Molecular SO2 (MSO2) is the form of SO2 that protects your wine from the spoilage organisms that want to do it harm.
Typically, 0.8 ppm MSO2 is considered the magic number for keeping your wine free of spoilage organisms. If you have a microbially stable wine (RS Dry and ML Dry), you might live dangerously and consider 0.5 ppm MSO2 your magic number.
