Animated Solution for Chemistry - Ionic Equilibrium: The strength of an aqueous $\text{NaOH}$ solution is most accurately determined by titrating : (Note : consider that an appropriate indicator is used)

The laboratory is a place of precision, and titration is one of its most elegant dances. In this problem, we are tasked with finding the exact strength of an aqueous $\text{NaOH}$ solution. It sounds simple, but the devil is in the details. Let's break down the chemistry, the logic, and the human biology that makes one specific setup the absolute best choice.

Sodium hydroxide ($\text{NaOH}$) is a strong base, but it has a flaw: it is highly hygroscopic. It absorbs moisture and carbon dioxide from the air. Because of this, you can never just weigh out some $\text{NaOH}$ pellets, dissolve them in water, and trust that you know the exact concentration. It is what we call a secondary standard.

To find its true strength, we must pit it against a primary standard—a substance that is highly pure, stable, and can be weighed with absolute certainty.

Looking at our options, we have two acids to choose from: concentrated sulfuric acid (${\text{H}}_2{\text{SO}}_4$) and aqueous oxalic acid (${\text{H}}_2{\text{C}}_2{\text{O}}_4$).

Could we use concentrated sulfuric acid? Absolutely not! Adding water or aqueous solutions to concentrated ${\text{H}}_2{\text{SO}}_4$ is a highly exothermic and dangerous process. It can cause the solution to boil and spatter violently. Furthermore, concentrated sulfuric acid is not a primary standard.

Aqueous oxalic acid, on the other hand, is a perfect primary standard. It is stable, safe to handle in dilute aqueous forms, and reacts cleanly with $\text{NaOH}$. So, oxalic acid is our chosen opponent.

Now that we have our acid and base, we need a referee—an indicator to tell us exactly when the reaction is complete.

We are titrating a weak acid (oxalic acid) against a strong base ($\text{NaOH}$). When they neutralize each other, the resulting salt undergoes anionic hydrolysis. The anions react with water to release hydroxide ions (${\text{OH}}^{-}$), making the solution basic at the equivalence point.

Because the equivalence point occurs at a $\text{pH}>7$, we need an indicator that changes color in the basic range. Methyl orange changes color in the acidic range, so it would trigger too early. Phenolphthalein, which transitions from colourless to pink between $\text{pH}$ 8.2 and 10, is the perfect choice.

Why Order Matters

Here is where the problem gets truly fascinating. We have our chemicals, but where do we put them? Does it matter if the $\text{NaOH}$ is in the burette or the flask? Yes, it matters immensely!

Imagine we pipette the $\text{NaOH}$ into the conical flask and add a few drops of phenolphthalein. Because $\text{NaOH}$ is a strong base, the solution immediately turns a deep, vibrant pink. Now, we start adding oxalic acid from the burette. Our goal is to find the exact drop where the solution becomes completely colourless.

Biologically, the human eye struggles with this. It is very difficult to pinpoint the exact moment a fading color completely vanishes. You might add an extra drop or two of acid just to be "sure" the pink is gone, leading to an overestimation and an inaccurate result.

Now, let's flip the script. We place the aqueous oxalic acid in the conical flask and add our phenolphthalein indicator. Because oxalic acid is acidic, the solution remains completely colourless.

We fill the burette with our aqueous $\text{NaOH}$ solution. We open the stopcock and let the base drip into the acid. Drop by drop, the acid is neutralized.

The moment the last molecule of oxalic acid is neutralized, the very next drop of $\text{NaOH}$ makes the entire solution slightly basic. Instantly, the phenolphthalein reacts, and the colourless solution flashes a beautiful, distinct faint pink.

The human eye is incredibly sensitive to the sudden appearance of color against a blank background. You will catch that first hint of pink immediately, allowing you to close the stopcock at the exact right moment.

Therefore, to minimize human error and achieve the highest possible accuracy, the optimal setup is to place the aqueous $\text{NaOH}$ in the burette and the aqueous oxalic acid in the conical flask. It is a beautiful intersection of chemical theory and practical human biology!