Calibrate at least once a week, preferred is daily. Calibrate if in doubt of your results being accurate or reproducible. When subject to temperature changes, mind that a difference of 1°C causes a measurement error of 2%. Check the manual for more information on each ISE about the expected stability.
The stock standard will last at least six months before discarding, whereas diluted standards treated with ISA/pH buffer should be prepared weekly.
It is best to store them dry when they are not to be used in the next week or so. Empty out the filling solution in the gas-sensing electrode. For shorter periods, store in dilute standard approximating the sample concentration, and ISA/pH buffer added when required.
Yes, it is possible, but somewhat difficult.
It is best to standardize and measure samples at the same temperature without using temperature compensation.
Yes, direct measurement is possible in many cases without ISA/buffer addition to the sample stream. However, Fluoride, Sulfide, Ammonia, and Sodium Electrodes do require pH adjustment and must have ISA/ buffer added to the sample stream.
The most obvious choice will be determined by what concentration units are desired (e.g. ppm as what?). Also, if an electrode is being used to measure another ion (e.g. sulfate with a lead electrode), use a stock standard of the ion to be measured (e.g. sulfate).