# How is NaOH able to clear a solution of Paraformaldehyde?

While making a solution of $4~\%$ para-formaldehyde for immunohistochemistry (to fix the tissue), we add para-formaldehyde into water and stir. It looks like a chalky supersaturated solution even after stirring with a magnetic stirrer. And as per protocol, you are supposed to add 2 drops of $2~\mathrm{N}\ \ce{NaOH}$ to clear the solution. It surprisingly turned crystal clear!

I was amazed at how just 2 drops could make such a huge difference. Does it react with para-formaldehyde? Does changing pH change solubility? But para-formaldehyde is a polymer, how could it suddenly become soluble? Does adding $\ce{NaOH}$ make it a monomer?

para-formaldehyde consists of long chains of the following type:

$$\ce{HO-CH2-O-[CH2-O]_{n}-CH2-OH}$$

When being dissoluted in water, this needs to be broken down into formaldehyde monomers (and then further into formalin):

$$\ce{(CH2O)_{n} + H2O <=> n CH2O + H2O <=>> CH2(OH)2}$$

Acids and bases are both able to speed up the first step. Bases by deprotonating one end and causing a domino effect of bonds being broken and formed with a hydroxide released on the other end. Acids by protonating one end and then the domino occuring in the other direction.

$$\ce{HO-[CH2-O]_{n}-CH2-O-CH2-O-CH2-OH ->[\ce{OH-}] \\HO-[CH2-O]_{n}-CH2-O-CH2-O-CH2-O- ->\\ HO-[CH2-O]_{n}-CH2-O-CH2-O- + H2C=O -> \\HO-[CH2-O]_{n}-CH2-O- + 2 H2C=O ->}\\ \dots\\\ce{HO- + (n+3) H2C=O}$$

• so ultimately every PF solution has to form a formalin solution? – Polisetty Jun 5 '16 at 19:22
• Yeah, it’s not going to stay para-formaldehyde. Indeed, any time you are using the stuff in chemistry, you require it to break down and actually want to use ortho-formaldehyde. But gases are hard to weigh @Polisetty ;) – Jan Jun 5 '16 at 19:24