Which inputs are you using? If e.g. best_salt_1 is not one of the options of the first switch block, the output of [] is expected.
The best way to examine such problems by your own is the debugger. Set a breakpoint in the first line of this function by clicking on the box left beside the text in the editor. Then step through the code line by line.
By the way, the names of your function and variables are extremely exhaustive. Programming allows to apply a more abstract view on the problem, so you can use the same function for a set of problems. Then "beregn_ionic_strength_best_salts_CaCl2_MgCl2_NH4l" is a cruel name of a function.
