Jun 03, 2021 · For samples collected in 40-mL VOA vials, the VOA vials will be pre-preserved with HCl if needed. If free chlorine exists in a sample (Section 3.1), sodium thiosulfate will need to be added to the VOC method 624 VOA vials (Table 1). If sodium thiosulfate is needed a few crystals (10 mg/40 mL is sufficient for up to 5 part-per-million (ppm) Cl