What are the grades of ethanol commonly used in the biology lab?

There are 3 grades of ethanol commonly used in the lab. 

1. 95% (95.6%) ethanol is the highest concentration of ethanol one can get by distillation. This is because 95.6% ethanol is an azeotrope, where the ratio of ethanol to water in its vapor state is identical to that in its liquid state. 
2. Denatured alcohol (at 95% or absolute form) includes substances like isopropanol and methanol, making it hazardous to drink. Denatured ethanol is commonly used for laboratory disinfection purposes. 
3. Absolute ethanol (purity of 99–100%) is often needed for procedures sensitive to water presence. To achieve concentrations beyond 95%, additives are introduced to disrupt the azeotrope composition, enabling further distillation. However, this process may result in trace amounts of additives like benzene in absolute ethanol. It is important to note that absolute ethanol is hygroscopic. Therefore, if left uncapped it won't maintain its 100% ethanol purity for an extended period.