A straightforward method for measuring the range of apparent density of microplastics

Highlights: • Bases of ethanol, ultrapure water and saturated NaI are optimal for the density gradient solutions. • Density gradient solutions are easy to be prepared in a density range of 0.8–1.8 g/cm³. • Density gradient solutions are feasible to measure the apparent density of microplastics. Density of microplastics has been regarded as the primary property that affect the distribution and bioavailability of microplastics in the water column. For measuring the density of microplastis, we developed a simple and rapid method based on density gradient solutions. In this study, we tested four solvents to make the density gradient solutions, i.e., ethanol (0.8 g/cm³), ultrapure water (1.0 g/cm³), saturated NaI (1.8 g/cm³) and ZnCl₂ (1.8 g/cm³). Density of microplastics was measured via observing the float or sink status in the density gradient solutions. We found that density gradient solutions made from ZnCl₂ had a larger uncertainty in measuring density than that from NaI, most likely due to a higher surface tension of ZnCl₂ solution. Solutions made from ethanol, ultrapure water, and NaI showed consistent density results with listed densities of commercial products, indicating that these density gradient solutions were suitable for measuring microplastics with a density range of 0.8–1.8 g/cm³.