How to Make a Drying Agent (Anhydrous Magnesium Sulfate)

Intro

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This is a comprehensive guide to making a drying agent (Anhydrous Magnesium Sulfate). Feel free to check out the above video to see it in action.

Outline (For the above video)

1. Purpose of experiment
2. Safety precautions and disclaimer
3. Material to be used
4. Pre-weigh multiple batches for different test configurations
5. Small-scale testing on different temperature
6. Start of experiment
7. Literature data on Magnesium sulfate dehydration
8. Calculation and analysis of the results
9. Recommended steps and configuration
10. Notable mentions
11. Tips and tricks for dehydration
12. Things to avoid
13. Demonstration of hygroscopic properties
14. Demonstration of dissolution properties
15. Crush the powder and bottle it up

For those who just need the instructions for making it but not the scientific details, please follow the steps below, some of the information (Background/Science) is skipped.

1. Epsom salt (1 pack)
2. Oven (Must be able to reach 250°C/482°F)
3. Over tray (Preferrably a disposable aluminium baking tray)
4. Air-tight storage container

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Safety precautions and disclaimer

All experiments should be carried out with extreme caution, please read till the end to fully understand this experiment before proceeding.

1. Don't touch the oven when it is hot.

(This experiment is actually pretty safe.)

Purpose of experiment

A desiccant is very useful in many experiments, we are going to make anhydrous magnesium sulfate by dehydration of Epsom salt, which is the heptahydrate form of magnesium sulfate.

The steps for this experiment are pretty straightforward and simple, I will however include many tips and tricks, things to avoid and a detailed explanation along with the experiment, feel free to skip to the part you need.

Preparing the Epsom salt

The bag of Epsom salt is poured onto the disposable oven tray and spread evenly across the surface.

Place the Epsom salt inside the oven

Place the tray inside the oven and turning on the heat to 250°C/482°F and bake it for 2 hours.

Collect the Anhydrous Magnesium Sulfate

After baking for 2 hours, the salt should turn into a solid chunk of anhydrous magnesium sulfate, you can then wobble or flex the tray slightly to break the chunks apart.

1. Disposable aluminium oven tray is highly recommended over normal oven tray (The reusable one that usually comes with your oven), since Epsom salt tends to harden and stick to the tray surface as it hardens, it will be very difficult to clean up.
2. A brand new/clean/lab-only oven is highly preferred, as oil and grease inside a used oven will easily deposit on the Epsom salt as it heats up, turning the magnesium sulfate brown which is contaminated with oil.

Literature data on Magnesium sulfate dehydration

According to this book*, magnesium sulfate starts losing its first 4 water molecules at 70-80°C, the 5th water at 100°C, 6th water at 120°C, the last one is pretty tough to lose though, it turns anhydrous at around 250°C.

*:O'Neil, M.J. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. 13th Edition, Whitehouse Station, NJ: Merck and Co., Inc., 2001., p. 1018

There are few ways you can store the product, but keeping it as a larger chunk form will prolong its shelf life by reducing the ability of it absorbing moisture from the air, and only crush the chunk before every use.