Application of liesegang ring formation on a gelatin film to the determination of sulfate concentration in individual rain droplets.

The determination of SO(4)(2-) concentration in individual rain droplets can be achieved using a gelatin film containing 5 mM barium chloride and 5 mM barium nitrate. SO(4)(2-) in droplets reacts with barium chloride in the film to form the characteristic Liesegang rings. The areal ratio of the ring to the droplet print (ARRD) on the film is a good indicator for determining the SO(4)(2-) concentrations in an individual droplet. Interference by NO(3)(-) in determining SO(4)(2-) concentration can be avoided by adding barium nitrate to the gelatin. Measurements of SO(4)(2-) in rainwater samples by this method agreed well with those from ion chromatography. The individual analysis of rain droplets by this method indicated that the concentrations of SO(4)(2-) were largely different with a 5-order magnitude, even among individual rain droplets collected simultaneously, and were higher in smaller rain droplets.