Systematic Analysis of Anions
It is thus constituting the cornerstone of qualitative inorganic chemistry by the systematic study of anions, making it possible to determine anionic parts of most compounds, mainly salts. Anions are negatively charged ions formed when atoms gain electrons; different chemical behaviors and interactions manifest. How these interactions are understood is cardinal to a chemist since anions are chief participants in so many chemical processes, for example, in acid-base, precipitation, and complex formation reactions.
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The identification of anions is generally done in the laboratory in a systematic order. These tests may broadly be classified under preliminary and confirmatory tests. Preliminary tests provide preliminary evidence about the presence of certain anions due to some observable reaction on the addition of some reagents, which may result in the evolution of gas or the formation of a precipitate. For example, when a sample containing carbonate ions, CO 32 is added to dilute sulphuric acid, effervescence occurs due to carbon dioxide gas formed. Confirmatory tests, however, are positive identifications of the anions by more specific reactions and observations.
Types and Aspects of Anion Analysis
Anion analysis can be broadly divided into preliminary tests and confirmatory tests. Preliminary tests are conducted to detect the presence of only a few anions by simple observations. For instance, the addition of dilute sulphuric acid to carbonate ions CO 32 results in effervescence due to carbon dioxide gas being produced. Another example is that sulfide ions, S2-, on reaction produce hydrogen sulfide gas with the characteristic smell of rotten eggs.
Confirmatory tests are based on more specific methods and clearly indicate the presence of an anion. For example, carbonate ions can be confirmed by the formation of a white precipitate formed through the reaction of limewater with the evolved carbon dioxide gas. Sulfide ions can be confirmed by the formation of a black precipitate of silver sulfide upon reaction with silver nitrate. Different anions form different reactions that are observed and measured, hence making the identification accurate.
The testing of anions forms the bedrock of most courses in chemistry but has numerous applications in environmental monitoring, clinical diagnostics, and industrial quality control. In these contexts, knowledge of reactions and characteristics of a given anion empowers practices that assure safety and compliance with regulatory standards.
In this test, the action of dilute sulphuric acid on the salt is noted at room temperature and on warming. Carbonate $\mathrm{CO}_3^{2-}$ sulphide $\left(\mathrm{S}^{2-}\right)^{\text {s }}$, sulphite $\left(\mathrm{SO}_3^{2-}\right.$, nitrite $\left(\mathrm{NO}_2^{-}\right)$and acetate $\left(\mathrm{CH}_3 \mathrm{COO}\right)$eact with dilute sulphuric acid to evolve different gases. A study of the characteristics of the gases evolved gives information about the anions.
Procedure:
Take 0.1 g of the salt in a test tube and add 1–2 mL of dilute sulphuric acid. Observe the change, if any, at room temperature. If no gas is evolved, warm the content of the test tube. If gas is evolved test it
| Observation | Inference | |
| Gas Evolved | Possible Anion | |
| A colorless, odorless gas is evolved with brisk effervescence, which turns lime water milky | $\mathrm{CO}_2$ | Carbonate $\left(\mathrm{CO}_3{ }^{2-}\right)$ |
| Colourless gas with the smell of rotten egg is evolved which turns lead acetate paper black. | $\mathrm{H}_2 \mathrm{~S}$ | Sulphide $\left(S^2\right)$ |
| Colourless gas with a pungent smell, like burning sulphur which turns acidified potassium dichromate solution green. | $\mathrm{SO}_2$ | Sulphite$\left(\mathrm{SO}_3{ }^2\right.$ |
| Brown fumes turn acidified potassium iodide solution containing starch solution blue. | $\mathrm{NO}_2$ | Nitrite $\left(\mathrm{NO}_2{ }^{-}\right)$ |
| Colorless vapors with the smell of vinegar turn blue litmus red | $\mathrm{CH}_3 \mathrm{COOH}$ Vapours | Acetate$\mathrm{CH}_3 \mathrm{COO}^{-}$ |
Confirmatory (wet) tests for anions are performed by using water extract when salt is soluble in water and by using sodium carbonate extract when salt is insoluble in water. Confirmation of $\mathrm{CO}_3^{2-}$ is done by using an aqueous solution of the salt or by using solid salt as such because sodium carbonate extract contains carbonate ions. Water extract is made by dissolving salt in water.
Preparation of sodium carbonate extract: Take 1 g of salt in a porcelain dish or boiling tube. Mix about 3 g of solid sodium carbonate and add 15 mL of distilled water to it. Stir and boil the content for about 10 minutes. Cool, filter, and collect the filtrate in a test tube and label it as sodium carbonate extract.
| Anion | Confirmatory Test |
| Carbonate $\mathrm{CO}_3{ }^{2-}$ | Take 0.1 g of salt in a test tube, and add dilute sulphuric acid $\mathrm{CO}_2$ gas is evolved with brisk effervescence which turns lime water milky. On passing the gas for some more time, milkiness disappears. |
| Sulphide $\left(S^{2-}\right)$ | Take 1 mL of water extract and make it alkaline by adding ammonium hydroxide or sodium carbonate extract. Add a drop of sodium carbonate extract. Add a drop of sodium nitroprusside solution. Purple or violet coloration appears |
| Sulphite $\left(\mathrm{SO}_3{ }^{2-}\right)$ |
|
| Nitrite $\left(\mathrm{NO}_2{ }^{-}\right)$ |
|
| Acetate $\mathrm{CH}_3 \mathrm{COO}^{-}$ |
|
If no positive result is obtained from dil. H2SO4 test, take 0.1 g of salt in a test tube and 3-4 drops of conc. H2SO4. Observe the change in the reaction mixture in cold and then warm it.
| Observation | Inference | |
| Gas | Possible | |
| A colorless gas with a pungent smell, which gives dense white fumes when a rod dipped in ammonium hydroxide is brought near the mouth of the test tube. | HCl | Chloride $\mathrm{Cl}^{-}$ |
| Reddish brown gas with a pungent odor has evolved. The intensity of reddish gas increases on heating the reaction mixture after the addition of solid $\mathrm{MnO}_2$ to the reaction mixture. The solution also acquires red color. | $B r_2$ Vapours | Bromide $B r^{-}$ |
| Violet vapors, which turn starch paper blue, and a layer of violet sublimate is formed on the sides of the tube. Fumes become dense on adding $\mathrm{MnO}_2$ to the reaction mixture. | $I_2$CO Vapours | Iodide $\left(I^{-}\right)$ |
| Brown fumes evolve which become dense upon heating the reaction mixture after the addition of copper turnings and the solution acquires blue color. | $\mathrm{NO}_2$ | Nitrate $\mathrm{NO}_3^{-}$ |
| Colourless, odorless gas is evolved which turns lime water milky and the gas coming out of lime water burns with a blue flame if ignited | CO and $\mathrm{CO}_2$ | Oxalate,$\left(\mathrm{C}_2 \mathrm{O}_4{ }^{2-}\right)$ |
Confirmatory tests for the anions that react with concentrated sulphuric acid are given below in the Table:
Confirmatory tests for $\mathrm{Cl}^{-}, \mathrm{Br}^{-}, \mathrm{I}^{-}, \mathrm{NO}_3^{-}$ and $\mathrm{C}_2 \mathrm{O}_4^2$
| Anion | Confirmatory Test |
| Chloride $\mathrm{Cl}^{-}$ | (a) Take 0.1 g salt in a test tube. Add a pinch of manganese dioxide and 3-4 drops of conc. Sulphuric acid, Heat the reaction mixture. Greenish-yellow chlorine gas is evolved which is detected by its pungent odor and bleaching action. (b) Take 1 ml. of sodium carbonate extract in a test tube. Acidify it with dil. $\mathrm{HNO}_3$ or take water extract and add silver nitrate solution. A curdy white precipitate is obtained which is soluble in ammonium hydroxide solution. (c) Take 0.1 g salt and a pinch of solid potassium dichromate in a test tube, add conc. $\mathrm{H}_2 \mathrm{SO}_4$, heat and pass the gas evolved through sodium hydroxide solution. It becomes yellow. Divide the solution into two parts. Acidify one part with acetic acid and add lead acetate solution. A yellow precipitate is formed. Acidify the second part with dilute sulphuric acid and add 1 ml of amyl alcohol followed by 1 ml of 10 % hydrogen peroxide. After gentle shaking the organic layer turns blue. |
| Bromide $B r^{-}$ | (a) Take 0.1 g of salt and a pinch of $\mathrm{MnO}_2$ in a test tube. Add 3-4 drops conc. sulphuric acid and heat. Intense brown fumes are evolved. (b) Neutralise 1 ml. of sodium carbonate extract with hydrochloric acid (or take the water extract). Add 1 ml. carbon tetrachloride $\left(\mathrm{CCl}_4\right)$ /chloroform $\mathrm{CHCl}_3$/carbon disulphide $C S_2$. Now add an excess of chlorine water dropwise and shake the test tube. A brown coloration in the organic layer confirms the presence of bromide ions. (c) Acidify 1 ml of sodium carbonate extract with dil. $\mathrm{HNO}_3$ (or take 1 mL water extract) and add silver nitrate solution. A pale yellow precipitate soluble with difficulty in ammonium hydroxide solution is obtained. |
| Iodide $I^{-}$ | (a) Take 1 ml. of salt solution neutralized with HCl and add 1 mL chloroform/carbon tetrachloride/carbon disulfide. Now add an excess of chlorine water dropwise and shake the test tube. A violet color appears in the organic layer. (b)Take 1 ml of sodium carbonate extract and acidify it with dil. $\mathrm{HNO}_3$ (or take water extract). Add, silver nitrate solution. A yellow precipitate insoluble in $\mathrm{NH}_4 \mathrm{OH}$ solution is obtained. |
| Nitrate $\left(\mathrm{NO}_3^{-}\right)$ | Take 1 mL of salt solution in water in a test tubeAdd 2 ml of conc. $\mathrm{H}_2 \mathrm{SO}_4$ and mix thoroughly. Cool the mixture under the tap. Add freshly prepared ferrous sulphate along the sides of the test tube without shaking. A dark brown ring is formed at the junction of the two solutions. |
| Oxalate$\left(\mathrm{C}_2 \mathrm{O}_4^{2-}\right)$ | (a) Take 1 ml. of water extract or sodium carbonate extract acidified with acid and add calcium chloride solution. A white precipitate insoluble in ammonium oxalate and oxalic acid solution but soluble in dilute hydrochloric acid and dilute nitric acid is formed. (b) Take the precipitate from test (a) and dissolve it in dilute. $\mathrm{H}_2 \mathrm{SO}_4$. Add a very dilute solution of $\Omega \mathrm{MnO} \mathrm{O}_4$ and warm. Color of $\mathrm{M} M \mathrm{O}_4$ solution is discharged. Pass the gas coming out through lime water. The lime water turns milky. |
Anion Analysis: Applications in Real Life
It is relevant to systematic anion analyses in so much as applied beyond the laboratory. In environmental science, anionic species like nitrates and phosphates in water bodies form part of the parameters that determine the quality of water and health in aquatic ecosystems. High nitrate contents, for example, lead to eutrophication, which greatly damages marine life and renders waters useless. Further, constant monitoring of these anions provides the base for the development of methods for controlling pollution and preservation of water resources.
Testing of anions in the pharmaceutical industry helps prove that drugs are safe and will work as expected. For instance, anions result in undesirable reactions in case they are in the formulation of certain drugs. Very stringent testing programs have been put in place to help identify and quantify these anions with a view to obeying the safety regulations.
Another important application area in anion analysis is in food safety. A few anions, such as sulfites, could be a potential cause for allergic reactions in some susceptible individuals. As such, regulatory bodies insist that manufacturers of such foods carry out rigorous analyses in detecting the anions for consumer safety.
Moreover, new developments in the techniques of analysis increase the level of effectiveness and precision in anion analysis. These methods can detect several anions from the same sample at a go, greatly cutting down on the time required for analysis and bringing out reliable data.
In other words, qualitative analysis is the systematized analysis of anions, which impacts almost every sector. The methodologies and importance of anion analysis give people an understanding and appreciation for its protection of health, and the environment, and assurance of quality in products and processes.
Some Solved Examples
Example 1
Question:
On mixing two colorless gases, a deep brown color is observed. The gases are
1)$\mathrm{N}_2 \mathrm{O}$ and $\mathrm{O}_2$
2)NO and $\mathrm{O}_2$
3) $\mathrm{N}_2 \mathrm{O}_3$ and $\mathrm{O}_2$
4) None of these
Solution:
Nitric Oxide reacts with oxygen gas to form reddish-brown nitrogen dioxide.
$2 \mathrm{NO}+\mathrm{O}_2 \rightarrow 2 \mathrm{NO}$
( Deep brown gas)
Hence, the answer is the option (2).
Example 2
Question:
Which of the following is a colorless gas with a smell of rotten egg?
1) $\mathrm{H}_2 \mathrm{~S}$
2) $\mathrm{PH}_3$
3) $\mathrm{SO}_2$
4) None of these
Solution:
Hydrogen sulfide (H2S) is a colorless, highly toxic gas with a distinct smell of rotten eggs. It is commonly found in natural gas, petroleum, volcanic gases, and some well waters. The characteristic odor of hydrogen sulfide is often associated with the breakdown of organic matter containing sulfur compounds, such as in sewage, swamps, or certain industrial processes.
Hence, the answer is the option (1).
Example 3
Question:
Which of the following salts will evolve sulfur dioxide gas along with the formation of yellowish turbidity when treated with dilute H2SO4?
1) Sodium sulphate
2) Sodium sulphite
3) Sodium sulphide
4) Sodium thiosulphate
Solution:
As we have learned,
$\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ is decomposed by $\mathrm{H}_2 \mathrm{SO}_4$, giving S and $\mathrm{SO}_2$. The reaction occurs as follows:
$\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+\mathrm{S}+\mathrm{SO}_2+\mathrm{H}_2 \mathrm{O}$
Hence, the answer is the option (4).
Summary
Systematic examination of anions is, therefore, an inseparable part of qualitative inorganic chemistry dealing with establishing the anionic composition of various compounds by a sequence of systematic tests. This paper has been aimed at describing the most important aspects related to anion analysis, starting from preliminary tests carried out with diluted and concentrated sulphuric acid that give preliminary indications of the presence of certain anions. The confirmatory tests will further fix these findings and give a correct identification regarding anions like carbonate, sulfide, sulfite, nitrite, and acetate.
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