Hydration number of alkali metal ions determined by insertion in a conducting polymer.

Steen Skaarup

Research output: Chapter in Book/Report/Conference proceedingConference abstract in proceedingsResearchpeer-review


In aqueous solutions, the alkali metals ions are associated with a number of H2O molecules. A distinction is made between a primary solvent shell, (or inner solvation shell), consisting of H2O molecules directly coordinated to the metal ion, and a secondary (or outer) solvation shell, consisting of all other water molecules whose properties are still influenced significantly by the cation. Knowing the hydration number is important when considering, for instance, the transport of Na+ and K+ in biological cell membranes, since their different behavior may depend on the details of ion hydration. The solvation of alkali metal ions has been discussed for many years without a clear consensus. This work presents a systematic study of the hydration numbers of the 5 alkali metal ions, using the electrochemical insertion of the ions in a conducting polymer (polypyrrole containing the large immobile anion DBS-). The technique of Electrochemical Quartz Crystal Microbalance (EQCM) has been used to simultaneously determine the mass entering a film of PPy(DBS), and the charge during the first reduction. The method determines the total mass of metal ions and H2O entering the film quite accurately. The charge inserted allows direct calculation of the number of M+ ions entering the film, and therefore the inserted M+ mass. The mass of the water molecules is calculated as a difference. The results yield the following primary hydration numbers: Li+: 5.5-5.6; Na+: 4.0-4.1; K+: 2.0-2.5; Rb+: 0.6-1.2; Cs+: ~0. The most important result is the clear distinction between all the 5 cations, including a clear difference between Na+ and K+. The method is attractive because of the relative simplicity of interpretation – it may also be akin to transport in a cell membrane.
Original languageEnglish
Title of host publicationBook of Abstracts ICSM 2008
Number of pages56
Publication date2008
Publication statusPublished - 2008
EventInternational Conference on Science and Technology of Synthetic Metals 2008 - Pernambuco, Brazil
Duration: 1 Jan 2008 → …


ConferenceInternational Conference on Science and Technology of Synthetic Metals 2008
CityPernambuco, Brazil
Period01/01/2008 → …


  • Hydration number
  • Solvation number


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