You're asking about the purpose of adding NaOH (sodium hydroxide) to a mixture before taking a polarimeter reading. This is a common practice in certain types of reactions, particularly when dealing with esters.

Here's why:

* Hydrolysis of Esters: Adding NaOH to an ester solution initiates a hydrolysis reaction. This means the ester bond is broken down, and the components of the ester – an alcohol and a carboxylic acid – are formed.

* Polarimetry and Chirality: Polarimetry measures the rotation of plane-polarized light by a chiral molecule (a molecule that is non-superimposable on its mirror image). The rotation angle is related to the concentration and specific rotation of the chiral compound.

* Measuring Rate of Reaction: Often, we use polarimetry to monitor the progress of a reaction involving chiral molecules. By adding NaOH to an ester, we create a reaction that can be followed with a polarimeter.

* Example: Let's say we are hydrolyzing an ester that contains a chiral alcohol. As the ester hydrolyzes, the chiral alcohol is released. Since the alcohol rotates polarized light, we can track the change in rotation over time using a polarimeter. This tells us how fast the ester is being hydrolyzed.

Important Note: The specific reason for adding NaOH depends on the specific reaction and the goal of the experiment. It's essential to understand the context of your experiment and the reaction you are monitoring.

Let me know if you have more details about the specific reaction you are working with. This will help me provide a more specific and accurate answer!