Neutralizing Solutions with Sodium Hydroxide

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

We often need to neutralize acidic solutions in order to dispose of them safely. In this lesson we will learn how to do this using sodium hydroxide and the precautions that we need to take.

Disposing of Hydrochloric Acid

You have just finished your experiment, written down the results, and now it is time to clean up. You realize that you have some hydrochloric acid left over. You wonder how to properly dispose of the strong hydrochloric acid (HCl). If you simply dump it down the drain, the acid could do a lot of damage to the pipes and kill the microbes necessary to breakdown organic matter in the sewage. You can't just dump it on the ground; the acid would soak into the soil, killing everything there. So, what is the proper way to dispose of the hydrochloric acid? It is actually quite simple - you neutralize it by turning it into salt water!

How are we supposed to turn hydrochloric acid into salt water? By what magic are we doing this? Well, neutralizing hydrocholoric acid is easier than you might think. We just need to add sodium hydroxide (NaOH), a strong base, into the mix. Let's think for a moment about how we would make salt water without acids and bases. The easiest way is to take water (H2 O) and add salt (NaCl). Now, let's think about what makes hydrochloric acid and sodium hydroxide - HCl and NaOH. If we rearrange these elements, we can easily form H2 O and NaCl by combining the Na, from the NaOH, and the Cl, from the HCl, to form the salt (NaCl) and by combining the OH, from the NaOH, and the H, from the HCl, to form water (H2 O).

The Chemical Formula

When we mix hydrochloric acid and sodium hydroxide, we form sodium chloride (salt) and water:

Acid base reaction

This is a common acid-base reaction. The acid and base react to form a salt and water. Most of the time in chemistry, when we say 'salt', we mean any positive and negative ion bonded with an ionic bond. In this case, it just so happens to also be table salt (NaCl).


In order to calculate how much sodium hydroxide to use, we need to know the molarity of the hydrochloric acid and sodium hydroxide. Let's say that in this experiment we were using 0.5 M HCl and 0.1 M NaOH. What does this mean? The 0.5 M and 0.1 M refer to the molarity. Molarity is the number of moles in a litre of solution. Therefore, molarity is equal to moles divided by volume. In order to reach neutrality, we need to have the same number of moles of hydroxide (OH) as hydrogen ions (H). So if we have 500 mL of 0.5 M HCl, then we can determine how many moles of hydrogen ions we have:

molarity = moles/volume

moles = molarity * volume

moles = 0.5 M * 0.5 L

We changed 500 mL into 0.5 L because molarity is per L NOT per mL.

moles = 0.25 hydrogen ions

Now we need to determine the volume of 0.1 M NaOH that will have 0.25 moles of hydroxide ions:

molarity = moles/volume

volume = moles/molarity

volume = 0.25 moles/0.1 M

volume = 2.5 L

Therefore, we will need about 2.5 L of sodium hydroxide to neutralize the hydrochloric acid.

The Process

In order to tell when the sodium hydroxide has completely reacted with all of the hydrochloric acid, we just need to monitor the pH. We can do this with a pH meter or with pH indicator paper. We start by slowly adding the 2.5 L of sodium hydroxide to the 0.5 L of hydrochloric acid. Once we are almost finished adding the 2.5 L of sodium hydroxide, we start checking the pH. Once the pH is neutral (a pH of 7) then we know that all of the hydrochloric acid has been turned into salt water.

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