And Epsom salt is MgSO4, is it MgSO4·7H2O?
Can you explain your error analysis in more detail?I'm not sure what was specifically meant, but CaCl at least has a tendency to suck moisture from the atmosphere and weigh more than it should on a ppm basis. If you leave it open and live somewhere humid . . . well, there's a reason it is used as a dehumidifier.
I use 1 and 5 gallon jugs, and know the levels in my kettles. I have a 100 gram capacity scale with a 0.01 gram resolution. My water volume and salt mass additions should be pretty close.
I hope Martin can lend some knowledge here.
Typically I think it is 7H2O, but whatever form it is, it is not hygroscopic like the CaCl is. Most salts are fine I think, unless maybe you live somewhere ridiculously humid. Even table salt will pick up moisture in a really humid area, or at least that is the explanation given for the grains of rice in the salt shakers at the beach.