How to safely dilute acids - why you need to add acid to water

Using concentrated acids and diluting them down as required helps to save space in the lab and gives you the flexibility to make up any concentration you need. The drawback is that working with concentrated acids can be very hazardous. When performing dilutions it is vital to know how to safely dilute acids and always add acid to water, not the other way around! [caption id="attachment_4217" align="aligncenter" width="297"] How to safely dilute acids[/caption] Fully understanding why will help you to remember, and ensure your dilutions are always performed safely.

Heat is generated

After measuring out your concentrated acid and water, the acid must always be added to the water. This is because when the two mix, heat is generated - this is called the "Enthalpy of solution" or "enthalpy of dissolution". On an atomic level this heat is caused by acid-water attractions being created in the solution as the two species mix. You can observe this when you perform the dilution. As you add the concentrated acid into the water, you will feel the solution getting warm.

Water absorbs the heat safely

As the heat is generated, it has to go somewhere. [caption id="attachment_9948" align="aligncenter" width="70"] How to safely dilute acids[/caption] If you add water into concentrated acid, the heat will go into the still very concentrated acid. This can cause it to fume, spatter or even boil - giving off corrosive fumes and droplets. If you add acid into water, the heat is absorbed by the water which just warms slightly but remains un-reactive. This is due to the hydrogen bonding in water, which means a lot of energy is needed to make it boil - the heat from a dilution is usually not enough to do this.

Always work safely!

Even when performing a dilution the right way, adding acid to water, you should still exercise caution. Any work with concentrated acids is hazardous and should be done carefully. [caption id="attachment_9946" align="aligncenter" width="152"] How to safely dilute acids[/caption]

• Goggles, gloves and a lab coat should be worn to protect you. Other PPE may be recommended depending on the task - consult the risk assessment or work method if you're unsure.
• Use suitable equipment - are the volumes small enough for simple lab glassware? For larger volumes, would a pump help you avoid carrying heavy liquids manually?
• Is any other equipment needed - if your concentrated acid is fuming, it should be handled in a fume hood to avoid inhalation of the corrosive vapours.

Remember that safety is different for each job. A risk assessment or work method should be performed to make sure you have everything you need to perform dilutions safely. For more information about the equipment and PPE you may need to perform a dilution, follow the links above or contact us;