The hydrogel-forming polymers are used as absorbents for collecting and concentrating solutions. A solution to be concentrated can be contained in a dialysis membrane, with the hydrogel-forming polymer, or mixture thereof, on the outer surface of the membrane. A hydrogel-forming polymer can be added directly to the solution to be concentrated and the resulting concentrate separated from the gel by filtration of decanting. A hydrogel-forming polymer can be placed in a container formed of porous material and the container can be added to the solution to be concentrated and physically removed after concentration has occurred. Hydrogel-forming polymer can be used as a liquid collection in diagnostic membrane assay cartridges.