Arm and Hammer Super Washing Soda vs. Soda Ash

Strictly BBB

Aug 26, 2010
I am new to the forum. And new to using forum all together. So I hope this question is in the right place. It may need to be in "The Deep End". I think this pool site is the best around. :-D I was wanting to know if Arm and Hammer Super Washing Soda and regular soda ash should have the same by weight or by volume as each other in the pool calculator? I am sure the pool calculator is right, but this "All about hand dying T-shirts" website has got me a little confused. Link Sorry, if the link did not work. I will repost it, if needed. The paragraphs in the web page titled Hydration and Where can you buy soda ash?, is where my question comes from. I know that I will get the right answer here. Thanks to all for the great info on the TFP site.


LifeTime Supporter
Nov 18, 2009
Sacramento, CA
For our purposes you can use the weights derived in the pool calculator (soda ash = washing soda).

Soda ash is used to raise both pH and TA.

If you only need to raise pH use Borax. If you only need to raise TA use baking soda.

Strictly BBB

Aug 26, 2010
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.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
The MSDS for Arm & Hammer Super Washing Soda indicates that it is >= 85% Sodium Carbonate and <= 15% Water so it is clearly not the decahydrate form (though could be the monohydrate form). They just don't guarantee that it didn't absorb some water, but it's basically similar to soda ash. You can compare this to BioGuard Balance Pak 200 which is listed as 100% sodium carbonate. So Arm & Hammer Washing Soda could be as low as 85% pure so maybe you might need to use a little more, but not very much.

We never had any reports of people using Arm & Hammer Washing Soda getting wildly different results than expected, but then again it's not used that often unless one is using Trichlor as their primary source of chlorine.