Joanne -

I am not sure if Sodium Bicarbonate ever expires...on the other side - I never tried it on AVs for whatever reason - but it is an old fungal preventative. Used it myself on gooseberry bushes in wet years to prevent Sphaeroteca - false powdery mildew - with mixed results... but it was in a stone age...

The rest - when you mix water and oil - you create a suspension - a mix of minuscule drops of oil in water. With time this delicate balance deteriorates - as in milk - cream goes to the top.

So - if they say - mix as much as you need, use and dump the rest - the reason - the active ingredient is not going to stay evenly distributed. Little droplets of oil will merge... and cream again will go to the top....