Formic and hydrochloric acids (HC1) have also been suggested as digestive agents. With scleractinian corals (Dipsastrea pallida), formic acid (3%, 72 h) has been used to decalcify polyps to assist in the visualization of ingested blue polypropylene shavings . HC1 has also be tried out as a digestant of microplastics from pelagic and sediment samples; however this non-oxidizing acid proved inconsistent and inefficient in digesting organic material .
Excised fish tissues, including the esophagus, stomach and intestines, have been successfully digested using potassium hydroxide (KOH, 10%) following a 2-3 week incubation [33,54]. The protocol has been adapted for the dissolution of gastrointestinal tracts of fish and mussel, crab and oyster tissues, either directly or following baking (450°C, 6 h), by incubating tissues in 10% KOH at 60°C overnight [21,97]. This latter method has proved to be largely efficacious in removing biogenic material. It is well suited to the dissolution of invertebrates and fish fillets, but it is less applicable for fish digestive tracts owing to the presence of inorganic materials. As with HN03, an oily residue and bone fragments may remain following digestion. Another strong base, sodium hydroxide (NaOH; 1 M and 10 M), has been successfully applied to remove biogenic material (e.g., zooplankton) from surface trawls, with 90% efficiency based on sample weight loss . Foekema et al. suggests polymers are resistant to KOH, and Dehaut et al.  showed no demonstrable impact on polymer mass or form, except in the case of cellulose acetate (CA). Testing the rapid KOH digestion protocol achieved a 100% microplastic recovery rate . PA, PE and polypropylene (PP) are resistant to 10% KOH, but polycarbonate (PC) and PET are degraded. Cole et al.  tested 40% NaOH (60°C) on a range of polymers and observed deformation of PA fibers, yellowing of polyvinyl chloride (PVC) granules and melding of polyethylene particles. Dehaut et al.  also noted that PC, CA and PET were degraded using this protocol. PE and PVC are resistant to NaOH, even at concentrations of 50% at 50°C.