Re: How to measure the Acidity of an atmosphere?

Great question! It would all depend upon what definition you are using for 
acidity and what is the structure of the Venusian atmosphere. I haven't 
been able to find anything specific in the way of details - especially 
about the probes and their instrumentation, so this is going to be very 
general.

"Acidity", as we normally define it, is a measure of the pH of an acidic 
or basic compound where pH is the hegative lograrthm of the hydrogen ion 
concentration. There is two ways that this can generate very acidic 
conditions - the first is by having a strong acid (i.e. sulfuric acid) and 
the second is by having a very concentrated weak acid (i.e. a high 
concentration of phosphoric acid). Both show up as a low pH and therefore, 
very acidic.
Working on the premise that spectroscopy tells us that the Venusian 
atmosphere has high concentrations of the strong acid, sulfuric acid, it 
would seem that we are looking for both of these conditions to be 
fulfilled - and this would give "high acidity".
On the assumption that there might be traces of water still in the 
atmosphere, that would mean that we are looking for something that is 
acidic in our conventional sense of the term and we could measure it as a 
pH based on traces levels of the hydrated acid - droplets or microdroplets 
in the atmosphere.

However, as the Venusian atmosphere is likely to be quite dry, and lacking 
free moisture, something like a pH probe and micro-droplets would seem to 
be unlikely. In that case, what we are really looking for is the 
concentration of ionized sulphuric acid (H+ + HSO4-) in the atmosphere and 
this becomes a surrogate for measuring pH directly. That is, be using a 
spectrometer to determine the concentration of the bisulphate ion, you can 
effectively estimate the concentration of free H+ in the atmosphere which 
is then a measure of the acidity or pH.

That said, a different definition of acidity relates to the ability to 
cleave bonds (i.e. methane is a very weak acid) and that could be measured 
using infrared spectroscopy. In this case, a whole variety of substances 
are open to interpretation as acidic compounds even though they might not 
produce H+ or be measureable with a pH meter. For example, CO2 is acidic 
because it can combine with water - not because it is combined with water 
if you see the difference. An atmosphere with a high CO2 content is acidic 
because the carbon dioxide molecule is capable of reacting with other 
species, such as water, in an acid/base type reaction.

In any case, an IR spectrometer or a Mass Spectrometer could be used to 
determine both the species present in the atmosphere and their bond 
strengths - which would lead to an assessment of their acidity. For 
example, the acidity of methane is measured by Mass Spec. I suspect that 
this might be what they are doing on the Venusian probe but I couldn't 
find anything to confirm it. However, it is hard to imagine that any 
scientists would send a probe to explore the Venusian atmosphere without a 
Mass Spectrometer on board!

Hope this helps.