Have you ever turned on the tap and seen red water gush out? It might have been due to high levels of iron. Iron is an essential nutrient, but like many other heavy metals, can be toxic in high concentrations. As a result, the UK has set strict limits for the maximum concentrations of certain metals in drinking water. For iron, the figure stands at 200µg per litre of water1.
We naturally find trace amounts of many heavy metals in the soil. However, certain modern industrial activities also release heavy metals as pollutants. For example, sources of heavy metals can include plastics, water pipes, processing plants, fertilisers, and paints. These pollutants leach into our wastewater, and if we are not careful, end up in the environment. There, they can harm both plant and animal life, as well as make their way into our own drinking water systems. If their levels get too high, the consequences could be fatal2.
To help keep ourselves and the world around us safe from heavy metal poisoning, we remove certain heavy metals from wastewater. This is often done using sodium hydroxide. Sodium hydroxide causes many metal ions to precipitate out of solution, and is the basis behind the sodium hydroxide test for cations.
- This article is about the sodium hydroxide test in chemistry.
- We'll start by exploring how the sodium hydroxide test works and why we use it.
- We'll then look more closely at the sodium hydroxide test for metal ions, alongside its results and colours.
- Along the way, you'll learn why some metals can't be identified using the sodium hydroxide test.
- To finish, we'll teach you how to write sodium hydroxide test equations.
What is the sodium hydroxide test?
The sodium hydroxide test is an analytical technique used to identify positive ions in solution.
In the article Chemical Analysis, you learned how chemical analysis plays an important role in our lives. It is used in all sorts of fields, from medicine and criminology to environmental science and production, to identify products, reduce pollution, and ensure safety. One type of analytical technique is the sodium hydroxide test. This test is used to test for positive ions (also known as cations) in solution.
The sodium hydroxide test works thanks to a precipitation reaction. Sodium hydroxide (NaOH) dissolves in solution to form sodium ions (Na+) and hydroxide ions (OH-). The hydroxide ions react with dissolved cations to form an ionic hydroxide salt, which precipitates out of solution. These precipitates vary in colour and solubility, and so they give us some clues as to the unknown cation's identity.
Sodium hydroxide test: Pros and cons
The sodium hydroxide test is relatively cheap and simple to carry out. However, it has its downfalls. Can you think of any?
For example:
- The sodium hydroxide test is not as accurate, sensitive, or precise as instrumental methods of chemical analysis, such as flame emission spectroscopy.
- The sodium hydroxide test can't be used on trace samples and isn't that useful at determining concentration.
- It can also give ambiguous results.
- For example, the sodium hydroxide test gives similar results for multiple different cations, meaning you need to carry out further tests to determine their identity.
- You could theoretically measure the mass of the precipitate formed and use this to work out the levels of cations in your solution. However, this is a tricky procedure, with many opportunities for error. As a result, the sodium hydroxide test is better used qualitatively, not quantitatively.
- On the other hand, because the test is only qualitative, you don't have to worry about measuring your sample and reagents out with 100% accuracy!
Sodium hydroxide test: Testing for metal cations
We commonly use the sodium hydroxide test to identify metal cations. It can help us detect the following metal ions:
- Calcium (Ca2+).
- Magnesium (Mg2+).
- Aluminium (Al3+).
- Copper (II) (Cu2+).
- Iron (II) (Fe2+).
- Iron (III) (Fe3+).
In all of these cases, the metal cations react with hydroxide ions from sodium hydroxide to form an ionic metal hydroxide salt. This precipitates out of solution.
Notice something about the metals above? None of them are from group 1 in the period table. This brings up a question: why can't we test for group 1 metal cations using the sodium hydroxide test? Well, as we learned earlier, the sodium hydroxide test works by forming a solid hydroxide salt. Because it is a solid, the salt is easy to identify. However, group 1 hydroxide salts are soluble in water and so don't precipitate out of solution. Therefore, there is .
Experiment
You might carry out the sodium hydroxide test as an experiment in class. You might use it to confirm the identity of a metal cation from a known solution. In addition, you could also use it to identify an unknown metal cation from a range of possible options. Here's how you go about the test:
- Pour approximately 10 cm3 of a metal cation solution into a test tube.
- Add a few drops of sodium hydroxide solution to the test tube using a pipette.
- Observe any changes, noting down the results.
- Gradually add more sodium hydroxide until it is in excess, once again noting any observations.
- Repeat the experiment with different metal cation solutions.
Try asking yourself the following questions during the experiment:
- Does a precipitate form?
- If so, what is its colour?
- Does the precipitate dissolve in excess sodium hydroxide?
- Alternatively, is the precipitate insoluble?
For example, in the diagram below, adding a few drops of sodium hydroxide solution causes a red-brown precipitate to form. This precipitate is insoluble in excess sodium hydroxide.
Fig. 1: An example of the sodium hydroxide test. In this experiment, adding sodium hydroxide to a metal cation solution causes an insoluble red-brown precipitate to form.Vaia Originals
Recording your results
It is useful to record your results and observations from the sodium hydroxide test in a table. For example, you could create a table like the blank example below. Try copying it out into your lab book and filling it in as you complete the experiment.
| Solution | Ion present | Observation |
| Small volume of NaOH | Excess NaOH |
| | | |
| | | |
| | | |
If you know the formulae of the solutions you have used, you can fill in the 'Ion present' column straight away. However, if you are testing unknown solutions, you'll have work out the identity of the ion yourself using your observations and results. Don't worry - we'll show you the results of the sodium hydroxide test next.
Sodium hydroxide test: Results
So, we've learned that you can use the sodium hydroxide test to identify metal cations in solution. But what changes should you actually observe? Here's a table showing the results you should expect to see when you add sodium hydroxide to various metal cations in solution. We've included both the colour of the precipitate that forms and further details about whether it dissolves in excess sodium hydroxide.
Ion present | Observation |
Small volume of NaOH | Excess NaOH |
| Ca2+ | White precipitate | Insoluble |
| Mg2+ | White precipitate | Insoluble |
| Al3+ | White precipitate | Precipitate dissolves |
| Cu2+ | Blue precipitate | Insoluble |
| Fe2+ | Green precipitate that slowly turns brown ('Dirty' brown) | Insoluble |
| Fe3+ | Red-brown precipitate | Insoluble |
Notice that Ca2+, Mg2+, and Al3+ ions all initially form a white precipitate when they first react with NaOH. You can identify the precipitate that contains Al3+ ions because it dissolves in excess NaOH. However, it is impossible to tell the Ca2+ and Mg2+ precipitates apart using this test as they are both insoluble in NaOH, no matter the concentration. To distinguish between the two, you must carry out some other type of chemical analysis. For example, you could use a flame test. Check out the article Test for Metal Ions for further information.
Sodium hydroxide test: Colour changes
Want a more visual guide to the sodium hydroxide test? Here is a flow chart that helps you identify the metal cation in solution from the colour of the precipitate formed when you add sodium hydroxide.
Fig. 2: A guide to the colours of the precipitates formed in the sodium hydroxide test and their respective metal cation.Vaia Originals
Sodium hydroxide test equations
Before we finish, we need to learn how to write ionic equations for the sodium hydroxide test.
Don't try to memorise the sodium hydroxide test for each metal cation individually - that's far too much work! Instead, learn how work out the ionic equation for the reaction between sodium hydroxide and any metal cation. It is simple, really, once you get the hang of it.
As we explored earlier, the sodium hydroxide test involves a reaction between negative hydroxide ions (OH-) and positive metal cations. They react to form an ionic metal hydroxide salt, which precipitates out of solution. To write an ionic equation, we need to know both the charges of the ions involved and the formula of the salt formed. We then must balance the equation. Fortunately, this isn't too tricky.
Here's the process:
- Use the charges of the hydroxide anion and the metal cation to work out the formula of the metal hydroxide salt formed.
- Write a basic chemical equation involving your ion reactants and metal hydroxide product. In this equation, an unknown quantity of hydroxide ions reacts with an unknown quantity of metal cations to form one unit of salt. There are no other products.
- Balance the equation, using the formula of the metal hydroxide salt to guide you.
The following tips might help you build and balance ionic equations for the sodium hydroxide test:
- To write a formula for the metal hydroxide salt formed, you need to know the charges of the ions involved. Remember that the superscript characters to the right of an ion tell us its charge. Note that if there is just a minus or plus sign but no number, we pretend that there is a 1 there too. For example, OH- ions have a charge of -1 whilst Fe2+ ions have a charge of +2. Likewise, Fe3+ ions have a charge of +3.
- Metal hydroxides are examples of ionic compounds. This means that they are neutrally charged overall. If you add up the charges of all the ions involved in the salt, the total should equal zero.
- To balance equations, we need to make sure that there is the same number of atoms of each element on each side of the equation. We do this by adding in more units of the species already involved in the reaction. For example, we could add in more units of reactants, or more units of products, or perhaps both!
- However, we can't change the chemical formula of any species. We also can't add in any new species.
If you feel like you need more advice or tips on any of these topics, check out Ions and Isotopes, Ionic Compounds, and Chemical Equations. They'll bring you up to speed with not only writing formulae for ionic salts, but also balancing chemical equations. On the other hand, if you think you're ready to dive straight into some practice questions, put your skills to the test with the following worked examples.
Let's have a go at writing sodium hydroxide test equations.
Write an ionic equation for the reaction that occurs when you add sodium hydroxide to solutions containing:
- Ca2+ ions.
- Fe3+ ions.
Let's start by tackling part a. First, we need to find the formula for the hydroxide salt formed. We do this using the charges of the ions involved.
Well, we know that our salt contains OH- ions and Ca2+ ions. The superscript characters tell us that OH- ions have a charge of -1, whilst Ca2+ ions have a charge of +2. We also know that the salt needs to have a neutral charge overall - if we add up the charges of all the ions involved, it should equal zero. Therefore, our salt must contain two OH- ions for each Ca2+ ion. This gives it the formula Ca(OH)2.
Next, we write a basic equation. In this reaction, unknown quantities of OH- ions and Ca2+ ions react to form one unit of Ca(OH)2, with no other products:
$$\_ OH^-+\_ Ca^{2+}\rightarrow Ca(OH)_2$$
Finally, we need to balance the equation to find the unknown quantities of OH- ions and Ca2+ ions. On the right-hand side, we have one unit of Ca(OH)2. We know that each unit of Ca(OH)2 is made from one unit of Ca2+ ions, but two units of (OH)- ions - that's what the subscript 2 in the salt's formula means. Therefore, we need one unit of Ca2+ ions and two units of OH- ions on the left-hand side of the equation as well. Here's our final answer:
$$2OH^-+Ca^{2+}\rightarrow Ca(OH)_2$$
We'll work through part b a little faster, now that you know what you are doing.
Our metal hydroxide salt contains OH- ions and Fe3+ ions. The OH- ions have a charge of -1, whilst the Fe3+ ions have a charge of +3. Our neutral salt must contain three OH- ions for each Fe3+ ion, giving it the formula Fe(OH)3.
Next, let's write and balance the equation. Here's the unbalanced version:
$$\_ OH^-+\_ Fe^{3+}\rightarrow Fe(OH)_3$$
Each unit of Fe(OH)3 on the right-hand side of the equation contains one unit of Fe3+ ions and three units of OH- ions. Therefore, we need one unit of Fe3+ ions and three units of OH- ions on the left-hand side too. Here's our final equation:
$$3OH^-+Fe^{3+}\rightarrow Fe(OH)_3$$
Well done! That's sodium hydroxide test equations sorted. In fact, that's the whole of the sodium hydroxide test topic covered. You'll be ready for your exams in no time!
Sodium Hydroxide Test - Key takeaways
References
- 'Testing Water for Metals & Heavy Metals'. WTS
- Linda Sims, 'Heavy Metals in Water: All You Need to Know to Be Safe'. Water Is A Right (17/01/2020)
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