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.

  • 3
    $\begingroup$ could you not just dry it in the sun or in the oven at a moderate temperature? $\endgroup$ – Fl.pf. Mar 27 '17 at 12:38
  • $\begingroup$ Related: chemistry.stackexchange.com/q/32588/40029 $\endgroup$ – Felipe S. S. Schneider Mar 27 '17 at 13:00
  • $\begingroup$ @Fl.pf. well I was worried about unexpected reactions. I will do it. $\endgroup$ – FarO Mar 27 '17 at 13:17
  • $\begingroup$ I can't think of something bad to happen. 50 degree C should be enough to dry it in a short time and there shouldn't be any chemistry happening i think $\endgroup$ – Fl.pf. Mar 27 '17 at 13:19

For a rough estimate, you could do the following:

  1. Weight a sample of it and heat it above 100°C for a while (say, one hour?).
  2. 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.

  • $\begingroup$ Heating it in a steel pot in the oven is safe? $\endgroup$ – FarO Mar 27 '17 at 13:19
  • 2
    $\begingroup$ Stainless steel is okay, oven safe ceramic would be better. If you have parchment paper or silicone baking mats, you can use those for the surfaces that touch the NaOH. $\endgroup$ – J. Ari Mar 27 '17 at 13:46
  • 1
    $\begingroup$ For @Felipe Schneider's hint pointing to $\ce{Na2CO3}$, especially if the sample were openly exposed to air while being heated, I'm skeptic if simple heating will dry NaOH or rather worsen the problem. Not only worsen the problem by creating more $\ce{Na2CO3}$ (my oven is not purged by Nitrogen or Argon ...), but that NaOH, already hygroscopic at ambient temperature, will catch even more humidity. $\endgroup$ – Buttonwood Mar 27 '17 at 20:50

As a speculation, perhaps following Armaregos' Purification of Laboratory Reagents may be adapted a little bit. By appropriate downscaling you might obtain a good sample of NaOH that then may used in new batch or a comparative analysis with the larger amount of NaOH kept aside. To quote the entry in question:

"Sodium hydroxide (anhydrous) [1310-73-21] M 40.0. Common impurities are water and sodium carbonate. Sodium hydroxide can be purified by dissolving 100g in 1L of pure EtOH, filtering the solution under vacuum through a fine sintered-glass disc to remove insoluble carbonates and halides. (This and subsequent operations should be performed in a dry, CO2-free box.) The soln is concentrated under vacuum, using mild heating, to give a thick slurry of the mono-alcoholate which is transferred to a coarse sintered-glass disc and pumped free of mother liquor. After washing the crystals several times with purified alcohol to remove traces of water, they are vacuum dried, with mild heating, for about 30h to decompose the alcoholate, leaving a fine white crystalline powder [Kelly and Snyder JACS 73, 4114, 1951.]"

For the eventual comparison, you may engage a volumetric analysis. Don't worry if you do not have a burette, a finely graduated cylinder will do so, too. In absence of pH paper I would resort to red cabbage, too.

I mean, if the transfer of the filtrate into the vessel for evaporation of EtOH under reduced pressure is done rapidly; and not at least, if there is (borrowed?) equipment to perform all this, if a purchase of new batch NaOH isn't an option, and, and, and ...

  • $\begingroup$ The procedure is interesting, but do you call those "household items/chemicals"? $\endgroup$ – FarO Mar 28 '17 at 12:14
  • 1
    $\begingroup$ @OlafM It was called a speculation, as repeated soap making could be read like there were some special equipment available. Hence all the "if" in the end, yet simultanously including the suggestion that proper disposal of the old and purchase of a new batch of NaOH were an alternative to consider -- both in investment of time, chemicals, and money. $\endgroup$ – Buttonwood Mar 28 '17 at 12:25
  • $\begingroup$ Ok now I see what you meant. $\endgroup$ – FarO Mar 28 '17 at 12:45

Measure out a known volume of NaOH and weigh it, then compare this mass to the theoretical mass calculated using the density of dry NaOH granules and the known volume.

  • $\begingroup$ How can I measure the volume, since it contains voids between granules? if I put it in water to fill them, it dissolves. $\endgroup$ – FarO Mar 27 '17 at 13:16
  • $\begingroup$ You would have to measure out NaOH that hasn't clumped yet. The assumption that I made was that the extra water from the air was absorbed uniformly enough the mass difference will be noticeable from any sample taken from your jar. $\endgroup$ – J. Ari Mar 27 '17 at 13:44

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