In this context substitution means eliminating or replacing hazardous chemicals and procedures with less hazardous alternatives.
What Is Substitution?
Substitution in experimental work means replacing hazardous chemicals and procedures with less hazardous alternatives to minimize the risk of hazards. A hazard is anything that can cause harm, such as hazardous chemicals, electricity, high temperatures, work equipment or a slippery floor. Our examples focus on replacing chemicals.
Substitution could make school laboratory work more in line with the green chemistry principles. This is discussed in more detail in our resources on green chemistry in school experimental work.
What Needs to Be Substituted?
It is sometimes necessary to substitute certain substances because they are restricted or prohibited by law. If you are using any of the substances considered as substances of very high concern (SVHC), prioritize to substitute these first.
Even though substitution is mandatory for certain substances, this is not necessarily well-known among all textbook authors. Furthermore, hazard classification may change when we acquire new knowledge about a substance. If you find experiments described in a textbook or on a website, you should consider whether you could use substances that are less hazardous than those recommended in the experiment.
How to Substitute?
You can prevent exposure to a hazardous substance by substituting it with another substance which presents less, or no risk; or using another process which doesn’t create a hazardous form of that substance. A risk assessment helps decide if alternative chemicals are appropriate choices.
If substitution is not possible, the risks could be minimized, for example by re-designing the experiment as a microscale experiment to reduce the quantities of the hazardous substance being used, or perhaps simply by using a lower concentration of solution. The CheSSE Label Generator contains information on the hazard classification for different concentrations of common solutions and can be used as a tool to assess how reducing the concentration affects risk. Doing the experiment as a demonstration is also a possibility.
Example 1:
Production of hydrogen gas by the reacting zinc with an acid is a common experiment in many schools. Using zinc requires a strong acid with high concentration. By choosing a more reactive metal, such as magnesium, a low concentration of a weak acid is sufficient for the reaction to occur. The learning outcome is the same, and the experiment has lower risk.
Example 2:
Phenolphthalein is commonly used as a chemical indicator for acid-base titrations. However, phenolphthalein is on the Candidate List of substances of very high concern (SVHC), and there are precautionary guidelines for its use.
Depending on the type of titration performed, one solution could be to substitute phenolphthalein with another less hazardous indicator, such as thymolphthalein. If this is not possible, risks can be minimized by using a lower concentration of the phenolphthalein solution or reducing the quantities of phenolphthalein.
Under the CLP Regulation, there are certain concentration limits used when classifying mixtures. These can help us to determine the concentration at which a carcinogenic, mutagenic and reprotoxic (CMR) chemical is diluted to the point where it is no longer classified as a CMR.
| Concentration limit | Threshold concentration above which classification will be triggered for a specific hazard class. |
|---|---|
| Specific concentration limit | A concentration limit that is specific to a substance and takes precedence over generic concentration limits. Specific concentration limits can be found in the CLP Regulation and on the SDS sheet. |
Phenolphthalein has the hazard statement May cause cancer (H350), but a specific concentration limit of ≥1 %, which is higher than the generic concentration limit (≥ 0.1 %). In other words, the solution of phenolphthalein is classified as carcinogenic only if it is ≥1 %. A solution of 1 g/L will work fine in most experiments. However, if you do not buy a pre-made, diluted solution from a supplier, regulations for handling carcinogenic substances will apply to those who make the solution.
Another way of eliminating the hazard could be changing the procedure and using a pH-meter instead of phenolphthalein.
The step-by-step model below shows the process of substituting hazardous chemicals and identifying better alternatives for products and processes.
