Editing Gravimetric analysis (section)

==Volatilization methods==

Volatilization methods can be either ''direct'' or ''indirect''. Water eliminated in a quantitative manner from many inorganic substances by ignition is an example of a direct determination. It is collected on a solid desiccant and its mass determined by the gain in mass of the desiccant.

Another direct volatilization method involves carbonates which generally decompose to release carbon dioxide when acids are used. Because carbon dioxide is easily evolved when heat is applied, its mass is directly established by the measured increase in the mass of the absorbent solid used.<ref>{{Cite book|title=Fundamentals of Analytical Chemistry|last=Skoog|first=Douglas A|publisher=Saunders and Harcourt Brace|year=1996|pages=97}}</ref><ref>{{Cite book|title=General Chemistry: Principals and Modern Applications|last1=Petrucci|first1=Ralph H|last2=Harwood|first2=William S|publisher=Macmillan Publishing Company|year=1993|isbn=978-0-02-394931-9|editor-last=Corey|editor-first=Paul F|location=New York|pages=265–268}}</ref>

Determination of the amount of water by measuring the loss in mass of the sample during heating is an example of an indirect method. It is well known that changes in mass occur due to decomposition of many substances when heat is applied, regardless of the presence or absence of water. Because one must make the assumption that water was the only component lost, this method is less satisfactory than direct methods.

This often faulty and misleading assumption has proven to be wrong on more than a few occasions. There are many substances other than water loss that can lead to loss of mass with the addition of heat, as well as a number of other factors that may contribute to it. The widened margin of error created by this all-too-often false assumption is not one to be lightly disregarded as the consequences could be far-reaching.

Nevertheless, the indirect method, although less reliable than direct, is still widely used in commerce. For example, it's used to measure the moisture content of cereals, where a number of imprecise and inaccurate instruments are available for this purpose.

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=== Types of volatilization methods ===
In volatilization methods, removal of the analyte involves separation by heating or chemically decomposing a volatile sample at a suitable temperature.<ref name=":0" /><ref>{{Cite web |date=January 8, 2017 |title=Introduction to gravimetric analysis |url=https://www.khanacademy.org/ |url-status=live |archive-url=https://web.archive.org/web/20190801125426/https://www.khanacademy.org/ |archive-date=August 1, 2019 |access-date=January 8, 2017}}</ref> In other words, thermal or chemical energy is used to precipitate a volatile species.<ref>{{Cite web |date=January 8, 2017 |title=Gravimetric Methods of Analysis |url=https://www.emu.edu.tr/mugarip/chem247/lectureppt/c12%20gravimetric%20analysis.pdf |url-status=dead |archive-url=https://web.archive.org/web/20171118061918/https://www.emu.edu.tr/mugarip/chem247/lectureppt/c12%20gravimetric%20analysis.pdf |archive-date=November 18, 2017 |access-date=January 8, 2017}}</ref> For example, the water content of a compound can be determined by vaporizing the water using thermal energy (heat). Heat can also be used, if oxygen is present, for combustion to isolate the suspect species and obtain the desired results.

The two most common gravimetric methods using volatilization are those for water and carbon dioxide.<ref name=":0" /> An example of this method is the isolation of sodium hydrogen bicarbonate (the main ingredient in most antacid tablets) from a mixture of carbonate and bicarbonate.<ref name=":0" />  The total amount of this analyte, in whatever form, is obtained by addition of an excess of dilute [[sulfuric acid]] to the analyte in solution.

In this reaction, nitrogen gas is introduced through a tube into the flask which contains the solution. As it passes through, it gently bubbles. The gas then exits, first passing a drying agent (here CaSO<sub>4</sub>, the common desiccant ''Drierite''). It then passes a mixture of the drying agent ''and'' sodium hydroxide which lies on asbestos or ''Ascarite II'', a non-fibrous silicate containing sodium hydroxide.<ref>{{Cite book |last1=Skoog |first1=Douglas A |title=Fundamentals of Analytical Chemistry |last2=West |first2=Donald M |last3=Holler |first3=F James |publisher=Saunders College Publishing and Harcourt Brace |year=1995 |edition=Seventh |location=Fort Worth |pages=96–97 |chapter=5.6 |lccn=95-067683}}</ref> The mass of the carbon dioxide is obtained by measuring the increase in mass of this absorbent.<ref name=":0" />  This is performed by measuring the difference in weight of the tube in which the ascarite contained before and after the procedure.

The calcium sulfate (CaSO<sub>4</sub>) in the tube retains carbon dioxide selectively as it's heated, and thereby, removed from the solution. The drying agent absorbs any aerosolized water and/or water vapor (reaction 3.). The mix of the drying agent and NaOH absorbs the CO<sub>2</sub> and any water that may have been produced as a result of the absorption of the NaOH (reaction 4.).<ref name=":2">{{Cite web |date=January 8, 2017 |title=Section 3-2: Volatilization methods |url=https://www.csudh.edu/oliver/che230/textbook/ch03.htm |url-status=live |archive-url=https://web.archive.org/web/20161125160038/http://csudh.edu/oliver/che230/textbook/ch03.htm/ |archive-date=November 25, 2016 |access-date=January 8, 2017}}</ref>

The reactions are:

Reaction 3 - absorption of water

NaHCO<sub>3</sub><small>(aq)</small> + H<sub>2</sub>SO<sub>4</sub><small>(aq)</small> → CO<sub>2</sub><small>(g</small>) + H<sub>2</sub>O<small>(l)</small> + NaHSO<sub>4</sub><small>(aq).</small><ref name=":2" />

Reaction 4. Absorption of CO<sub>2</sub> and residual water

CO<sub>2</sub><small>(g)</small> + 2 NaOH<small>(s)  →</small> Na<sub>2</sub>CO<sub>3</sub><small>(s)</small> + H<sub>2</sub>O<small>(l)</small><small>.</small><ref name=":2" />