Baking Soda: Is it hygroscopic?

[Silver_Is_Money]

Does the percentage of sodium bicarbonate by weight change with time due to exposure to humidity?

Just as for CaCl2, does one need to be aware of the water content (and compensate for it) in NaHCO3 when trying to determine how much of it to add in order to raise mash pH by a calculated amount? And if so, what typical moisture compensation factor should be applied?

 

[dmtaylor]

Yes, I believe it absorbs moisture. Not sure how much. Probably <10% under most circumstances, or not enough to be concerned about. No worries.

 

[ajdelange]

While I'm sure that it adsorbs to some extent as almost everything does (so that an analytical chemist would heat it in an oven before weighing it out) it doesn't absorb to the extent calcium chloride does. It is commonly sold for household use in boxes and you can leave an open box in the pantry or put one inside a humid refrigerator and it won't cake nor form a puddle as CaCl2 does. When weighing out CaCl2 in humid conditions on a sensitive balance (0.1 mg precision) you can watch the numbers roll. This doesn't happen with NaHCO2. If you look up the chemical form there is only one, NaHCO3 - no NaHCO3.nH2O i.e. no forms listed that show water of hydration.

Now while the above is fine for practical purposes NaHCO3 does, in fact absorb water from the air at high humidity. For more detail than you will ever really want see:
http://seniorchem.com/sodiumbicarb_stability.pdf

 

[Silver_Is_Money]

Now while the above is fine for practical purposes NaHCO3 does, in fact absorb water from the air at high humidity. For more detail than you will ever really want see:
http://seniorchem.com/sodiumbicarb_stability.pdf

The water absorption doesn't seem to be all that bad sans for the case of 25 degrees C. and 93% humidity, wherein sodium bicarbonate gains weight at the rate of 2.6% per day. Moral of the story: Don't store it at 93% humidity.