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ELECTRICAL PHENOMENA ACCOMPANYING THE FREEZING OF AQUEOUS SOLUTIONS
In a paper published in 1950, Workman and Reynolds[^1] report the discovery of a new electrical effect that is observed during the freezing of certain dilute aqueous solutions.
The essence of this phenomenon is as follows.
If an aqueous solution containing a small amount ($10^{-3}$–$10^{-6}$ normal concentration) of certain substances is undergoing ordered, “unidirectional” solidification, i.e., if heat removal is carried out from one side and the boundary of the ice formed during freezing advances parallel to itself, then a considerable potential difference arises between the liquid and solid phases, reaching, for some solutions, tens and even hundreds of volts.
This effect is essentially associated with the process of freezing. It appears simultaneously with the onset of freezing and disappears when freezing ceases.
On melting, no potential difference is detected at the boundary.
The potential difference and electric charge that arise are, within wide limits, independent of the rate of freezing.
In the work, 43 different solutions of various concentrations were investigated. It was shown that the sign and magnitude of the potential difference, as well as the amount of electricity released during freezing, depend on the nature of the dissolved substance and on the concentration of the solution.
The table gives some of the results reported by the authors.
As is seen from the table, the greatest potential difference, reaching 230 volts, arises upon freezing of a solution containing \(3\cdot 10^{-5}\) normal concentration of \(\mathrm{NH_4OH}\).
The observed effect is manifested most strongly when ammonium or fluorine ions are present in the solution. This is explained, in the authors’ opinion, by the fact that the indicated ions are strongly electronegative, and also by the fact that the ammonium ion is isomorphous with the hydrogen ion, while the fluorine ion has a structural similarity to the hydroxyl ion.
| Dissolved substance | Concentration in fractions of normal concentration | Potential of the water relative to the ice in volts | Charge in CGSE released upon freezing \(1\ \mathrm{cm^3}\) |
|---|---|---|---|
| \(\mathrm{NaF}\) | \(20\cdot 10^{-6}\) | \(+\,21\) | 530 000 |
| \(\mathrm{NaCl}\) | \(100\cdot 10^{-6}\) | \(+\,30\) | 92 000 |
| \(\mathrm{CsF}\) | \(30\cdot 10^{-6}\) | \(+\,34\) | 440 000 |
| \(\mathrm{NH_4Cl}\) | \(70\cdot 10^{-6}\) | \(-105\) | 104 000 |
| \(\mathrm{NH_4OH}\) | \(30\cdot 10^{-6}\) | \(-232\) | 260 000 |
| \(\mathrm{NH_4NO_3}\) | \(30\cdot 10^{-6}\) | \(-185\) | 83 000 |
| \(\mathrm{Ca(OH)_2}\) | \(60\cdot 10^{-6}\) | \(+\,75\) | 12 500 |
| \(\mathrm{CaCO_3}\) | \(50\cdot 10^{-6}\) | \(+\,20\) | 9 000 |
The ice formed as a result of such ordered freezing of the solution is a semiconductor and possesses rectifying properties in the direction of freezing. In the direction perpendicular to the direction of freezing, no rectifying action was found.
The authors believe that the phenomenon they established is connected mainly with structural changes accompanying freezing. (Some authors indicate that during the change of state of water up to 15% of the hydrogen bonds are broken.) Therefore one may expect that this effect should be observed for those substances in which a strong change of bonds occurs during the transition from the liquid state to the solid state. Such a substance is, for example, salol. And indeed, for salol in the process of ordered solidification, a potential difference was observed between the liquid and solid phases of 45 volts, with the liquid phase being negatively charged relative to the solid.
The existence of an electrical effect accompanying the freezing of dilute aqueous solutions was confirmed in work \(^{2}\).
Ribeiro reports the discovery of an analogous effect, which he called “thermodielectric.” He found that if a capacitor contains the solid and liquid phases of one and the same substance, with the phase boundary parallel to the plates of the capacitor, then upon melting
or solidification of a substance, a small electric current is produced. This was observed with various waxes, sulfur, naphthalene, and ionic solutions.
Workman and Reynolds¹ attempt to apply the effect they discovered to explain the origin of electric charges in clouds that give rise to atmospheric lightning discharges. They suppose that water droplets present in a cloud, upon striking ice pellets (hailstones), partially freeze on their surface and, in accordance with the experiments described above, become negatively charged; the remaining part of the water droplet, on which a positive charge remains, rises upward. As a result, the upper part of the cloud acquires a positive charge, and the lower part a negative one.
In the same work, the possibility is indicated of applying the discovered effect for purposes of chemical analysis. Measurement of the potential difference during freezing can be used to determine impurities in amounts down to \(10^{-7} \div 10^{-8}\) gram.
V. S.
CITED LITERATURE
- E. J. Workman, S. E. Reynolds, Phys. Rev. 78, 254 (1950).
- V. J. Schaefer, Phys. Rev. 77, 721 (1950).
- J. C. Riberio, Anals. Acad. Brasilq Ciencias 22, 325 (1950).