How can I estimate how much humidity is present in a NaOH granulate? I have a container with NaOH granulate. I want to use it for making soap again, as I did in the past.

When I checked it I saw big clumps because the seal was not good and humidity got in since last time. Yesterday I used it anyway, increasing the amount arbitrarily by 10%, and still was not enough.

I have the following tools:


scale (sensitivity 1 gram),
arbitrary glass or plastic volume meter calibrated with said scale,
temperature probe (up to 70C with 0.5C tolerance and 0.1C sensitivity),
white vinegar,
concentrated lemon juice,
concentrated HCl (I think 36%).


More common "kitchen" chemicals may be available, if you ask in the comments.

How can I estimate how much humidity is present in the NaOH granulate using household items/chemicals?

A measurement performed directly on the NaOH solution used for the soap (around 300 g NaOH and 750 g water) is acceptable.
For a rough estimate, you could do the following:


Weight a sample of it and heat it above 100°C for a while (say, one hour?).
Weight the sample afterwards.


The difference should be the amount of water given away and is an estimate (from below) of how much water there was in the sample.

But be aware that you may have some sodium carbonate depending on storage conditions.