View Full Version : Pot/Sod Carbonate substitutions

John Cahill
8-Apr-2007, 18:17
It is my understanding that Sodium Carbonate (anhydrous) may be substituted for Potassium Carbonate (anhydrous) in solutions except those using glycin. If a formula calls for n weight of potassium carbonate (anhydrous), what amount of sodium carbonate (anhydrous) could be substituted by weight? TIA

8-Apr-2007, 20:47
I hope this helps.

Eric James
8-Apr-2007, 21:59
It's difficult to say without knowing the role that potassium carbonate plays in this solution.

K2CO3 has a molecular weight of roughly 138g/mole; Na2CO3 has a molecular weight of approximately 106g/mole. You can use the "equal proportions" method to calculate the amount of sodium carbonate to substitute for potassium carbonate if your goal is to have a solution of equal molarity.

e.g. If your recipe calls for 100g of potassium carbonate, solve for X to determine how much sodium carbonate to use for an equimolar solution:

106/138 = X/100

So X = 76.8

9-Apr-2007, 04:51
For the Pyrocats, I just substitute the sodium carbonate 1:1 and make my tests. The sodium carbonate works just fine and so long as I am consistent, my times stay the same. It seems to me that practical photography requires balancing a lot of variables and that consistency is more important than just following a formula. YMMV.

John Cahill
9-Apr-2007, 11:23
Many thanks one and all. Now I know where to go with this. BTW, J.E. What means YMMV?

Donald Qualls
9-Apr-2007, 19:00
This substitution is complicated by the fact that most of the sodium carbonate you can easily obtain is in the monohydrate form, while potassium carbonate is usually sold and shipped in anhydrous (and called out that way in formulae) -- if potassium carbonate has been exposed to air for any prolonged time, it deliquesces (that is, draws water out of the air to increase its hydration, to the point it dissolves).

So, if you know the hydration state of your sodium carbonate, you simply have to add the appropriate number of H2O to the molecular weight. Last time I did this calculation, I found myself using about 88% as much sodium carbonate monohydrate as the potassium carbonate called out (which substitution, BTW, worked *magnificently* in Bath B of a 2-bath C-41 color developer).

YMMV, BTW, means "your mileage may vary" -- derived from automobile advertising of the 1970s when fuel economy was first a big selling point.

9-Apr-2007, 19:10
These is no substitution for pot.