Supersaturation
- Page ID
- 131427
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Required Training |
Required PPE |
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UC Lab Safety Fundamentals |
Lab coat, safety glasses/goggles, nitrile gloves |
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Equipment |
Chemicals |
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2L beaker |
175g Sodium acetate trihyrate, NaC2H3O2 . 3H2O |
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hotplate |
50 ml DI H2O |
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500ml Erlenmeyer flask (clean & scratch free) |
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100ml beaker |
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spatula |
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cardboard |
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Procedure:
I. Preparation (2 per demo)
- Provide a boiling water bath by filling a 2L beaker ¾ full with water and heating it on the hot plate.
- In a 500ml Erlenmeyer flask, place 175g Sodium acetate trihydrate, NaC2H3O2 . 3H2O in 50ml DI H2O.
- Heat mixture in the boiling water bath and swirl the flask occasionally until a clear, homogeneous solution is obtained.
- Then, invert the 100ml beaker over the mouth of the flask and allow to cool undisturbed to room temperature (1-3 hours).
- After the demonstration, The super saturation is restored by heating in a boiling water bath; it can be re-used repeatedly unless contaminated. After several cycles, a small amount of water may have to be added to compensate for evaporation.
II. Instruction Demonstrate Crystallization
- Avoid bumping during transport as it may cause crystallization to occur.
- Place a few crystals of NaC2H3O2 . 3H2O on the cardboard.
- Slowly pour the solution onto the crystals. Crystallization from the supersaturation should occur immediately, forming a mound of white solid sodium acetate. If crystallization does not occur add a few more crystals
- The shape of the mount will depend on the manner in which the solution is poured; if desired, pillars of different shapes and heights can be produced. Another method is to add the “seed” crystals to the flask and watch the crystallization form with in the flask.
- After the demonstration, the solid can be cut up with the spatula and returned to the Erlenmeyer flask to be used again.
Discussion:
How it works: all the water is trapped within the solid, leaving no visible trace of liquid. Note: the solid feels warm to the touch.
When the solution is cooled to 20oC, it is unsaturated with respect to NaC2H3O2 but supersaturated with respect to NaC2H3O2 . 3H2O. “Seeding” the solution with NaC2H3O2 . 3H2O thus causes formation of crystals of the hydrated sodium acetate. Since there is no residual water, higher hydrates than trihydrate may be formed. The composition of the solution corresponds to NaC2H3O2 . 5.2 H2O.
Hazards:
If crystallized sodium acetate touches the skin, wash with water.
SOP:
N/A
Disposal (by Storeroom)
The material can be recycled. When disposal is warranted, the solid should be flushed down the drain with water.
Acknowledgement and adapted from:
B. Z. Shakhashiri Chemical Demonstrations; A Handbook for Teachers of Chemistry Vol.1, Wisconsin, 1989, Vol.1, p.27-30