Hi,
I am looking for wearable sodium and sweat sensors. Do you know where I can purchase or how to build them?
I really appreciate your help!
Most people would call that a t-shirt - wear a gray one, watch it change color!
Otherwise, look up "galvanic skin sensor", basically measuring change of resistance between 2 probes.
Tape 5V from arduino and an analog input to your arm, take analog Readings. High = low resistance = lots of skin conductivity. Low = dry skin, high resistance.
Probably need like a 10K resistor from input to Gnd also so probe level does not 'float" & yield false readings.
Thank you very much for your suggestions! 
I was hoping to find something like this: http://www.crunchwear.com/biotex-body-fluid-analyizer/
But I think it is the same principle as your suggestion and I will try it.