Glauber’s Salt — Thermal Energy Storage

Sodium sulfate decahydrate as a phase change material: retrograde solubility, supersaturation, and latent heat

Difficulty: Medium | Time: 45–90 minutes | Visual Impact: Medium

Historical Context

Johann Rudolf Glauber (1604–1670) was a German-Dutch chemist who isolated the decahydrate of sodium sulfate from mineral spring water and named it sal mirabile — the wonderful salt. He sold it as a panacea and laxative to wealthy clients across Europe, becoming moderately famous in his lifetime. Scientifically, he was among the first to systematically study the preparation and properties of mineral salts, bridging alchemy and early modern chemistry.

The unusual solubility of Glauber’s salt — rising sharply up to 32.4°C then falling as temperature continues to rise — puzzled chemists for two centuries. The explanation lies in the phase transition at 32.4°C: the decahydrate melts congruently at this temperature, dissolving in its own water of crystallisation. Above this point, the stable solid phase is the anhydrous salt (thenardite), which is far less soluble. The apparent decrease in solubility is therefore a change in which form of the salt is in equilibrium with the solution.

Modern interest in Glauber’s salt focuses on thermal energy storage. Its melting point of 32.4°C and high latent heat (~254 kJ/kg) make it suitable for solar thermal systems and passive building temperature regulation — storing heat during the day and releasing it at night when the building cools below 32°C.

Materials

  • Sodium sulfate — 70 g anhydrous (or ~180 g Glauber’s salt decahydrate)
  • Distilled water — 100 mL
  • Small saucepan or heatproof beaker
  • Thermometer
  • Very clean glass jar with a tight lid (250 mL mason jar works well)
  • Seed crystal of sodium sulfate (save a few before starting)

Procedure

Part 1 — Making and Observing the Retrograde Solubility

  1. Dissolve 30 g of anhydrous sodium sulfate in 100 mL of water at room temperature (~20°C), stirring well. Some solid will remain — this is a saturated solution with excess
  2. Warm the slurry gently while stirring. Note the temperature at which all solid just dissolves (~26°C). This is near the solubility maximum
  3. Continue warming past 35°C — if using the decahydrate form, more solid appears as the anhydrous phase becomes the stable form; if using anhydrous and excess has dissolved, solubility appears to decrease
  4. Allow to cool to room temperature and observe crystals reforming — the characteristic large flat plates of the decahydrate

Part 2 — Supersaturation and Triggered Crystallisation

  1. Dissolve 70 g anhydrous sodium sulfate in 100 mL water at 40°C with gentle stirring until fully clear
  2. Filter through a clean paper towel or coffee filter directly into the very clean, pre-warmed glass jar — remove any dust or undissolved particles that would trigger premature crystallisation
  3. Cap the jar and allow to cool slowly, undisturbed, to room temperature
  4. The solution should remain liquid below 32.4°C — it is now supersaturated
  5. Touch a seed crystal to the surface of the liquid through a briefly opened lid. Crystallisation propagates outward from the nucleation point; the solution warms noticeably as latent heat is released

If premature crystallisation occurs while cooling, reheat gently to redissolve and try again.

Part 3 — Thermal Energy Storage Demo

  1. Fill a small sealed container (e.g., a resealable bag or a small jar) with Glauber’s salt decahydrate (either purchased or made by allowing Part 2 crystallisation to complete)
  2. Place the container in water at 40°C until fully melted (the salt becomes a clear liquid)
  3. Remove from the water bath; place the container in a room-temperature environment
  4. Record temperature every 2–3 minutes using a thermometer or thermometer strip taped to the outside
  5. Plot temperature vs time. A flat plateau at ~32°C confirms that energy is being released as latent heat at a constant temperature — the signature of a phase change material

Variation — 18°C Eutectic with Table Salt

Adding sodium chloride to Glauber’s salt depresses the melting point, and at the eutectic composition the mixture freezes and melts at approximately 18°C — close to comfortable room temperature and well below the pure decahydrate’s 32.4°C.

  1. Dissolve approximately 3 parts Glauber’s salt (by weight) with 1 part table salt in the minimum amount of warm water needed to form a homogeneous melt
  2. Seal in a small container and allow to cool
  3. The mixture will transition between solid and liquid near 18°C, absorbing heat as it melts and releasing it as it freezes

Because the phase change occurs near room temperature, this mixture is potentially useful for passive cooling — packs that stay cold by absorbing heat at 18°C rather than requiring ice temperatures. As with pure Glauber’s salt, the mixture tends to supercool, so seeding with a small crystal may be needed to trigger solidification.

(Variation demonstrated by NightHawkInLight.)

The Science

Congruent Melting and Retrograde Solubility

At exactly 32.4°C, sodium sulfate decahydrate (Na₂SO₄·10H₂O) melts into a solution that has exactly the composition needed to re-form the decahydrate on cooling — this is congruent melting. Above 32.4°C the equilibrium solid is anhydrous sodium sulfate, which has a much lower solubility. The apparent reversal in solubility reflects this shift in equilibrium phase, not a decrease in the intrinsic solubility of the salt.

\[\ce{Na2SO4 \cdot 10H2O(s) <=> Na2SO4(aq) + 10H2O(l)} \quad T_\text{m} = 32.4°C\]

Supersaturation

Like sodium acetate in the “hot ice” experiment, sodium sulfate solution can be cooled below 32.4°C without crystallising if the solution is clean and undisturbed. The supersaturated state is metastable — the solution “wants” to crystallise but lacks a nucleation site. A seed crystal provides the template needed to start the cascade.

Latent Heat

Crystallisation releases the latent heat of fusion — energy stored in the disordered liquid arrangement that is released as ions settle into the ordered crystal lattice and as water molecules bind into the hydrate structure. For Glauber’s salt this is ~254 kJ/kg, comparable to many commercially used phase change materials. The temperature plateau in Part 3 is direct evidence of this stored energy: the system maintains 32°C against the temperature gradient of the surroundings for an extended period.

Tips for Success

  • Cleanliness is critical for supersaturation — rinse the jar with distilled water before use; any particle will nucleate crystals prematurely
  • If using Glauber’s salt from a pharmacy, check that it is pure sodium sulfate decahydrate (some formulations contain additives)
  • The supersaturated state is more reliably achieved if you heat the solution to ~45°C and let it cool to room temperature inside an insulated box — slow, even cooling minimises disturbance
  • For the thermal energy storage demo, an inexpensive digital thermometer with a probe on a wire gives the best results for tracking the phase change plateau
  • Glauber’s salt can be reused indefinitely — simply remelt and repeat