Should the 'a' in the acid dissociation constant (Ka or pKa) be capitalised? In science, the negative logarithm of the acid dissociation constant is denoted $\mathrm{p}K_\mathrm{a}$ or $\mathrm{p}K_\mathrm{A}$ depending on the source (lowercase "a" or uppercase "A"). Since it is related to the acid dissociation constant, defined as

$$K_\mathrm{A} = \frac{[\ce{A-}][\ce{H3O+}]}{[\ce{AH}]}$$

I would write $\mathrm{p}K_\mathrm{A}$ (the LaTeX package chemmacros renders it so, but doesn't give any source it in its documentation). It seems to me that $\mathrm{p}K_\mathrm{a}$ is the "modern" version of pKa, which was written like this in the last century because of the lack of subscript character on typesetting machines. 

I cannot find any source or styling guide saying explicitly which one should be used today (like the IUPAC Green Book), and why.
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.