Dear Mr Emad is possible to determine the expiry date using Arrhenius equation. You need accelerate the degradation process in a convenient temperature and you determine the kinetic constants for each temperature. Using Arrhenius equation, you extrapolate at room temperature. Follow, a good reference for this is: J. L. Manfio, A. Dal’Maso, A. M. Pungens, L. Brum Jr., and M. Steppe, Determination of carbocysteine syrup shelf life by Arrhenius method. Braz J Pharm Sc., 43(4), 563-570 (2007). This reference is in portuguese, but you can traduce easily using an electronic translate. You can download it using web of science. I hope that this suggestion helped you.

It depends upon the physico-chemical properties of the solute and solvent. If the compound is stable in the solution then it can be stored at low temperature. Concentration/strength of the solution can be measured after specific time interval to check any degradation of the compound. Accelerated storage test can also be done for some products.

Dear Mr. Amad, firstly it would be easier to answer by knowing what kind of solution you have and/or its property you need (pH, oxidizing / reducing agent, stock solution for calibration etc.). I would do as follows:

1. preparing a solution

2. apply it against freshly prepared solution after a time you consider (daily, weekly, monthly), eg. measure its pH, use for calibration etc., compare the results you get by fresh and non-fresh solution (eg. t-test, ANOVA, control-chart)

3. when you can find a systematic error caused by non-fresh solution the time has spent since its preparation may consider as its expiry date

I hope I could help you. :)