Making Anhydrous Magnesium Sulfate from Magnesium Sulfate Heptahydrate (Epsom Salts)
MgSO4 • 7H2O = Magnesium Sulfate Heptahydrate (Epsom Salts)
MgSO4 = Anhydrous Magesium Sulfate
Anhydrous Magnesium Sulfate will absorb water from the atmosphere, acting as a very effective desiccant or drying agent.
The drying power of Anydrous Magnesium Sulfate is superior to that of Anhydrous Calcium Sulfate (Drie-Rite).
Epsom Salts can be easily converted to Anhydrous Magnesium Sulfate by baking in an oven at temperatures between 400°F and 482°F
150°C = 302°F At this temperature Magnesium Sulfate Heptahydrate is converted to Magnesium Sulfate Monohydrate
200°C = 392°F At this temperature Magnesium Sulfate Monohydrate is converted to Anhydrous Magnesium Sulfate
250°C = 482°F Loses all waters of hydration. yielding Anhydrous Magnesium Sulfate
1124°C = 2055°F Decomposes to MgO, SO2, SO3 before or as it reaches melting point.
MgSO4 7H2O = 246.4564 molecular weight
MgSO4 anhydrous = 120.36 molecular weight
120..36 / 246.45 = 48.83%
246.45 / 120.36 = 204.76%
Anhydrous MgSO4 can absorb 204.7% it's weight in water
From the article "sodium and calcium sulfates are surprisingly ineffective...[drying agents]"
| % of drying after 15 minutes with 10% w/v desiccant |
|---|
| Desiccant | % of drying |
|---|
| 4A Molecular Sieves | 99 |
| Calicium Chloride (powder) | 86 |
| Calcium Chloride ( pellet) | 84 |
| Magnesium Sulfate | 80 |
| Molecular Sieve (beads) | 65 |
| Calcium Sulfate | 22 |
| Sodium Sulfate | 18 |
Given anhydrous Calcium Chloride would be a superior choice,
and is cheaper, I'd suggest that was a preferred alternative to
anhydrous NaSO4 or Magnesium Sulfate ( note that Epsom
Salts is the heptahydrate, and can't be used ).
One common item that can be purchased at your local grocery store is Epsom Salt, which has the chemical formula MgSO4*7H2O.
This compound is considered hydrated, meaning that the salt is in combination with water.
Magnesium sulfate heptahydrate loses its water of hydration in steps.
The heptahydrate, which exists as transparent crystals, will readily lose one molecule of water already at room temperature when the air has low humidity.
The resulting hexahydrate has the appearance of dull,white, fractured crystals.
The hexahydrate will lose water of hydration and form the resulting monohydrate if heated to approximately 150 °C.
At 200 °C, the monohydrate loses its water and forms anhydrous magnesium sulfate.
At much higher temperatures,decomposition to magnesium oxide and sulfur trioxide occurs.
Therefore, the conversion of the heptahydrate to the monohydrate must be carried out at temperatures below 200°C but above 150°C.
Equipment:
250-mL beaker
Bunsen burner
Ring Stand
Stir stick(2)
Iron rings
Weighing paper
Balance
Spatula
Procedure:
Prior to the laboratory period, students should go to a grocery store and purchase a one pound carton of Epsom Salt. You will only need 10 g of this salt.
The appearance of the crystals must be checked for a transparent look.
If the crystals are opaque and white, you will be starting with MgSO4*6H2O.
Measure out 20 g of Epsom Salt and place inside a 250-mL beaker.
Determine the mass of your sample to 0.01 g.
Construct a ring stand with two iron rings.
A wire gauze will have to be placed between the two iron rings.
Place the beaker between the two rings on top of the wire gauze.
Begin to gently and cautiously heat the beaker with the crystals.
Stir the crystals to allowthe water to escape. If you heat too strongly and do not stir the crystals sufficiently, too much water is released to quickly.
This water might dissolve some of the crystals andcertainly will cause your material to cake. If this happens, your final product will not be an attractive white powdery substance.
After you have gently heated the material for a few minutes without any clumpingoccurring, slowly intensify the heating but make sure that your product does not begin to melt.
Continue to heat until you have a fine, white powder.
Too much heat will cause the product to take on an unattractive gray coloration and expel sulfur trioxide.
After the dehydration is complete, allow the product to cool.
Transfer the product ontoweighing paper and determine your mass.
Calculate the percent yield based upon thenumber of waters of hydration lost.
Consider the starting material to be hexahydrated orheptahydrated as determined by your initial visual inspection.
Consider the product to bemonohydrated.
Procedure
Mass a crucible and its cover and record in a data table. Place about 2 grams of the hydrate in the crucible and record the mass of the crucible, cover and hydrated salt. Gently heat the crucible and its contents for about 10 minutes. During the last minute of heating remove the cover so that any moisture which has collected on the underside of the cover can evaporate. When the crucible has cooled determine the mass of the crucible, cover, and contents.
Replace the cover and heat the crucible for five more minutes removing the cover during the last minute of heating as you did above. Cool and determine the mass of the crucible, cover, and contents. This last mass should agree with the previous mass to within plus or minus 0.005 grams. If it does not repeat this heating until a constant mass is reached. This is called heating to a constant mass and is the only way of insuring that the reaction is complete.
Magnesium Sulfate is Hygroscopic
A hygroscopic material (literally "water seeking") is one that readily absorbs water (usually from the atmosphere).
In most cases, the water can be removed from the material by heating (sometimes under vacuum or under a flow of dry gas such as nitrogen).
Hygroscopic materials are fairly common. Some may absorb a finite amount of water (such as magnesium sulfate, MgSO4) while others may attract so much water that they form a puddle and dissolve (deliquesce). For example, solid sodium hydroxide (NaOH) pellets will form a small corrosive puddle in less than an hour in moist air.
Drie-Rite is a brand of Anhydrous Calcium Sulfate commonly used as a desiccant or drying agent.
Cost is approximately $4.07 per pound.
Epsom Salt is the generic name for Magnesium Sulfate Heptahydrate.
Epsom Salts typically sell for less than $1 a pound.