  When plotted in linear transformations of the Michaelis-Menten equation, ion
uptake in plants has been shown to be multiphasic, i.e. to be represented by a
series of straight lines separated by discontinuous transitions (Nissen 1971,
1974, 1991, 1996). Reanalysis of data for other transport, binding and enzyme
systems has also given such kinetics (Nissen and Mart\'in-Nieto 1998).
Recently, such profiles have been found for a variety of biological as well as
non-biological processes and phenomena, including activation of ion channels,
binding, pH, folding/unfolding, effects of various interactions and chain
length (Nissen 2015a,b, Nissen 2016a-d). These biological and nonbiological
processes have little in common beyond the ions involved. Here I show that the
multiphasic property seen in so many systems is present in the fundamental
physical properties of the ion themselves.
  The characteristics of multiphasic profiles have now also been found in
profiles for activities of ions in simple inorganic solutions, indicating that
they are somehow causing the multiphasic profiles in more complex systems. A
set of extensive and precise data (Lee et al. 2002) for the activities of the
biologically very important ions Na+, K+ and Cl- (in NaCl and KCl) at four
different temperatures (15, 25, 35 and 45oC) will be reanalyzed in the present
paper. The activities of NaBr and KBr will be plotted in a forthcoming paper.
Extensive data for these and other ion activities Wilczek-Vera et al. (2004)
can also be precisely represented by multiphasic profiles (in preparation).
