Calcium nitrate in cold packs and heat storage: what the tetrahydrate can and cannot do
Dissolving calcium nitrate tetrahydrate in water is highly endothermic, and Wikipedia notes it is “sometimes used for regenerable cold packs”.[1] In practice this is a niche, mostly research-stage use: the mass-market instant cold pack is built on ammonium nitrate, calcium ammonium nitrate or urea.[3][17] The same hydrate, Ca(NO3)2·4H2O, melts at about 43–44 °C with an enthalpy of fusion of 36.6 ± 0.2 kJ/mol,[10] so it is also studied as a low-temperature phase-change material (PCM) for heat storage. Its main practical problem is supercooling of up to about 60 K, which patents and a University of Edinburgh thesis address with nucleating additives.[5][11] It is also strongly hygroscopic (deliquescence at about 50.5% relative humidity at 25 °C),[13] so sealed handling matters.
Key facts at a glance
| Item | Value / answer |
|---|---|
| Cold-pack role | Dissolution of the tetrahydrate is highly endothermic; “sometimes used for regenerable cold packs”[1] |
| Typical commercial instant cold-pack salts | Ammonium nitrate, calcium ammonium nitrate or urea[3][17] |
| Melting point of the tetrahydrate | 42.7 °C (Wikipedia); 43.9 ± 0.3 °C measured by calorimetry[1][10] |
| Enthalpy of fusion, tetrahydrate | 36.6 ± 0.2 kJ/mol[10]; about 155 kJ/kg by our calculation from the molar mass (236.15 g/mol) |
| Literature spread for fusion | Melting 42–47 °C and fusion enthalpy 28.7–36.1 kJ/mol across earlier reports[10] |
| Supercooling of the pure hydrate | Fails to nucleate; sub-cools up to 60 K below its freezing point[11]; one patent example sub-cools to about 27 °C[5] |
| Solubility of the tetrahydrate | 1.44 g per g of water (literature value quoted by Sigtryggsson et al.)[13] |
| Deliquescence relative humidity | 50.5% at 25 °C for calcium nitrate (value from Adams & Merz, 1929, quoted in a 2020 study)[13] |
| Water uptake, 24 h at high humidity | Laboratory-grade calcium nitrate absorbed 1.80 g water per g in 24 h; complete dissolution in 4.3 h at 90–99% RH[13] |
| Stable solid phase at 0 and 25 °C in Ca–Mg nitrate brines | Ca(NO3)2·4H2O[15] |
| Heat of solution, ammonium nitrate (reference) | +25.69 kJ/mol (CRC values quoted in a cold-pack patent)[6] |
| Heat of solution, urea (reference) | +15 kJ/mol[6] |
| Typical salt-hydrate PCM latent heat | Often 200–300 kJ/kg (general figure, not specific to calcium nitrate)[20] |
Why does calcium nitrate tetrahydrate get cold when it dissolves?
When a salt dissolves, energy is needed to break the crystal lattice and energy is released when ions are hydrated. If the lattice cost exceeds the hydration gain, the solution takes heat from its surroundings and cools. Wikipedia describes the dissolution of calcium nitrate tetrahydrate as highly endothermic, which is why it is sometimes used in regenerable cold packs.[1] The cold-pack effect is a property of the tetrahydrate: the tetrahydrate is the form calcium nitrate takes in contact with moist air.[1] Textbook-style demonstrations show the same principle with ammonium nitrate: a cold pack of this type holds about 4.4 mol of water per mol of NH4NO3 and can fall to about −11 °C.[18]
We could not find a calorimetric enthalpy-of-solution value for Ca(NO3)2·4H2O in a source we could read in full, so this page does not quote one. The NIST Thermodynamics Research Center publishes ThermoML records for calcium nitrate systems,[14][15][16] and a 1976 study reported the standard enthalpies of solution and formation of the anhydrous magnesium and calcium nitrates;[19] anyone designing a pack should take the tetrahydrate value from those primary data sets, not from a vendor sheet.
How does it compare with ammonium nitrate and urea cold packs?
A cold-pack patent lists heats of solution taken from the CRC Handbook (90th edition).[6] Calcium nitrate tetrahydrate is not in that table.
| Salt | Heat of solution (kJ/mol) | Per gram (kJ/g, calculated) | Note |
|---|---|---|---|
| Ammonium nitrate, NH4NO3 | 25.69 | 0.32 | Most common instant-pack salt[2][3][6] |
| Potassium nitrate, KNO3 | 34.89 | 0.35 | Higher per mole but much less soluble; listed for comparison[6] |
| Sodium nitrate, NaNO3 | 20.5 | 0.24 | [6] |
| Potassium chloride, KCl | 17.22 | 0.23 | [6] |
| Urea, CO(NH2)2 | 15 | 0.25 | Lower toxicity; often blended with ammonium nitrate[6][9][17] |
| Ammonium chloride, NH4Cl | 14.78 | 0.28 | [6] |
| Calcium nitrate tetrahydrate | Described as highly endothermic[1]; no sourced numerical value available to us | ||
Per-gram values are our arithmetic from the tabulated molar values and standard molar masses. A commercial pack is also limited by solubility and dissolution rate, not just heat per mole. An extended-life patent, for example, reports that adding urea to ammonium nitrate roughly tripled the time a pack stayed cold in a simulated-use test (patent example, not a specification).[9] Another cold-pack patent lists ammonium nitrate as the preferred salt and urea among alternatives.[8] Calcium ammonium nitrate, a different material from calcium nitrate, is also named as a pack ingredient.[3][17]
Is calcium nitrate actually used in commercial cold packs?
Only marginally, as far as published sources show. Wikipedia mentions regenerable packs,[1] but the cold-pack patents and consumer-safety sources we read name ammonium nitrate, calcium ammonium nitrate and urea, not calcium nitrate.[3][6][8][17] Treat calcium nitrate cold packs as a niche or experimental idea. A buyer wanting one needs their own formulation data and trials.
What is calcium nitrate tetrahydrate as a heat-storage material?
Ca(NO3)2·4H2O melts congruently near 43–44 °C. Calorimetry gives 43.9 ± 0.3 °C and 36.6 ± 0.2 kJ/mol, and earlier reports range from 42 to 47 °C and 28.7 to 36.1 kJ/mol.[10] Storing heat as latent heat of melting in this temperature window suits low-grade heat storage; the Modine patent, for instance, targets automotive heat storage.[4] Another patent presents it as a PCM candidate.[5] The spread of literature values is itself a warning: purity, water content and measurement method change the answer.
This is a different regime from the high-temperature molten-salt work in molten-salt thermal storage. For the anhydrous salt, a 2019 NIST ThermoML dataset reports an enthalpy of fusion of 33.4 ± 1 kJ/mol at 823 K.[14]
What goes wrong with calcium nitrate tetrahydrate as a PCM?
- Supercooling. The hydrate “melts congruently, but fails to nucleate” and can sub-cool up to 60 K.[11] A patent example shows the pure hydrate sub-cooling to about 27 °C.[5] Stored heat is then not released at the expected temperature.
- Phase segregation and incongruent melting. Salt hydrates generally lose capacity over repeated cycles unless thickeners, extra water or encapsulation are used.[12][20]
- Density and water balance. A Modine patent on density-stabilised PCMs gives examples of calcium nitrate tetrahydrate blended with lithium, potassium or sodium nitrate in which at least roughly 28–30 wt% of the PCM must be water (patent examples, not specifications).[4]
- Corrosion. Inorganic PCMs can attack metals, so container and heat-exchanger materials must be chosen carefully.[20]
- Moisture exchange. An unsealed hydrate can pick up or lose water, shifting its melting behaviour (see below).
How is supercooling fixed?
A 2022 University of Edinburgh thesis describes a “cascade nucleation” scheme in which strontium nitrate nucleates magnesium nitrate hexahydrate, which in turn nucleates calcium nitrate tetrahydrate, plus a mechanical seeding design; it reports consistent nucleation over repeated cell-scale cycles.[11] A granted patent (US11891561B2) similarly reports that strontium nitrate tetrahydrate and magnesium nitrate hexahydrate together reliably nucleate the PCM with minimal sub-cooling.[5] A broader review counts 38 nucleator and salt-hydrate combinations and 11 phase-separation mitigation techniques across salt hydrates.[12] Older cold and heat-pack patents also list calcium nitrate tetrahydrate among salts whose solutions can be supercooled and then triggered, as a background statement.[7] The evidence is mainly laboratory and patent-stage; we found no independent field data.
How hygroscopic is it, and do the hydrates change form?
Calcium nitrate is among the more hygroscopic fertilizer salts. A 2020 study from the Swedish University of Agricultural Sciences quotes a deliquescence relative humidity of 50.5% at 25 °C for calcium nitrate (versus 62.7% for ammonium nitrate), and measured laboratory-grade calcium nitrate absorbing 1.80 g of water per gram in 24 h, with complete dissolution in 4.3 h at 90–99% RH.[13] In Ca–Mg nitrate solutions the stable solid at 273.15 and 298.15 K is the tetrahydrate.[15] Related double salts exist: calcium ammonium nitrate decahydrate, 5Ca(NO3)2·NH4NO3·10H2O, shows a solid–solid transition between about 300 and 330 K.[16] Practical consequence: material exposed to air above roughly 50% RH takes up water, so for a PCM or a pack the water content must be controlled and sealed packaging is essential.
Is it safe to use in a consumer product?
Not without a dedicated hazard assessment. Supplier safety data sheets typically classify calcium nitrate as an oxidising solid that causes serious eye damage and is harmful if swallowed; check the SDS for the exact material. The comparison is instructive: poison-control guidance says ammonium nitrate is the most toxic common cold-pack ingredient, calcium ammonium nitrate moderately toxic and urea the least.[17] Calcium nitrate is not a food-contact or skin-contact material, and a leaking pack would expose users to an oxidiser. Industrial heat-storage units are sealed systems, which avoids most of this.
What should a buyer specify?
- Hydration state and water content (tetrahydrate vs anhydrous), because latent heat per kg depends on it.
- Calcium and nitrogen assay and a batch Certificate of Analysis.
- Insoluble residue and trace impurities; these affect nucleation, corrosion and cycling.
- Particle form and moisture packaging (sealed, moisture-barrier bags).
- Your own DSC cycling and corrosion tests on the exact material and container.
When calcium nitrate is NOT the right choice
- Consumer or first-aid cold packs. Published cold-pack products use ammonium nitrate, calcium ammonium nitrate or urea; calcium nitrate is oxidising and an eye-damage hazard.[3][17]
- Unmodified PCM use. The pure hydrate sub-cools by tens of kelvin and needs nucleators and cycling validation.[5][11]
- Open or humid storage. It deliquesces at roughly 50% relative humidity at 25 °C.[13]
- Unprotected metal containers without corrosion testing.[20]
- Food, medical or skin-contact products. This page does not support any such use.
What HRSU supplies — and what it does not
HRSU Indore supplies fertilizer-grade calcium nitrate powder: 18.5–19% calcium, 15.5% minimum nitrogen, 100% water-soluble with 0% insoluble residue, in 25 kg and 50 kg bags with a batch COA, for bulk and export, with TDS and SDS available. HRSU does not claim this grade is a cold-pack, PCM, food, pharmaceutical or consumer-product grade, and does not supply a defined tetrahydrate PCM material with stated latent heat. If you are researching thermal-storage or cooling formulations, share your required hydration state, assay and impurity limits, request the TDS and a sample, and validate it with your own calorimetry and corrosion tests.
Request a calcium nitrate sample and COA for your thermal trial
100% water-soluble · 0% insoluble residue · 25 kg & 50 kg bags · bulk/export available.
Buy online WhatsApp usFrequently asked questions
Is calcium nitrate used in cold packs?
What is the melting point of calcium nitrate tetrahydrate?
What is the heat of solution of ammonium nitrate versus urea?
A cold-pack patent quoting CRC data gives +25.69 kJ/mol for ammonium nitrate and +15 kJ/mol for urea.[6]
Why does calcium nitrate tetrahydrate supercool as a PCM?
Is calcium nitrate hygroscopic?
Yes. A study quotes deliquescence at 50.5% relative humidity at 25 °C, and laboratory-grade material dissolved fully within 4.3 h at 90–99% RH.[13]
Is calcium nitrate safe in a cold pack?
Supplier SDSs generally classify it as an oxidiser and serious eye-damage hazard. Even the common cold-pack salts carry ingestion risks, with ammonium nitrate the most toxic of the usual three.[17] A dedicated risk assessment is needed.
Does calcium nitrate store heat at high temperature too?
The anhydrous salt is studied for molten-salt storage; one NIST ThermoML dataset reports its enthalpy of fusion as 33.4 ± 1 kJ/mol at 823 K.[14] See the molten-salt page for that regime.
Can I use HRSU calcium nitrate for a PCM or cold pack?
HRSU supplies fertilizer-grade, 100% water-soluble powder with batch COAs but makes no PCM or cold-pack claim. Define your hydration state and impurity limits, request a sample and validate with DSC cycling.
Related guides
- Calcium nitrate in molten-salt thermal storage
- Calcium nitrate safety, handling and storage
- Uses of calcium nitrate: the complete guide
- Powder vs granular calcium nitrate
References
- Calcium nitrate — Wikipedia. Encyclopedia
- Enthalpy change of solution — Wikipedia. Encyclopedia
- Ice pack (instant cold packs) — Wikipedia. Encyclopedia
- US 6,083,418 A: Density stabilized phase change material (Modine Manufacturing Co) — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 11,891,561 B2: Metal nitrate based compositions for use as phase change materials — Patent, via Google Patents. Patent
- US 9,039,924 B2: Cooling agent for cold packs and food and beverage containers (heat-of-solution table from CRC Handbook, 90th ed.) — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 4,077,390 A: Reusable heat pack containing supercooled solution and means for activating same — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 6,099,555 A: Gelling cold pack — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 6,233,945 B1: Extended life cold pack — U.S. Patent and Trademark Office, via Google Patents. Patent
- Calorimetric study of calcium nitrate tetrahydrate and magnesium nitrate hexahydrate (Sadovska et al.) — Journal of Thermal Analysis and Calorimetry, 2016 (University of Pardubice record). Peer-reviewed
- Improved phase-change material formulations for heat-storage applications (E. Goddard) — University of Edinburgh, doctoral thesis, 2022. University thesis
- Review of stability and thermal conductivity enhancements for salt hydrates (Kumar et al.) — Journal of Energy Storage 24, 100794, 2019, via Oak Ridge National Laboratory. Peer-reviewed / national lab
- Laboratory studies on dissolution of nitrogen fertilizers by humidity and precipitation (Sigtryggsson, Hamner, Kirchmann) — Agricultural & Environmental Letters, 2020, Swedish University of Agricultural Sciences. Peer-reviewed
- Comprehensive analysis of thermodynamic properties of calcium nitrate (Sergeev et al.) — Journal of Chemical Thermodynamics 134, 2019; NIST Thermodynamics Research Center ThermoML. NIST data record
- Solubility phase diagram of the Ca(NO3)2-Mg(NO3)2-H2O system (Yin et al.) — Journal of Chemical & Engineering Data 59, 2014; NIST ThermoML. NIST data record
- Thermodynamic properties of calcium-ammonium nitrate decahydrate (Grishchenko et al.) — Journal of Chemical Thermodynamics 122, 2018; NIST ThermoML. NIST data record
- What is inside an ice pack? — National Capital Poison Center (poison.org). Poison-control guidance
- Endothermic reactions: the enthalpy of solution (ammonium nitrate cold-pack demonstration) — Purdue University chemistry demonstrations. University teaching resource
- Standard enthalpies of solution and formation at 298.15 K of anhydrous magnesium and calcium nitrates — Journal of Chemical Thermodynamics, 1976, Elsevier. Peer-reviewed
- Phase-change material — Wikipedia. Encyclopedia