It may be better to let first ...

It may be better to let first showers not be collected for a few minutes as these will have maximum acidity. For balance rain water, you may need to add a few minerals/ sodium/ potassium/calcium salts in traces as they do mineralisation of RO water..to make Ph as well as mineral salt content better for water to taste good as pure RO water without minerals is also bitter. Must also check Rainwater for any microbes that float in air and come down with rain.