Rain water has few if any ...

Rain water has few if any minerals and hence very little buffer capacity. 

 

to determine normal rainwater pH

First, we calculate the amount of  CO 2 dissolved in water under an atmosphere of pressure from Henry’s Law

Since  CO 2 makes up 0.0355% of the atmosphere (on the average) and

K CO 2 =2x10-3

Since is in equilibrium with  H 2 CO 3 (aq), the first acid equilibrium is normally given by

is predominant. Also since

CO 2 (aq) +  H 2 O « H + (aq) +  HCO 3- (aq)

The proton and bicarbonate concentrations are equal. Thus

When we substitute the carbon dioxide concentration, and solve for pH, we get

p H  = 5.65

Since rain is in equilibrium with the atmosphere, this is the pH expected for natural rain. It is also the pH expected if the body of water is in equilibrium with the atmosphere, and does not contact limestone (e.g.,  CaCO 3).  Adding some limestone to your water tank should increase the pH overtime and restore proper balance.

 Yours is a bit lower than 5.65 so it may contain some sulfur compounds forming a bit of sufluric acid but shouldn't be harmful.