Yup.
To get all anal-technical about it, the solvent can affect the measured pH. Most of the stuff we are used to are measured in water, so ammonia in water, H2S in water, etc.
Another common solvent used to measure the pKa's and pKb's is DMSO.
To get really anal about it, it is probably easiest to use the pka's and pkb's. pKa measures at above what pH is necessary to rip a proton off, and pKb measures at below what pH is necessary to stick a proton on. (It's a log scale, so at the pH things will be at 50% protonated/50%deprotonated. At 1 log unit below, you get about 10%/90%, and at 2 pH units below/above you get 1%/99%.)
[Medicinal chemists gotta worry all the time about what state molecules are in to get absorbed in the stomach or duodenem, or transported in the bloodstream (physiological pH 7.4).]
Here's a real example about pH: Your eyeball is at about pH 7.2-7.4. You can tolerate eyedrop formulations down to about pH 5, but anything below 6.5 will start to sting. On the basic side, if you get around pH 8 or pH 9, the basic conditions can actually cause hydrolysis of key stuff in the eye and can cause permanent damage - not good. Most eyedrop therapeutics avoid basic formulations and shoot for acidic formulations if they can't use a pH "neutral" formulation.
Depending on the biochemistry of your alien (especially the skin or other sensitive membranes), an different solvent or a different pH could cause damage. [A salt-dude visiting from the surface of Titan might not be too happy in his first terrestrial rainstorm.]
-Mike
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