The best thing is to measure the water content of the carbonate you have in hands. F.e. weigh 10 gramms, put it in the oven and heat to 120 °C or slightly more for about 30 minutes. Weigh again and you you have the difference that was water, now you have plain anhydrous. Then you can adjust your recipe. If stored in tight containers it won't change significantly. I have a tight box with anhydrous carbonate that didn't change at all in about 1 year. If it's very old you may do the test again. Other substances may behave different.
Imo it's important to determine exactly which kind of soda you have. My anydyrous carbonate has 2 % water, that's neglectible. For Arm&Hammer washing soda (waterfree) I have reports containing 2 - 9 % water ex factory. In France you get a soude cristeaux that has 50 % water, that doesn't match neither mono- nor decahydrate, it's in between. So you better check.
Measuring the pH may be much worse, because a pH-meter that is pretty exact as your scale needs frequent calibration and will coast much much more than a neat scale with 0.01 g resolution. And checking the scale is very easy with a cheap syringe and water. 1 ml = 1 gramm.
Again, I got a lot of inquiries why f.e. my Caffenol-C-L recipe didn't work at all. If the pH goes below 9 it doesn't work. Almost always(!) the wrong soda was the cause.
Maybe you want to read this: http://caffenol.blogspot.de/2011/02/...-shooting.html
Best - Reinhold
Last edited by grommi; 01-20-2013 at 05:26 PM. Click to view previous post history.