Abstract: The invention relates to a kit for the measurement of anion, cation or zwitterion availability in a medium or for the measurement of free inorganic ions in disrupted cells from plant or animal tissue. The kit comprises ion-exchange material having a measurable ion uptake potential for immobilizing available ions from the medium on the material following contact on the material with the medium or suspension. It also comprises means to remove particulate solids from the material after having contacted the medium with the material. Preferably, the ion-exchange material is cation and/or anion-exchange material in the form of rigid or flexible films, sheets, membranes or the like. The invention also relates to a method for the on-site in situ measurement of anion and/or cation availability in a solid medium and to a method for measuring free organic ions in cells from plant or animal tissue.