What is the role of potassium acetate in plasmid extraction?
The alkaline lysis method of plasmid purification involves the addition of sodium dodecyl sulfate to lyse the cells and sodium hydroxide (a strong base) to denture the DNA. Potassium acetate is then added for two reasons:


The acidic acetate buffer neutralizes the solution and allows catenated plasmids to renature. 
Potassium dodecyl sulfate is poorly soluble in water. Adding potassium to solutions of dodecyl sulfate precipitates it, thus facilitating its removal.