 When the membrane is chosen so that is it more permeable to one constituent than another, then the permeate will be richer in that first constituent than the retentate.The rules for positive and negative azeotropes apply to all the examples discussed so far, but there are some examples that don't fit into the categories of positive or negative azeotropes.The best known of these is the ternary azeotrope formed by 30% acetone, 47% chloroform, and 23% methanol, which boils at 57.5 °C.