This element was discovered by H. Davy in London in 1807. The name comes from the English word potash (pot ashes); in Latin the element is called kalium.

Ionization energies
KI 4.3 eV, KII 31.6 eV, KIII 50.3 eV, KIV 67.0 eV, KV 90.9 eV.

Absorption lines of KI

Table 1. Equivalent widths of KI 7699(1)

Group V III Ia

G2 0.15   0.18
S 0.154    
K2   0.21  
M2     0.45

The KI resonance lines are 7664 and 7699(1), but 7664 falls near a strong atmospheric line of O2 and is therefore difficult to observe. The 7699 line shows a complicated luminosity effect, in the sense that it is strong in dwarfs and supergiants and weaker in giants (Keenan 1957). In subdwarfs KI is even stronger than in dwarfs. The interpretation of the observations is complicated because the lines lie in the region of TiO bands. If TiO bands are missing - for instance in C stars - the K lines become apparently much stronger. The same happens if the TiO bands are weaker, as occurs in supergiants (because for equal spectral type they have a higher temperature than dwarfs). A further complication of the interpretation resides in the fact that, in high-luminosity objects, part of the K I resonance lines may be due to interstellar or circumstellar absorption. For the latter case see for instance Sanner (1976), who also provides line profiles of the resonance lines for several stars.

The K lines are also useful as a classification criterion for very late (M 8-9) type stars (Solf 1978, Kirkpatrick et al. 1991). Kirkpatrick et al. (1993) have used also the lines at 1.169, 1.177, 1.243 and 1.2521 µm (M.5 and 6).

Emission lines of KI
KI lines (M.1) were seen in emission in one S-type Mira variable (Bretz 1966, Wallerstein 1992) and those from multiplet 3 by Deutsch and Merrill (1959).

Forbidden lines
Forbidden lines of KIV and KV were observed in one recurrent nova Joy and Swings 1945).

Two stable isotopes exist, namely K39 and K41, plus one long-lived isotope, K40, with a half life of 1.3 × 109 years. These three isotopes occur in the solar system with frequencies 93%, 7% and 0.01% respectively. There also exist seven short-lived isotopes and isomers. K40 could be used for radioactive dating.

K 39, 40 and 41 are all produced by explosive nucleosynthesis, but K40 can also be produced by the s process and by Ne burning.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.