Emma, you are right to be concerned about the concentration and balance of electrolytes in your system. But with a juice fast there is probably nothing to worry about, as most vegetables and fruits contain enough natural Sodium (Na) and Potassium (K) to provide a healthy balance.
Typically, where electrolyte problems arise during a fast is with taking in too much plain water, either drinking it or when using enemas and/or colonics.
If you are taking regular enemas while fasting, you should add 1 teaspoon of Sea Salt or non-iodized table salt for each pint of water, or 4 teaspoons for a full bag enema. Never drink distilled water. If you are using bottled water, use spring water. Your body needs the minerals naturally present in most spring waters.