In this explainer, we will learn how to identify aqueous negative ions based on their reactivity and the color and solubility of their salts.
A laboratory chemical test will generally rely on the occurrence of one of the following. These are fundamental indications that a chemical reaction has occurred:
- a color change,
- the production of an odor,
- a change in temperature,
- the production of a gas (such as the formation of bubbles),
- the production of a precipitate.
A test can be positive or negative.
A positive test result (A) means that we observe the change we expected to get if the substance we were testing for was present. Test results B, C, and D show negative results; however, some chemical tests give the same results for other substances too (E), so we should use other methods to confirm our initial ideas:
A negative test result means that something else happened, including nothing at all. Generally, a negative result means that we can be sure that the substance we were testing for was not there:
By combining appropriate tests and using both positive and negative results, we can narrow down the candidates until we are sure what was and what was not there to begin with.
In this kind of qualitative analysis, different chemical substances are used to identify unknown anions, also known as acidic radicals. Negative monatomic and polyatomic ions are commonly referred to as anions. Anions have a net negative charge as the total number of electrons in the species outnumber the total number of protons.
When hydrogen ions are lost from an acid, the anion remaining is referred to as an acidic radical. Similarly, when hydroxide ions are lost from a hydroxide compound, the cation, known as a basic radical, remains. When acidic radicals and basic radicals combine chemically, a salt is formed.
We will examine three separate groups of anions in this explainer:
- anions detected using dilute hydrochloric acid,
- anions detected using concentrated sulfuric acid,
- anions detected by barium chloride solution.
|Concentrated Sulfuric Acid Group
|Barium Chloride Group
This order of testing in these groups has consequences. Any unknown anion should initially be tested using dilute hydrochloric acid. Should hydrochloric acid prove to be ineffective at determining the identity of the unknown acidic radical, then concentrated sulfuric acid should be used. Similarly, if concentrated sulfuric acid proves to be insufficient to determine the identity of the unknown anion, then barium chloride solution should be used.
The order of this sequence is determined by the strength of the acidic radicals. In the first group, the main reagent is dilute hydrochloric acid. This acid is more stable than the anions that it is used to test for, such as the carbonate anion. In this case, the hydrochloric acid will replace the less stable anions resulting in the evolution of gases that we can test for.
In the second group, concentrated sulfuric acid is used. The sulfuric acid is more stable than the anions that it is testing for, which include the chloride anion that was the testing reagent in the first group.
In the final group, we do not have a suitable reagent that is more stable than the sulfate and phosphate anions, and as such this group is tested using barium chloride solution.
Dilute hydrochloric acid can be used to identify the following anions:
- carbonate (),
- sulfite (),
- bicarbonate (),
- sulfide (),
- thiosulfate (),
- nitrite ().
In this explainer, we will focus on
- carbonate (),
- sulfite (),
The carbonate anion () is generally poorly soluble in water; however, sodium carbonate, potassium carbonate, and ammonium carbonate are soluble. The following primary test will work with solid carbonates and solutions.
Carbonate ions decompose if they react with acids, producing carbon dioxide and water.
Reaction: Carbonate Ions and Acid
Therefore, if we treat a substance with dilute hydrochloric acid and bubbles of gas are produced, the substance could have contained carbonate ions. We can test if the gas is carbon dioxide using the limewater test. Almost any acid will work, but hydrochloric acid is preferred because it is generally redox stable and cheap. This is the full test.
Example 1: Recall the Solution Used to Test the Gas Given Off by a Carbonate Anion Reacting with an Acid
When testing for the carbonate anion using dilute hydrochloric acid, through which of the following solutions is the gas produced commonly passed?
- Acidified potassium permanganate
- Acidified potassium dichromate
- Barium chloride
- Silver nitrate
If carbonate ions react with acids, they decompose into carbon dioxide and water according to the following equation:
Limewater is a saturated solution of calcium hydroxide. When carbon dioxide gas is passed through limewater it turns cloudy, as white calcium carbonate is formed in the solution of calcium hydroxide.
Acidified potassium permanganate and acidified potassium dichromate are both solutions that can be used to test for the presence of reducing gases such as sulfur dioxide. Barium chloride solution is used to test for sulfate and phosphate anions. Finally, silver nitrate is used in multiple tests throughout the testing protocol used to identify unknown acidic radicals. However, none of these substances are linked to a chemical test for a carbonate via the generation of carbon dioxide gas.
Taking all this into consideration, the correct answer is C, limewater.
However, bicarbonate ions also react in a similar way and so a confirmatory test is required. Initially, the unknown substance can be mixed with water; most carbonates are insoluble and if the salt is insoluble in cold water, then this can indicate the presence of carbonate. However, some carbonates dissolve, and all bicarbonates dissolve, so this may not be enough. If a solution is formed, a confirmatory test using magnesium sulfate will be necessary.
If magnesium sulfate is added to a cold solution in liquid form, the magnesium ions will react with the soluble carbonate ions instantly forming a white precipitate, of , which is soluble in cold dilute hydrochloric acid:
In the case of a bicarbonate solution, a white precipitate will not form unless the solution is heated. During the heating process, the bicarbonate anion decomposes forming a carbonate anion, which reacts as shown above.
Testing with dilute hydrochloric acid is also used to detect the presence of sulfite anions.
In the primary test, sulfite ions can be converted to sulfur dioxide () by treatment with hydrochloric acid:
is colorless, toxic, and acidic, and it is responsible for the smell of burned matches.
Not only is acidic, but also it can be oxidized. The primary test for a sulfite is to test for using oxidizing agents such as potassium permanganate () or potassium dichromate ().
The gas produced when is added to the unknown solid is tested with strips of filter paper wetted with acidified solutions of either or .
will cause the color of a solution of potassium permanganate to change from deep purple to pale pink:
It will also cause the color of a solution of potassium dichromate to change from bright orange to green:
Therefore, if treating a solution with hydrochloric acid and heat produces an acidic and oxidizable gas, the original solution probably contained sulfite ions.
A confirmatory test for the sulfite anion involves dissolution of the unknown solid followed by mixing the solution with silver nitrate. In the case of a sulfite being present, a white precipitate of silver sulfate is formed, which turns black upon heating:
If testing with hydrochloric acid proves to be ineffective, the second stage of testing involves using concentrated sulfuric acid.
Concentrated sulfuric acid can identify the following anions:
- halides (, , , and ),
- nitrate ().
The primary test for halide ions is generally used on solids, while the second confirmatory test is generally used on solutions of those solids.
When concentrated sulfuric acid () is used to detect halide ions, concentrated sulfuric acid is added dropwise to a solid sample. Any gas produced can initially be tested with damp blue litmus paper.
Salts that contain fluoride or chloride ions produce results that are visually identical. The reaction for these types of salts is simply an acid–base reaction:
The difference in the equations is because is a weaker acid than but a stronger acid than . is weaker than both.
The gas evolved produces white steamy fumes when introduced to a glass rod, wet with ammonia solution, due to the production of ammonium chloride, :
When concentrated sulfuric acid reacts with bromide or iodide salts, the same acid–base reaction occurs:
However, a redox reaction also occurs. Sulfuric acid is not a strong enough oxidizing agent to oxidize or , but it is strong enough to oxidize or :
Sulfuric acid is reduced by to form sulfur dioxide (). ions are stronger reducing agents than ions and reduce sulfuric acid further to form hydrogen sulfide ().
and are both smelly, toxic gases and the steamy acidic fumes of , , , and are all hazardous and so great caution is required when performing these tests that should always be carried out under a fume hood.
The following are the results of the silver nitrate confirmatory tests for common halides.
Fluoride ions do not react with the test solution and no precipitate is produced. In other words, there is no positive test for fluoride ions using silver nitrate.
For the other halides, the following reaction occurs:
We can simplify this by omitting the nitrate ion, which is a spectator ion:
The chloride, bromide, and iodide salts of silver are all insoluble in water, so they will precipitate. However, silver fluoride is highly water soluble, so it will not precipitate.
|White precipitate (turns violet in sunlight)
|Pale yellow precipitate
If, for example, we had a solution that contained a sodium halide salt but we did not know which halide it was, we could use this test. If we got a cream-colored precipitate, we know that the salt is sodium bromide.
One major concern for this test, even if done correctly, is that it is sometimes difficult to tell the difference between the possible precipitates. In some lighting, white can look cream, and cream can look yellow. The similarity between the color of the different precipitates can be seen in the photograph below (from left to right: , , and ).
One way of handling this problem is to do the confirmatory test on our unknown solution side by side with known solutions that contain the halides (e.g., a solution of , a solution of , and a solution of ). We can then compare the colors of the precipitates under the same conditions.
Another way is to use a solution of ammonia. Ammonia forms a complex ion with silver(I) ions and when we add ammonia to a solution containing a silver precipitate it can pull the silver back into the solution, making the precipitate disappear.
With the silver halide salts, we get different results when we add dilute ammonia and excess ammonia solution.
If we use concentrated ammonia, we may reach excess very quickly. However, if we add concentrated ammonia carefully in small quantities, we can get the same results as with dilute ammonia solution.
Example 2: Identifying Silver Halide Salts through the Use of Ammonia Solutions
A student wants to confirm the identity of a silver halide precipitate by using an ammonia solution.
- Which silver halide precipitate dissolves only when a dilute ammonia solution is added?
- Which silver halide precipitate does not dissolve when a concentrated ammonia solution is added?
When a solution containing silver ions is mixed with a second solution containing halide ions (, , and ), a solid silver halide precipitate is formed. The color of the solid silver precipitate is sometimes difficult to distinguish as silver chloride, silver bromide, and silver iodide are all similar light-colored solids.
We can confirm the identification of the silver halide salt by using dilute ammonia solution to test whether or not the precipitate dissolves.
Silver chloride dissolves in a small quantity of dilute ammonia solution, whereas silver bromide and silver iodide will not dissolve and as such the answer to part 1 is silver chloride, .
Adding a small quantity of dilute ammonia solution to silver bromide has no effect; however, if we add excess dilute ammonia solution, then the silver bromide precipitate dissolves.
In the case of silver iodide, even an excess quantity of dilute ammonia solution will not dissolve the precipitate. The same is true even if we use a concentrated ammonia solution; using a concentrated ammonia solution merely accelerates these reactions, causing them to happen with smaller volumes of chemicals. This means that the correct answer to part 2 of this question is silver iodide, .
Testing with concentrated sulfuric acid is also used to detect the presence of nitrate anions.
The nitrate anion () is highly water soluble in the presence of many different cations, so it is difficult to detect using precipitation.
- A primary test is the generation of brown nitrogen dioxide gas through the addition of concentrated sulfuric acid on the nitrate salt.
- A confirmatory test is formation of a brown ring through the reaction of nitrate salt solution with iron(II) sulfate and concentrated sulfuric acid.
The primary test is to react a solid, potentially containing nitrate ions, with concentrated sulfuric acid to form nitric acid:
The nitric acid formed will then start to decompose when heated, releasing nitrogen dioxide which has a characteristic brown color:
The rate of generation of the gas can be increased through the addition of copper filings:
The confirmatory test is to reduce nitrate ions with iron(II) ions (. One of the products of the reduction of nitrate ions is nitrogen monoxide (), which can complex with residual iron(II) ions to form a brown solid. Sulfuric acid is necessary for the reaction. If concentrated sulfuric acid is not mixed in with the solution, it will instead form a layer at the bottom of the reaction vessel (concentrated sulfuric acid is much denser than water). A brown ring of the iron(II) complex will form between the solution layer and the sulfuric acid layer; this is why this test is often known as the brown ring test.
The main reactions involved are the reduction of nitrate ions by iron(II) ions: and the formation of the brown iron(II) complex:
It is vital that the iron(II) ions in the sulfate solution do not be oxidized by the air to iron(III) ions; otherwise, the test cannot work. It is recommended that the solution of iron(II) sulfate is prepared fresh, just before the test is performed.
If testing with dilute hydrochloric acid and concentrated sulfuric acid proves to be ineffective, the third stage of testing involves using barium chloride solution.
Testing with barium chloride solution is used to identify the following anions:
- sulfate (),
- phosphate ().
It is not possible to use another acid to displace the sulfate and phosphate anions, which originate from very strong acids themselves. For example, hydrochloric acid is less stable than sulfuric acid, so it cannot replace the sulfate anion in salt solutions. Consequently, in this miscellaneous group, we will not be testing for a liberated gas, but instead we will be testing for the formation of a precipitate.
In this explainer, we will focus on sulfate ().
We can detect the sulfate anion in solution by introducing barium ions (). These react with sulfate ions to produce a white precipitate of barium sulfate (). Barium sulfate is insoluble even in dilute mineral acids like hydrochloric acid and nitric acid. Barium phosphate is a similar white precipitate to barium sulfate; however, this white precipitate is soluble in dilute hydrochloric acid:
The confirmatory test for the sulfate anion is to add lead acetate solution. In this precipitation reaction, the white precipitate of lead sulfate is formed, as shown in the chemical equation below:
Example 3: Deriving the Net Ionic Equation for the Reaction between Barium Nitrate and a Metal Sulfate
What is the net ionic equation for the reaction between barium nitrate and a metal sulfate, which is used as a test for the sulfate anion?
In a net ionic equation, we do not include any spectator ions. These are ions that appear on both sides of the reaction equation. Rather than jumping straight to this form, it can be easier to start from the full equation first.
The formula for barium nitrate is .
For the metal sulfate, we can use sodium sulfate ().
When these react in solution, the product will be a white precipitate of barium sulfate ().
This is the equation:
To convert this to the ionic equation, we can dissociate the soluble salts:
and appear on both sides of the equation in equal amounts; they are spectator ions. So, the net ionic equation is
Let’s summarize what we have learned about testing for anions.
- Carbonate ions can be detected by treatment with dilute hydrochloric acid, which produces carbon dioxide gas; this gas can be passed through limewater, turning it cloudy.
- To distinguish between carbonate ions and bicarbonate ions, magnesium sulfate is added as a confirmatory test for carbonates and bicarbonates. In cold solution, an immediate white precipitate indicates a carbonate; however, if no precipitate is produced initially but a white precipitate appears after heating, then bicarbonate ions are present.
- Sulfite ions can be detected by treatment with hydrochloric acid, which produces acidic, oxidizable sulfur dioxide gas, which can be detected by the decolorization of acidified (purple pink) or acidified (orange green).
- A confirmatory test for sulfite anions includes the addition of silver nitrate to form white silver sulfite that
turns black upon heating.
Anion Test Positive Result Carbonate ()
- Limewater ()
Effervescence due to the evolution of colorless gas turns limewater cloudy. Confirmatory test: White precipitate is formed in cold solution, which is soluble in dilute hydrochloric acid. Sulfite ()
Oxidizable, colorless gas turns purple potassium permanganate solution pale pink.
Oxidizable, colorless gas turns orange potassium dichromate solution green. Confirmatory test: White precipitate is formed that turns black upon heating.
- The presence of halides in solids can be detected using concentrated sulfuric acid.
Anion Primary Test Positive Result Fluoride () Concentrated Colorless fumes turning litmus paper red Chloride () Colorless fumes forming white steamy fumes with Bromide () Orange fumes that turn a starch indicator yellow Iodide () Purple fumes that turn a starch indicator blue
- The confirmatory test for halides uses silver nitrate solution, with a little nitric acid. If precipitates are
produced and the color is hard to describe, ammonia solution can be added to try to dissolve the precipitate.
Anion Confirmatory Test Positive Result Follow-Up Test Positive Result Fluoride () None None Chloride () White precipitate Precipitate dissolves in dilute . Bromide () Cream precipitate Precipitate does not dissolve in dilute .
Precipitate dissolves in excess .
Iodide () Pale yellow precipitate Precipitate does not dissolve in dilute or excess .
- The primary test for nitrate ions is the addition of concentrated sulfuric acid that generates nitric acid and in turn nitrogen dioxide, a brown gas.
- In the confirmatory test, nitrate ions can be detected using the brown ring test, where iron(II) sulfate and concentrated sulfuric acid react with nitrate ions, producing a brown ring of an iron(II) complex containing nitrogen monoxide.
- Sulfate ions can be detected using a solution of barium chloride that produces a white precipitate of barium sulfate that is insoluble in dilute hydrochloric acid.
- The confirmatory test for the sulfate anion is to add lead acetate solution, producing a white lead(II) sulfate
Anion Test Positive Result Nitrate () Concentrated Brown gas
A brown ring forms between the solution layer and sulfuric acid layer. Sulfate () White precipitate insoluble in dilute hydrochloric acid Lead acetate(aq) White precipitate