This study describes an extremely sensitive and simple assay to measure small volumes of solutions, <1 nL. The assay takes advantage of the Sandell-Kolthoff reaction in which yellow cerium(IV) is reduced to colorless cerium(III) in the presence of arsenic(III) and catalytic quantities of iodide ion. The reaction is linear with respect to the rate of Ce(IV) reduction and the quantity of I(-) present. Typical assays can measure 10-100 picomoles of iodide in a sample. When I(-) is substituted for chloride ion in standard biological buffers, such as Tris-buffered saline, the assay can be used to determine the volume of solution printed in a microarray.