Potassium chloride
| Potassium chloride | |
|---|---|
| Properties | |
| Chemical formula | KCl |
| Molar mass | 74.55 g/mol |
| Appearance | Colorless or white crystals |
| Density | 1.98 g/cm³ |
| Melting point | 770 °C |
| Solubility in water | About 34 g per 100 g of water at 20 °C |
| Conductivity 1 M | About 11 S/m at 25 °C |
Potassium chloride (KCl) is the salt in almost every nanopore experiment. A solution of 1 M, which is 74.55 g in a liter, conducts about 11 S/m at 25 °C, enough to carry nanoamperes through a pore a few nanometers wide, and its two ions travel at nearly the same speed. That second property is why chemists reached for it first. It makes the salt the standard filling for salt bridges and reference electrodes, and electrophysiologists and nanopore groups inherited the habit.
The two ions
The speed of an ion in a field is its mobility. At infinite dilution and 25 °C the limiting molar conductivities of the ions are 73.5 S cm2/mol for potassium and 76.3 for chloride, so chloride carries 51 percent of the current and potassium 49. Sodium is much slower, at 50.1, and lithium slower still, at 38.7.[1] The match comes from the water. Bare, potassium is the smaller ion, but it wears a coat of water, and the effective hydrated radii that Nightingale derived in 1959 are 3.31 Å for K+ and 3.32 Å for Cl−, as close as the method can tell apart.[2]
Equal speeds matter wherever two solutions of different concentration touch. When one ion outruns the other, a small voltage builds at the boundary, the liquid junction potential, and it adds error to every measurement taken across it. A bridge of agar saturated with KCl keeps the error to a few millivolts, which a bridge of sodium chloride would not.[3]
In a nanopore experiment
In a Coulter counter or nanopore the salt does three jobs. It carries the ionic current that the instrument measures. It sets the Debye length, which is 0.30 nm at 1 M and 3.0 nm at 10 mM, and with it how far the electric charge of the pore wall reaches into the liquid (see Surface charge (nanopore)). And it supplies the chloride that the Ag/AgCl electrodes need: the electrode reaction AgCl + e− ⇌ Ag + Cl− fixes the potential of the silver wire to the chloride concentration, and a silver wire in saturated KCl sits at +0.197 V against the standard hydrogen electrode.[3]
A salt concentration of 1 M is common because current, and with it the signal, scales with concentration. The 1996 experiment that detected single DNA and RNA strands in a protein pore used 1 M KCl.[4] Conductivity does not fall quite in proportion to the concentration, because ions in a dilute solution are less hindered by their neighbors: the molar conductivity of KCl at 25 °C is about 112 S cm2/mol at 1 M and about 141 at 10 mM.[5] The conductivity also rises by about 2 percent for each degree of warming, so a stated value is only as good as the stated temperature. The MR1 Nanopore Reader's simulations use 10.5 S/m, where other company materials use 11, a difference that a bath about 2 °C cooler would produce.[6]
Other salts are chosen for what they do to the molecule. Lithium chloride slows DNA in a solid-state pore, because lithium binds the strand more tightly than potassium does, and Kowalczyk and colleagues found a gain in resolution of at least tenfold.[7] Cells are potassium-rich inside, near 140 mM, which gives KCl a second claim on the nanopores that imitate their channels.
History
The mineral form of the salt is sylvite, named for the sal digestivum Sylvii or digestive salt of Sylvius, an early name for the substance.[8] Potassium itself was first isolated in 1807 by Humphry Davy, who passed a current through molten potash and described the result to the Royal Society that November.[9] Most potassium chloride mined or made today goes to fertilizer, where it is sold as muriate of potash.
At Molecular Reality
Molecular Reality Corporation has used the salt at both ends of the concentration scale. A video from its early years is titled "wet dpore EDH 1 100 mM KCl".[10] The example recording that ships with the MR1 Nanopore Reader names a 4 nm pore in silicon nitride, 1 M KCl with 10 mM Tris buffer at pH 8, and 180 mV, and the pore table of the design is computed for 1 M KCl at 200 mV.[6] The company describes its solid-state approach as one that needs no reagents beyond salt, in contrast to protein pores in thin membranes, which depend on enzymes kept cold.[6] The guide to the company's live molecular stream argues for dirtier liquids than the clean salt solutions of the literature. "A pristine salt solution is polite", it says. "It flows, it wets, it behaves."[11] Its reference sample is tap water, and ketchup is on the shelf beside it (see Condiment trials).
See also
References
- ^ "Ionic conductivity and diffusion at infinite dilution". In Haynes, W. M. (ed.). CRC Handbook of Chemistry and Physics. CRC Press. 97th ed. 2016.
- ^ Nightingale, E. R. (1959). "Phenomenological theory of ion solvation. Effective radii of hydrated ions". Journal of Physical Chemistry 63 (9): 1381-1387.
- ^ a b Bard, A. J.; Faulkner, L. R. (2001). Electrochemical Methods: Fundamentals and Applications. 2nd ed. Wiley.
- ^ Kasianowicz, J. J.; Brandin, E.; Branton, D.; Deamer, D. W. (1996). "Characterization of individual polynucleotide molecules using a membrane channel". Proceedings of the National Academy of Sciences 93 (24): 13770-13773. doi:10.1073/pnas.93.24.13770.
- ^ Pratt, K. W.; Koch, W. F.; Wu, Y. C.; Berezansky, P. A. (2001). "Molality-based primary standards of electrolytic conductivity (IUPAC Technical Report)". Pure and Applied Chemistry 73 (11): 1783-1793. doi:10.1351/pac200173111783.
- ^ a b c "Company records". Molecular Reality Corporation. October 2026.
- ^ Kowalczyk, S. W.; Wells, D. B.; Aksimentiev, A.; Dekker, C. (2012). "Slowing down DNA translocation through a nanopore in lithium chloride". Nano Letters 12 (2): 1038-1044. doi:10.1021/nl204273h.
- ^ "Sylvite". Handbook of Mineralogy. Mineralogical Society of America.
- ^ Davy, H. (1808). "The Bakerian lecture: on some new phenomena of chemical changes produced by electricity, particularly the decomposition of the fixed alkalies". Philosophical Transactions of the Royal Society 98: 1-44.
- ^ wet dpore EDH 1 100 mM KCl (Video). EpicQuestBio.
- ^ "Keltar's Field Guide to Molecular Reality". Molecular Reality Corporation. August 2026.