation and evaporation.
Figure 2: setup for fractional distillation in the laboratory.
The temperature of the column gradually decreases along its length. Components with a higher boiling point condense on the column and return to solution.
Components with lower boiling points (more volatile) pass through the column and are collected near the top.
Theoretically, having more beads or plates improves separation, but adding plates also increases the time and energy required to complete a distillation (Helmenstine, 2016).
References
- Anne Marie Helmenstine. (2014, May 16). Vapor Pressure Definition. Recovered from thoughtco.com.
- Encyclopædia Britannica. (2017, February 10). Vapor pressure. Recovered from britannica.com.
- Harvey, D. (2016, March 25). Volatilization Gravimetry. Recovered from chem.libretexts.
- Helmenstine, AM (2016, November 8). Fractional Distillation Definition and Examples. Recovered from thoughtco.com.
- Jim Clark, IL (2017, March 3). Raoult's Law. Recovered from chem.libretexts.
- Khan, S. (2009, August 27). Introduction to gravimetric analysis: Volatilization gravimetry. Recovered from khanacademy.
- Louis Gordon, RW (2014). Recovered from accessscience.com.
- Vapor Pressure. (SF). Recovered from chem.purdue.edu.