http://www.pburch.net/dyeing/FAQ/sodaash.shtml
The above link is were this come from.
Some forms of soda ash (e.g. that labeled as 'washing soda') contain more water molecules than others, which makes them weigh more and be larger for a given number of sodium carbonate molecules - this means that you need to measure out a larger quantity of the hydrated form in order to get the same results.
The type of sodium carbonate used in
washing soda is a decahydrate, so you need to use a lot more washing soda than you would anhydrous soda ash. One should
use 2.7 times as much washing soda as a substitute for soda ash, if measuring by weight, or 4.6 times as much if measuring by volume.
Although you can buy washing soda in the grocery store, this is usually advised against, because some US brands in the past were known to contain optical brighteners, salt, and/or fragrances.
However, Arm & Hammer brand "Super Washing Soda" does not now contain any additives, though you must use a larger quantrity due to its greater hydration level (see above). A better source is a swimming pool supplies store, or a hardware store that carries some pool supplies, which will carry pure sodium carbonate (a popular brand is pH Up), sold for the purpose of increasing the pH of pool water.
I am just trying to find out if the above statement is true.
Is Arm and Hammer washing soda (decahydrade) and Pool store Soda Ash (anhydrous)
Is the 2.7 times by weight and 4.6 times by volume for washing soda over pool store soda ash
a true statement?
The pool calculator shows the weight and volume amounts of washing soda and soda ash to be the same.