Practical, not theoretical apporach
I am pretty sure that I saved this from a Pat Gainer post.
Anyways - away we go...
The biggest problem I have faced when mixing metaborate is how pure is the hydroxide? It will absorb humidity and carbonic gas, so it's weight/activity will change as it ages. So, when one mixes the traditional borax/hydroxide formula, the results may vary significantly.
After lots of experimentation, I've devised the following to be able to mix it without the help of a pH meter: The trick is to make a concentrated solution, and mix it in a clear glass container (the concentrate will give visual clues).
25% metaborate solution (as per Michael Gudzinowicz tetrahydrate formula):
Water (room temp - some 20C/68F) - 400cc
Sodium hydroxide - 19g (pour slowly, stirring; usual precautions apply)
Borax - 90g (pour slowly, stir until dissolved, may take a couple of minutes).
Two things may happen:
-If the hydroxide is pure, there will be no borax left and the solution will be clear when stirred. And there will be no metaborate (transparent) crystals in the bottom.
-If the hydroxide is not pure (my case), there will be borax left, the solution will be cloudy when stirred, due to being a mix of dissolved metaborate and un-dissolved borax.
Start to add hydroxide slowly, stirring, 1g a time (or the equivalent in cc of an 20~50% hydroxide solution, easier) until the solution clears
When standing and there are just some borax crystals in the bottom of the solution when not stirred. DO NOT OVERDO.
If the crystals loose the borax whitish look and become kind of transparent (metaborate crystals), you've overdone the hydroxide, and the solution pH will be higher than it should be.
This is like a titration in which borax is the indicator. Let it stand for a couple of minutes (decant) and slowly pour the liquid in another container. Do not pour the crystals that are at the bottom.
Add water to 500cc.
This 25% solution has been mixed more than once, and it passes the basic (ouch!) test - dilute it to 1% and pH is 11.1 (within my pH meter precision). I've frozen it to 0C/32F and left it stand for 2 days at room temp and no crystallization.