What's the plural form of phenolphthalin? Here's the word you're looking for.
Answer
The noun phenolphthalin can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be phenolphthalin.
However, in more specific contexts, the plural form can also be phenolphthalins e.g. in reference to various types of phenolphthalins
or a collection of phenolphthalins.