Freezing Potentials in the System H2O–NH3–CO2at Controlled Concentrations

Abstract

Ammonium hydroxide solutions were frozen in a CO₂-free atmosphere and freezing potentials were measured. Carbonate-free solutions in the range 5 × 10⁻⁶–5 × 10⁻³ M did not develop appreciable freezing potentials, and neither did solutions with NH₃ concentrations ≥4 × 10⁻⁴ M that had absorbed CO₂ from air. However, if these exposed solutions were diluted, potentials of up to 100 V were observed, depending on the dilution factor and CO₂ content. An alternative method of achieving somewhat lower freezing potentials of up to 50 V consisted in reducing the pH with a few drops of relatively concentrated HCl or HF. In the latter case, a reversal of the potential sign was observed when the fluoride concentration was in excess of the dissolved CO₂ content. In the system H₂O-NH₃-CO₂ charge separation is influenced by both the hydrogen-ion and overall solute concentrations. The highest potentials occur only when CO₂ concentration equals or exceeds that of NH₃.