Test for Anions

If you visit one of the freshwaters in the southern United States, you might come across an alligator snapping turtle. On the other hand, you might not - these turtles have evolved some pretty ingenious features that help them blend into their surrounding environment. For example, their scaly backs look much like the rocks they hide upon, and to passing fish, their tongues closely resemble a wriggling worm.

Of course, once you know what you are looking for, it is easy to distinguish an alligator snapping turtle from the rocks around it, and you certainly wouldn't mistake its tongue for a worm! These physical traits simply make the turtle even more identifiable.

But some things aren't so easy to tell apart, and this is often the case in chemistry. For example, many negative anions, such chloride ions, bromide ions, and iodide ions, are both colourless and odourless when aqueous. How can we distinguish between them? To help identify negative ions in solution, scientists use tests for anions.

Ah, blackberry picking - that quintessential autumnal British tradition. Wild blackberries are tasty, abundant, easy to spot, and hard to confuse with anything else - they make the perfect forager's treat. But picking other wild foods can come with a little more of a risk. For example, you should never pick mushrooms you find when out and about in the countryside unless you are accompanied by an expert. This is because many toxic types of fungi look much like perfectly edible ones. Two seemingly identical mushrooms could actually be entirely different species - one safe to eat, the other extremely deadly. The consequences of a misidentification could be fatal. Do you really want to risk it?

We often find similar situations in chemistry. Many species look just like each other in certain situations, and it's important to be able to tell them apart. For example, multiple anions, such as sulfate, carbonate, and halide ions, are all colourless and odourless when aqueous - how do we distinguish between them? To help identify negative ions in solution, scientists use tests for anions.

• This article is about testing for anions in chemistry.
• We'll start with an overview of the tests for anions in aqueous solution, including their importance and uses.
• We'll then look more closely at the tests for carbonate, sulfate, and halide anions.
• After that, we'll pull together our learning with a handy testing for anions table.
• Finally, we'll compare tests for anions and cations.

Test for anions in aqueous solutions

If you've read the article Chemical Analysis, you'll know that analytical techniques play a big role in many areas of our lives. For example, they're used in manufacturing industries, medicine, sewage works, and more. Chemical analysis is used to identify certain species in a compound or solution. This can help us work out the products of a reaction, detect pollutants, and improve the safety of products.

An example of chemical analysis is testing for negative ions, which are known as anions. All the tests involve adding a certain reagent to an aqueous solution and observing the results. Like all suitable analytical techniques, we try to choose tests that give distinct and unique results for just one ion. For example, the test might form a coloured precipitate or release a certain gas, which can then be identified.

In this article, we'll look at how you test for the following anions:

• Carbonate ions (CO₃^2-).
• Sulfate ions (SO₄^2-).
• Halide ions. In particular, we'll focus on identifying chloride, bromide, and iodide ions (Cl^-, Br^-, and I^-).

Test for anions: carbonates

First up, let's explore the test for carbonate ions (CO₃^2-). Most carbonate salts are insoluble, and can't be identified in this way, but some carbonates do dissolve in aqueous solution and so can be tested for. Soluble carbonate salts include sodium, lithium, potassium, ammonium, and many uranium carbonates.

Here's the process:

1. Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
2. Use a pipette to add a few drops of a dilute acid, such as hydrochloric acid (HCl), to the test tube.
3. If the solution contains carbonate ions, the reaction will give off bubbles of carbon dioxide gas, which we can test for:
   • Attach a bung and tubing to the top of the test tube to collect the gas given off.
   • Bubble the gas through clear limewater.
   • If the gas is carbon dioxide, the limewater will turn cloudy.

The release of carbon dioxide gas, which we confirm using the limewater test, tells us that our solution contains carbonate ions.

Fig. 1: A diagram showing the test for carbonate ions. Carbon dioxide gas is released, which turns clear limewater cloudy.StudySmarter Originals

Test for anions: sulfates

Next up: testing for sulfate ions (SO₄^2-). Instead of releasing a gas, this test makes use of a precipitation reaction. We add specific reagents to an ionic solution, and if a white precipitate forms, we know that we have sulfate ions in our hands.

To carry out the test for sulfate ions:

1. Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
2. Use a pipette to add a few drops of hydrochloric acid (HCl) to the test tube.
3. Next, add in a few drops of barium chloride solution (BaCl₂).
4. If the ionic solution contains sulfate ions, a milky white precipitate will form.

Fig. 2: A diagram showing the test for sulfate ions. A white precipitate indicates a positive result.StudySmarter Originals

Test for anions: Halides

Last up, let's consider how you identify halide ions in solution. You can do this in multiple ways, but today we'll learn about their reaction with silver nitrate solution. This particular method is another example of a precipitation reaction and tests for chloride (Cl^-), bromide (Br^-), and iodide (l^-) ions. We can even use the test to distinguish between the three!

Here's how:

1. Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
2. Use a pipette to add a few drops of dilute nitric acid (HNO₃) to the test tube.
3. Next, add in a few drops of silver nitrate solution (AgNO₃).
4. If the ionic solution contains halide ions, a precipitate will form. The precipitate generally clouds the whole test tube, and its colour depends on the halide ion itself:
   • A white precipitate means that the solution contains chloride ions.
   • A cream precipitate means that the solution contains bromide ions.
   • A yellow precipitate means that the solution contains iodide ions.

Fig. 3: A diagram showing the test for halide ions. A precipitate indicates a positive result. We can use the colour of the precipitate to identify the halide ion.StudySmarter Originals

Test for anions and cations

This article has focused on testing for anions, such as those found in an aqueous solution of a dissolved ionic compound. Anions are negative ions. But remember that ionic compounds contain positive ions, too. These are known as cations. How can we test for these?

There are two main methods used to test for cations:

• We can identify some metal cations using a flame test. This involves dipping a nichrome wire in an ionic solution and holding it in a Bunsen burner flame. The flame changes colour depending on the metal ion within the solution. For example, potassium ions cause the flame to burn a brilliant purple, whilst calcium ions produce a brick-red flame.
• Not all metal cations change the colour of a flame. However, we can detect other cations using the sodium hydroxide test. This test involves gradually adding sodium hydroxide (NaOH) to an ionic solution until it is in excess. The hydroxide ions react with the metal ions in solution and form a precipitate. We can use the colour and solubility of these precipitates to help determine the metal cation's identity.