Or, Pa, is Element 91, Protactinium.
http://en.wikipedia.org/wiki/Protactinium
http://chemistry.about.com/od/elementfacts/a/protactinium.htm
column is
http://www.straightdope.com/columns/read/939/in-chemistry-what-do-the-letters-pa-stand-for
only the first letter of an elemental symbol is upper case.
As Cecil points out, from the context of the question, clearly the asker is referring to pH.
I thought maybe they were checking the pressure in Pascals (Pa), but just as with the element, both letters of the question were capitalized
The original letter writer appears to be relating what he heard, so his sense of capitalization isn’t really relevant – the amateur scientist he’s referring to may have meant “PA”, “Pa”, “pA” or even “pee-aye”, but since the letter writer doesn’t know what the scientist was referring to, he just wrote “PA” in his letter to Cecil.
Though I think Cecil’s belief that he misheard and the amateur scientist was saying “pH” makes the most sense.
After some thought, I think there’s some misinterpretation going on and its obvious that the question relates in some way to drug manufacturing, as the second response picks up on. However, that response refers to lead acetate, and Cecil rightly points out that “PA” makes no sense for lead acetate.
Allow me to refer the curious to a substance known as Phenylacetic acid, known as “PAA”, an organic compound used to make phenylacetone, which is used to make methamphetamine.
Given the context of the question that refers to “amateur chemists” (wink wink nudge nudge) and " I’m in jail awaiting transfer to the Texas Department of Corrections", I think a connection to drug manufacturing is likely. Breaking Bad is all the fad these days, so the question is timely. And its not beyond thought that the questioner, being of criminal mind and perhaps not being the sharpest knife in the drawer (he got caught, didn’t he?) misheard “PAA” as “PA”. Or maybe his amateur chemist friends like to take shortcuts and drop the last A.
I’m not one of Cecil’s assistants, so I can use Wikipedia:
Enjoy 
And the amateur chemist certainly wasn’t referring to protactinium.
I’m no kind of chemist, but it’s possible that both Slither and Cecil were right, in a sense. Since PAA is an acid, one could imagine ‘checking’ it with any of the standard methods of determining pH.
Checking the pH of the reaction mixture would be a really lousy way of monitoring the degree of conversion in this case (synthesis of phenylacetone from phenylacetic acid), as acetic acid / acetate ion would also be present, presumably in great excess.
Also, p.a. stands for pro analysi, the highest purity grade normally available for commercial chemicals (above puriss. and purum). Not the stuff I’d expect meth cooks to use (well, actually, I suppose meth cooks would use whatever grade they could come by).
Quick google search on lead acetate to verify this responders plea of trust leads to the Lead acetate page of wikipedia which shows two forms of Lead acetate, one “nicknamed” plumbic acetate, which could clearly be abbreviated PA. Whether this is correctly done in chemistry I don’t know but we’re talking about “amateur chemists” who may have little or no formal education. I’m not going to bother getting into how much people like to shorten things in general.
what da hell cooking meth from phenylacetic acid :eek:… is plastic synthesis, can’t use for crack’in or etc… coz it’s not soluble in water and even thou it’s still hard plastic , that means not for smoke or inject. i think it’s “just for replica only”