TL;DR: for the documents written in English use $\mathrm{p}K_\mathrm{a}$ (upright lowercase "a").
Current (2017Q3) chemmacros' documentation (p. 9) includes a reference for how acid-base module handles \pKa[...]:
\pKa: $\mathrm{p}K_\mathrm{a}$, \pKa[1]: $\mathrm{p}K_\mathrm{a1}$, depends on language settings, see section 6.5 starting on page 30. The translations can be adapted. [...]
As you can see the default subscripts of \Kw, \Ka and \Kb are lowercase letters. The literature is inconclusive about if this is the right way or if uppercase letters should be preferred. In textbooks the uppercase variant usually seems to be used while journals seem to prefer the lowercase variant. chemmacros’ default follows the usage in The IUPAC Green Book [Coh+08].
From IUPAC Green Book [1, p. 59]:
The equilibrium constant of dissolution of an electrolyte (describing the equilibrium between excess solid phase and solvated ions) is often called a solubility product, denoted $K_\mathrm{sol}$ or $K_\mathrm{s}$ (or $K_\mathrm{sol}^⦵$ or $K_\mathrm{s}^⦵$ as appropriate). In a similar way the equilibrium constant for an acid dissociation is often written $K_\mathrm{a}$, for base hydrolysis $K_\mathrm{b}$, and for water dissociation $K_\mathrm{w}$.
Additionally, "a" should be typed as an upright letter [1, p. 5]:
Subscripts and superscripts that are themselves symbols for physical quantities or for numbers should be printed in italic type; other subscripts and
superscripts should be printed in Roman (upright) type.
References
- IUPAC “Green Book” Quantities, Units, and Symbols in Physical Chemistry, 3rd ed.; Cohen R. E., Ed.; IUPAC Recommendations; RSC Pub: Cambridge, UK, 2007. ISBN 978-0-85404-433-7.
