Add Acid to Water or Water to Acid? Safely Diluting Acids


Always add acid to water, not water to acid.
Always add acid to water, not water to acid. Otherwise, the acid can splatter and splash up. (photo: Hans Reniers)

When you mix strong acids and water, it makes a difference whether you add acid to water or water to acid. Always add acid to water and not the other way around.

If you add a little water to a lot of concentrated acid, the resulting solution is still concentrated. The hydration reaction uses all the water (acid is the limiting reactant), generating a lot of heat. The solution violently boils, spitting concentrated acid out of its container. If you add a little concentrated acid to a lot of water, water is the limiting reactant and the resulting solution is more dilute. Here, all the acid reacts, but there is extra water to absorb the heat, lessening the chance of boiling.

Why Add Acid to Water

There are a few factors that make it better to add acid to water. Diluting acid with water is exothermic, so it’s easier to boil and splash water added to acid than acid added to water because water has a high heat capacity and can absorb a lot of heat. Also, some strong acids have a higher specific gravity or density than water. So, if you put water on top of acid, the heat that’s generated is at the acid surface. In this situation, the liquid readily boils and spits. On the other hand, if you pour acid on top of or into water, the water rises over the acid before mixing and contains the reaction.

Diluting Sulfuric Acid

Sulfuric acid (H2SO4) is the most dangerous common acid to dilute. Partly, this is because it reacts so violently with skin and clothing. Sulfuric acid quickly dehydrates proteins and carbohydrates in skin and muscle. The acid is much heavier than water, so water added to it reacts with the top layer first. A lot of heat gets generated, even when sulfuric acid and water are mixed properly. Mixing 100 ml of concentrated sulfuric acid and 100 ml of water at 19 °C reaches a temperature over 131 °C (well past the boiling point of water) in under a minute.

It’s the hydration reaction that generates all that heat:

H2SO4 + H2O → H3O+ + HSO4

Similarly, sulfuric acid readily strips water from organic molecules, dehydrating them.

Dilution Safety Tips

In general, the higher the concentration the acid, the greater the greater the heat increase and the higher the chance of boiling and splashing. Be extremely careful diluting any concentrated strong acid. Always wear proper safety gear and work under a fume hood.

No matter what acid you’re diluting, remember reaction rates double for every 10 degree Celsius temperature increase. So, if you make a serial dilution of an acid, the change of boiling or splashing the acid increases because the stock solution is already hot. There are a couple of ways to avoid this. One is to cool the stock solution before diluting it. You can rest the container in an ice bath before making the dilution. Another method is to pour the hot acid solution over ice made from de-ionized water and then diluting it with room temperature water to reach the final volume.

Finally, choose your glassware wisely. An Erlenmeyer flask or volumetric flask is a better choice than a beaker or graduated cylinder because the flask shape acts as a sort of splash shield (also, graduated cylinders are notoriously unstable and easy to tip over).

Tips to Remember to Add Acid to Water

Here are some easy mnemonic devices to help you remember to add acid to water:

  • AA: Add Acid
  • Acid to Water, like A&W Root Beer
  • Drop acid, not water
  • If you think your life’s too placid, add the water to the acid
  • First the water, then the acid, otherwise it won’t be placid

Diluting Strong Bases

Like strong acids, strong bases react with water in an exothermic reaction. Also, like the strong acids, strong bases tend to be corrosive and able to cause chemical and thermal burns on skin. It’s best to add base to water, just like you would acid.

References

  • Bishop, Michael L.; Fody, Edward P.; Schoeff, Larry E. (2004). Clinical Chemistry: Principles, Procedures, Correlations. Lippincott Williams & Wilkins. ISBN 0-7817-4611-6.
  • Cleapss.org. (2007). Student Safety Sheets – Sulfuric(VI) Acid.
  • Pauling, L.C. (1988) General Chemistry. Dover Publications.

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