Desiccants, Drying agents

Laboratory Practice of Organic Chemistry, G. Ross Robertson, 3rd ed. (1954), p. 147-152:

 

Water vapor pressure of hydrates of Magnesium Sulfate
MgSO4 . 7H2O 12 mm
MgSO4 . 6H2O 10 mm
MgSO4 . 5H2O 9 mm
MgSO4 . 4H2O 5 mm
MgSO4 . 3H2O 5 mm
MgSO4 . 2H2O 2mm
MgSO4 . 1H2O 1mm

Magnesium Sulfate. The best neutral drying agent of good capacity obtainable; compatible with most compounds, being a compound of a strong base and strong acid. Low cost, but not in common use; often difficult to obtain commercially, probably because it is more difficult to preserve than calcium chloride. Readily prepared, however, from ordinary clean technical crystalline Epsom salt ("bath salt") of good grade by preliminary heating at 150o to 175o in an oven, then to low red heat for complete desiccation. [temperatures are in Celsius]

 

Dessicant Next hydrate Mg. per liter
  Phosphorous pentoxide   (HPO3) ? Not detectable
  Sulfuric acid   (uncertain) Less than 0.001 mg.
  Magnesium perchlorate   Mg(ClO4)2 3H2O 0.002 mg.
  Calcium sulfate   2CaSO4  H2O 0.005
  Potassium hydroxide, fused   KOH H2O 0.014
  Calcium chloride   CaCl2 H2O 0.036
  Sodium hydroxide, sufed   NaOH H2O 0.80
  Zinc chloride   ZnCl2 H2O 0.98
  Magnesium sulfate   MgSO4 H2O 1
  Cupric sulfate   CuSO4 H2O 2.8
  Sodium sulfate   12

Calcium Sulfate (Drierite, patented product). Excellent from standpoint of intensity, but very low capacity; only 6.6% of water absorbed at equilibrium. Used often to dry organic compounds of the "water class" (with high hydroxyl content) which have already been partially dried by other methods.  ( http://en.wikipedia.org/wiki/Drierite , http://en.wikipedia.org/wiki/Calcium_sulfate#Dehydration_reactions )