Abstract: The ionic strength or the specific gravity of an aqueous test sample is determined by mixing the sample with a reagent composition on a test strip. The strip includes a bibulous carrier containing an organic phosphonic acid, such as 1-hydroxyethlidene-1,1 -Diphosphonic acid and aminotris(methylenephosphonic acid), or inorganic phosphate having one of the structure formulas, ##STR1## where M stands for one equivalent of an alkali metal, ammonium, lower alkyl ammonium, or hydrogen ion, with the proviso that at least one of the M is hydrogen; x is an integer ranging from zero to two and y is also an integer ranging from zero to one, and serving to buffer the metal ions, and a chromogenic indicator capable of providing a detectable response of the ions being sequestered and coordinated within the complex in the mixture.