Lesson Explainer: Further Tests for Anions Chemistry

In this explainer, we will learn how to identify a selection of aqueous negative ions based on their reactivity and the color and solubility of their salts.

In chemistry, qualitative analysis involves identifying the components of an unknown chemical substance. A chemical substance may be pure, in which case physical measurements such as boiling point or melting point can be used to identify the unknown compound. If the unknown substance is a mixture, then the component parts must be identified using chemical methods.

The chemical tests chosen to analyze an unknown substance have a specific order and methodology behind them designed to easily and accurately identify anions, also known as acidic radicals, and cations, also known as basic radicals.

When a chemical reaction takes place, several possible changes may be observed, or even a combination of changes such as those seen in the diagram below.

While a primary test may give us a good indication of a particular anion, often other negative tests may give the same result. As such, we use confirmatory tests that confirm our primary tests in order to accurately identify unknown ions in solid substances or solutions thereof.

Different chemical compounds are used in this form of qualitative analysis to classify unknown anions, also known as acidic radicals. Anions are monatomic and polyatomic ions that are negatively charged. Since the total number of electrons in the species exceeds the total number of protons, anions have a net negative charge.

The anion that remains after hydrogen ions have been removed from an acid is referred to as an acidic radical. A cation, also known as a basic radical, remains when hydroxide ions are separated from a hydroxide compound. A salt is formed when acidic and basic radicals interact chemically.

We can use different chemical substances to identify unknown acidic radicals in qualitative analysis. We are able to detect anions first by using dilute hydrochloric acid, then by using concentrated sulfuric acid, and finally by using barium chloride solution if the anion has still not been identified.

The order in which these reagents test the unknown anions has ramifications. Dilute hydrochloric acid can be used to test any unknown anion. If hydrochloric acid is unsuccessful in determining the identity of the unknown acidic radical, concentrated sulfuric acid should be used. If concentrated sulfuric acid is insufficient to establish the identity of the unknown anion, barium chloride solution should be used instead.

Dilute HCl GroupConcentrated Sulfuric Acid GroupBarium Chloride Group
-Carbonate (CO32)
-Sulfite (SO32)
-Bicarbonate (HCO3)
-Sulfide (S2)
-Thiosulfate (SO232)
-Nitrite (NO2)
-Halides (F, Cl, Br, and I)
-Nitrate (NO3)
-Sulfate (SO42)
-Phosphate (PO43)

The strength of the acidic radicals determines the order of this sequence. The primary reagent in the first group is dilute hydrochloric acid. This acid is more stable than the anions it is used to detect, like the carbonate anion. The hydrochloric acid would replace the less stable anions in this situation, resulting in the evolution of gases that we can test for.

Concentrated sulfuric acid is included in the second category. The anions it is testing for, such as the chloride anion, which was the testing reagent in the first group, are more stable than the sulfuric acid.

We do not have an appropriate reagent for the final group that is more stable than the sulfate and phosphate anions, so this group is tested with barium chloride solution.

We will examine two separate groups of anions in this explainer:

  • anions detected using dilute hydrochloric acid,
  • anions detected by barium chloride.

Dilute hydrochloric acid can be used to identify the following anions:

  • carbonate (CO32),
  • sulfite (SO32),
  • bicarbonate (HCO3),
  • sulfide (S2),
  • thiosulfate (SO232),
  • nitrite (NO2).

In this explainer, we will focus on

  • bicarbonate (HCO3),
  • sulfide (S2),
  • thiosulfate (SO232),
  • nitrite (NO2).

Hydrochloric acid is used as this acid is more stable than the relative acids of these anions. For example, hydrochloric acid is more stable than carbonic acid (HCO23). During the chemical reactions used to identify the unknown anions, the more stable chloride anion replaces the unknown anion, liberating a gas or forming a precipitate. In the case of tests where a gas is produced, often gentle warming of the test tube is necessary.

The overall reaction occurring in the first group of anions, detected using dilute hydrochloric acid, involves dilute HCl being added to a solid salt.

Bicarbonate ions react with acids to produce carbon dioxide and water.

Reaction: Bicarbonate Ions and Acid

HCO()+H()CO()+HO()3+22saqgl

The bicarbonate ion (HCO3) is also known as the hydrogen carbonate ion. It behaves like the carbonate ion (CO32) in some respects, but not in others. One initial method to distinguish between a metal carbonate and a metal bicarbonate is to try to dissolve them in water: the stable metal bicarbonate salts are all water soluble, but many metal carbonates are not. All carbonate salts are insoluble in water except sodium carbonate, potassium carbonate, and ammonium carbonate.

For a metal with carbonate and bicarbonate salts that are both soluble (e.g., sodium), the tests for bicarbonate ions look identical to those for carbonate ions.

Therefore, if we treat a substance with dilute hydrochloric acid and effervescence occurs or bubbles of gas are produced, then the substance could have contained bicarbonate ions or carbonate ions. While we can test if the gas is carbon dioxide using the limewater test, we need a confirmatory test for the bicarbonate.

The confirmatory test we can perform involves adding ions of a metal whose bicarbonate salt is soluble but whose carbonate salt is insoluble. This can be done by introducing magnesium ions by adding magnesium sulfate (MgSO4) in solution form.

In the case of bicarbonate ions, once mixed with the magnesium ions, the solution mixture is heated. Magnesium bicarbonate will decompose, and a white precipitate of magnesium carbonate is produced.

Reaction: Bicarbonate Ions and Magnesium Ions

2HCO()+Mg()Mg(HCO)()32+32aqaqaq

Reaction: Thermal Decomposition of Magnesium Bicarbonate

Mg(HCO)()MgCO()+CO()+HO()32322aqsgl

This is the full test.

We can tell the difference between a carbonate and a hydrocarbon carbonate salt using this test. If we wait to heat the mixture, and the white precipitate appears instantaneously, then carbonate ions are present. If the white precipitate only appears after heating, then hydrocarbon carbonate ions were present.

Testing with dilute hydrochloric acid is also used to detect the presence of sulfide anions.

The sulfide ion (S2) is the sulfur equivalent of the oxide ion (O2). Many metals that form oxides also form sulfides.

In the primary test, sulfide ions are detected by treatment with dilute hydrochloric acid (HCl()aq). Sulfide ions are converted to hydrogen sulfide, which can be detected by its reaction with lead(II) ethanoate (also known as lead(II) acetate).

Reaction: Sulfide Ions with Acid

S()+2H()HS()2+2saqg

Reaction: Hydrogen Sulfide with Lead(II) Acetate

HS()+Pb(CHCOO)()PbS(,black)+2CHCOOH()2323gaqsaq

Lead sulfide is insoluble and strongly colored, so it gives a safe visual indication of the presence of hydrogen sulfide.

This is the full test.

A confirmatory test for sulfide ions in solution is to add a solution of silver nitrate. If sulfide ions are present, a black precipitate of silver sulfide will form.

Reaction: Sulfide Ions with Silver Ions

S()+2Ag()AgS(,black)2+2aqaqs

This is the full test.

Other precipitates may be produced if anions other than sulfide are present; for instance, silver ions will combine with carbonate ions to produce a white precipitate of silver carbonate. However, the distinctive color of the silver sulfide precipitate should still be visible if sulfide ions are present.

Example 1: Identifying the Counterion of Sodium in a Salt Based On Chemical Tests

The image shows a series of tests carried out on an unknown sodium salt, X.

What formula is the unknown sodium salt likely to have?

Answer

In the image, a solution of X is being treated with silver nitrate (AgNO3), which produces a black mixture. Another sample of the solution of X is first treated with hydrochloric acid (HCl), which produces a smelly gas. After this, the mixture is treated with lead acetate (Pb(CHCOO)32), which produces a different black mixture.

These chemical tests are consistent with the presence of sulfide ions (S2). If X contains S2 ions, the addition of silver nitrate will produce a black precipitate of silver sulfide (AgS2).

Also, in the presence of hydrochloric acid, sulfide ions react to produce hydrogen sulfide gas that smells like rotten eggs. The addition of lead acetate will produce a black precipitate of lead acetate (if enough hydrogen sulfide remains, or if there are sulfide ions remaining).

Therefore, the sodium salt X must contain sulfide ions.

The likely formula for X is therefore NaS2.

Testing with dilute hydrochloric acid is also used to detect the presence of thiosulfate anions.

The thiosulfate ion (SO232) is structurally similar to the sulfate ion (SO42) and sulfite ion (SO32). We can think about a thiosulfate ion as sulfate ion with one oxygen replaced with a sulfur. However, the oxidation state of the sulfur in thiosulfate is actually the same as that of sulfur in sulfite:

This can be useful to remember because the test for sulfite and the test for thiosulfate are very similar: both begin with the addition of hydrochloric acid. If thiosulfate ions are present, sulfur dioxide gas will be produced, in addition to the formation of a yellow precipitate of sulfur, which is characteristic of the thiosulfate ion.

As SO2 can be oxidized, we test with potassium dichromate. Initially, the presence of a yellow precipitate is sufficient to demonstrate that thiosulfate ions may be present.

Reaction: Thiosulfate Ions with Acid

SO()+2H()HO()+SO()+S()232+22saqlgs

This is the full test.

A confirmatory test to detect the thiosulfate ion relies on its ability to reduce iodine to iodide ions. If we add a solution of iodine to the test solution and sufficient thiosulfate ions are present, the iodine solution will decolorize and the thiosulfate ions (SO232) will be oxidized to form tetrathionate ions (SO462). As iodine is actually not particularly water soluble, iodine solutions usually contain a little potassium iodide as well, which helps I2 molecules dissolve.

Reaction: Thiosulfate Ions with Iodine

SO()+I(,orange/brown)SO()+I(,colorless)2322462aqaqaqaqSO462 is the formula for the tetrathionate.

This is the full test.

Testing with dilute hydrochloric acid is also used to detect the presence of nitrite anions.

Nitrite ions (NO2) are not stable in acidic conditions. They can be detected by treatment with hydrochloric acid. One of the products, nitrous acid, will decompose and release nitrogen monoxide gas that is not particularly water soluble. However, nitrogen monoxide will react with oxygen in air to produce nitrogen dioxide that has a strong orange color.

Reaction: Nitrite Ions with Acid

NO()+H()HNO()3HNO()HNO()+HO()+2NO()2+2232saqaqaqaqlg

Reaction: Nitrogen Monoxide with Oxygen

2NO()+O()2NO()ggg22

This is the full test.

As a confirmatory test, the gas produced when HCl()aq is mixed with the unknown solid can be passed through a solution of KMnO4 acidified with concentrated sulfuric acid. The deep purple color of the permanganate will fade to a pale pink, almost colorless, solution as the manganate ions are reduced, as can be seen in the diagram below.

Reaction: Nitrogen Monoxide with Permanganate Ions

5NO()+2MnO(,purple)+6H()5NO()+2Mn(,palepink)+3HO()24+32+2aqaqaqaqaql

Therefore, the production of a gas that turns brown and is oxidizable is the confirmatory test for the nitrite ion.

If testing with hydrochloric acid proves to be ineffective, the second stage of testing involves using concentrated sulfuric acid. Concentrated sulfuric acid is used to identify halides (F, Cl, Br, and I) and nitrates (NO3).

If the first two stages of testing with dilute hydrochloric acid and concentrated sulfuric acid return negative results, the third stage proceeds with testing using barium chloride solution.

Barium chloride solution can be used to identify the following anions:

  • phosphate (PO43),
  • sulfate (SO42).

Other acids cannot be used to displace the sulfate and phosphate anions that are both anions of very strong acids. Hydrochloric acid, for example, is less stable than sulfuric acid and thus cannot substitute for the sulfate anion in salt solutions. As a result, we will not be checking for a liberated gas in this miscellaneous group, but rather for the formation of a precipitate.

In this explainer we will focus on phosphate (PO43).

We can detect phosphate ions by treatment with barium chloride. Barium ions will react with phosphate ions, producing a white precipitate of barium phosphate. A similar white precipitate of barium sulfate is also formed when barium chloride is added to an unknown solution containing sulfate ions. As such, the following step of adding dilute hydrochloric acid and checking for dissolution is a very important part of our testing protocol.

Reaction: Phosphate Ions with Barium Ions

2PO()+3Ba()Ba(PO)()432+342aqaqs

This is the test.

We can confirm the nature of this precipitate by adding dilute hydrochloric acid. A white barium phosphate precipitate will redissolve as shown below. However, a white barium sulfate precipitate will not be affected by the mineral acid.

Reaction: Barium Phosphate Ions with Acid

Ba(PO)()+6H()HPO()+3Ba()342+342+saqaqaq

This is the full test.

A confirmatory test we can do instead involves treatment with silver nitrate solution. Phosphate ions react with silver ions to produce a yellow precipitate of silver phosphate, which can be redissolved by the addition of nitric acid or ammonia.

Reaction: Phosphate Ions with Silver Ions

PO()+3Ag()AgPO(,yellow)43+34aqaqs

This is the full test.

Reaction: Silver Phosphate with Nitric Acid

AgPO()+3HNO()3AgNO()+HPO()343334saqaqaq

Reaction: Silver Phosphate with Ammonia

AgPO()+6NH()3[Ag(NH)]()+PO()34332+43saqaqaq

Example 2: Identifying and Describing the Precipitate Resulting from the Reaction of Aqueous Phosphate Ions and Barium Chloride

One test that can be used to detect the possible presence of phosphate ions is the reaction with barium chloride. The symbol equation for this reaction is shown: 2NaPO+3BaClX+6NaCl342

  1. What solid precipitate (compound X) does the reaction produce?
  2. What color is the precipitate produced?

Answer

Phosphate ions in solution can be detected using barium chloride. This test is effective because the product of the reaction of barium chloride and phosphate ions is a precipitate. If phosphate ions are present in a solution, this precipitate will form. This demonstrates that phosphate ions may be present.

Part 1

When we examine the equation, we can see that the other product is sodium chloride (NaCl). As this is a substitution reaction, the barium ions and the phosphate ions are unaccounted for; they must be present in X. The formula of the salt formed from Ba2+ and PO43 is Ba(PO)342.

The answer to part 1 is Ba(PO)342.

Part 2

Like many salts composed of main-group elements, a precipitate of barium phosphate will be white.

The answer to part 2 is: white.

Example 3: Evaluating Results from Chemical Tests of Unknown Solids to Determine Which Contain Specific Salts

A student is given five unknown sodium salts (AE). The student only has dilute HCl()aq, some AgNO()3aq, distilled water, and no other lab equipment. The student adds dilute HCl()aq to one sample of each salt, and they add AgNO()3aq to the aqueous solutions of the salts. The table below lists the observations seen for each reaction.

SaltReaction of Solid Salt with Dilute HCl()aqReaction of Salt Solution with DiluteAgNO()3aq
ANo observationsYellow precipitate formed
BColorless gas producedNo precipitate formed
CColorless, smelly gas producedBlack precipitate formed
DYellow precipitate formedNo precipitate formed
EBrown gas producedNo precipitate formed

  1. Which salt is likely to contain NaHCO3?
  2. Which salt is likely to contain NaNO2?

Answer

Before exploring which salts are likely to be in which solutions, we will examine the tests.

Silver nitrate solution (AgNO()3aq) is colorless. Here, we can see that solutions A and C yield a precipitate when AgNO()3aq is added, while the others show no change. We expect a precipitate to form from any of the following ions: Cl (AgCl, white), Br (AgBr, cream), I (AgI, pale yellow), CO32 (AgCO23, white), SO42 (AgSO24, white), S2 (AgS2, black), and PO43 (AgPO34, yellow).

The second test involves adding dilute hydrochloric acid (HCl()aq). For solutions B, C, and E, gases are produced, for solution D, there is a precipitate, and for solution A, we see no change.

We expect a colorless, odorless gas when HCl()aq reacts with carbonate ions or bicarbonate ions; the gas would be carbon dioxide, which we could detect using limewater.

A colorless, smelly gas would be produced if the salt contained sulfide ions (S2). Hydrochloric acid reacts with these to produce hydrogen sulfide gas (HS2) that smells like rotten eggs and is colorless.

A yellow precipitate would be produced if thiosulfate ions (SO232) were present. Along with sulfur dioxide gas (which smells like burned matches), a yellow precipitate of sulfur would form.

A brown gas would be produced if nitrite ions (NO2) were present. The reaction between hydrochloric acid and nitrite ions produces nitrogen monoxide that reacts with oxygen in air to form brown nitrogen dioxide.

Part 1

The ion we are interested in within sodium bicarbonate (NaHCO3) is the bicarbonate ion (HCO3). The results we expect from these two tests if hydrogen carbonate ions were present are the following: AgNO():nochangeHCl():acolorlessgasisproduced3aqaq,.

This matches solution B.

Silver hydrogen carbonate is soluble in water, so no precipitate would be produced if we added silver nitrate solution. The test with HCl()aq gives the same results if carbonate ions are present, but we would expect a white precipitate of silver carbonate from the silver nitrate test if carbonate ions were present. As this is not the case, we can be confident that the salt in solution B is AgHCO3.

The answer is B.

Part 2

The ion we are interested in within sodium nitrite is the nitrite ion (NO2). The results we expect from these two tests if nitrite ions were present are the following: AgNO():nochangeHCl():abrowngasisproduced3aqaq,.

This matches solution E.

Silver nitrite is soluble in water, so no precipitate would be produced if we added silver nitrate solution. The brown gas produced from the addition of HCl()aq is characteristic of the nitrite ion, so we can be confident in our identification.

The answer is E.

Let’s summarize what we have learned about further testing procedures for anions.

Key Points

  • Bicarbonate ions (HCO3) can be detected by treatment with an acid; if a gas is produced that turns limewater cloudy, then bicarbonate ions may have been present.
  • To distinguish between carbonate ions and bicarbonate ions, we can treat the unknown solution with magnesium sulfate; if no precipitate is produced initially but a white precipitate appears after heating, then bicarbonate ions are present.
  • Sulfide ions can be detected by treatment with dilute hydrochloric acid (HCl()aq); sulfide ions are converted to hydrogen sulfide that can be detected by its reaction with lead(II) ethanoate (also known as lead(II) acetate) that produces black lead(II) sulfide.
  • In the confirmatory test, sulfide ions can also be detected by treatment with silver nitrate that produces a black precipitate of silver sulfide.
  • Thiosulfate ions can be detected by the addition of hydrochloric acid; if thiosulfate ions are present, a yellow precipitate of sulfur will be produced, along with sulfur dioxide gas (which will turn filter paper, wet with potassium dichromate, from orange to green).
  • In the confirmatory test, thiosulfate ions can also be detected using a solution of iodine, which will lose its orange/brown color.
  • Nitrite ions can be detected by treatment with hydrochloric acid, producing neutral nitrogen oxide gas, which can be oxidized using potassium permanganate.
  • Phosphate ions can be detected by treatment with barium chloride to form a white precipitate, which dissolves again upon addition of the hydrochloric acid.
  • In the confirmatory test, phosphate ions can also be detected by treatment with silver nitrate, forming a yellow precipitate of silver phosphate that will redissolve if nitric acid or ammonia solution is added.
    AnionTestPositive Result
    Bicarbonate (HCO3)- HCl()aq
    - Limewater (CaOH()2aq)
    Colorless gas turns limewater cloudy.
    - MgSO()4aq
    - heat
    White precipitate forms after heating.
    Sulfide (S2)- HCl()aq
    - Paper wet with Pb(CHCOO)32
    Colorless gas turns colorless lead(II) acetate solution black.
    AgNO()3aqBlack precipitate
    Thiosulfate (SO232)- HCl()aq
    - Paper wet with KCrO()227aq
    Yellow precipitate
    Acidic, colorless gas turns litmus paper red.
    I()2aqOrange/brown iodine solution decolorizes.
    Nitrite (NO2)- HCl()aq
    - Acidified KMnO()4aq
    Oxidizable, colorless gas that turns brown
    Turns purple potassium permanganate solution pale pink
    Phosphate (PO43)- BaCl()2aq
    - Dilute HCl()aq
    White precipitate redissolves with hydrochloric acid.
    - AgNO()3aq
    - HNO()3aq
    Yellow precipitate redissolves with nitric acid.
    - AgNO()3aq
    - NH()3aq
    Yellow precipitate redissolves with ammonia solution.

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