Quote Originally Posted by juan
john_s, I'm no chemist, but I don't think you're quite right. I'm saying add the carbonate to 200ml of water, not add 100ml of water to the 130ml containing the carbonate. By my way of thinking, that means double the solution contains the same amount of carbonate.
If you add the carbonate to twice the specified amount of water, you don't have twice the volume of solution. And consequently, you don't have exactly half-strength Part_B.

How much does this matter? As you say, if one is consistent, there is unlikely to be a problem. In fact, you might have found a slightly better formula than Sandy! At least for your chosen film. I don't know. I was just talking about the mixing concentration.