|MadSci Network: Chemistry|
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.
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