"I substituted Sodium Carbonate for Potassium Carbonate since I already had it on hand for other uses. I spoke with Sandy King prior to doing this. The only change is I mix it in five times the amount of water for the stock solution. I then change the mix to 1-5-100. The reason is that Sodium Carbonate reaches solution saturation much more rapidly then Potassium Carbonate."
Yes, I want to reiterate what Donald writes. At the same weight of dry chemical sodium carbonte can be substituted for potassium carbonate with no difference in development charactericstics. A Pyrocat-HD working solution of 1:1:100 using the recommended formulat of 100g of potassium carbonate per 100ml of water gives same results as a working solution of 1:5:100 using a 20% solution of sodium carbonate.