Measuring the pH of air

A. Matthew,
As Ted said below and a few years back, pH is defined as the H+ and OH- ion concentration in solution (i.e. dissolved in a liquid). Therefore, the idea of the "pH of air" is meaningless.

HOWEVER, we do have to be aware that contents of real world air is not 100% gas. Of course there is numerous small particulate matter floating around, and some great portion of that are various chemicals released from the exhaust of various vehicles, factories, etc. (Though the bulk of combustion products are water vapor, carbon dioxide, and particulate hydrocarbons, of course impurities in the fuel result in smaller amounts of other things releasing into the air.) And lest we forget, natural sources exist as well, such as volcanic activity pumping huge volumes of noxious material into the atmosphere. While floating around in the air, these chemicals still do not have a pH, but if they drift onto a moist surface or get caught up in falling rain, now you have a solution, and the potential for some non-neutral pH to occur, which may cause damage over time to sensitive surfaces and so on. This would certainly include the moist airways of animals and humans.

Therefore I suspect most people discussing the "pH of air" are actually referring to this potential of a non-neutral pH to develop on SURFACES that are exposed to said air.

Also, a quick web search on the topic reminded me that pure water exposed to air ALWAYS becomes slightly acidic due to ambient carbon dioxide naturally entering into solution, where it hydrates (joins with water molecules) thus becoming carbonic acid. However, no serious chemist would call carbon dioxide gas "acidic" as is has no hydrogen atoms. (I would expect something similar could be said of nitrogen oxides and sulfur oxides, which when hydrated would result in nitric and sulfuric acid.)

A "test device" for any of this would be relatively difficult. The paper mentioned by John Mayor below no doubt involves the air pollutants being collected over a long period of time. I suppose one could, at best, collect material off an air filter and testing that for acid or alkaline forming compounds. (Quite possibly as simple as letting the filter soak in distilled water for a while, then checking the pH of that water. However, it would have to be a clean-room level filter to collect the kind of acid-forming chemicals we are after.) The idea of sticking a paper strip or a meter into the air and getting an instant pH reading though, is not feasible.

A. Hi, Mr. Arain.

I'm not sure that "air" per se really has a pH because I don't think there are hydronium ions in dry air, and pH is defined as the negative log of the hydronium ion concentration. If you are talking about the exhaust air from a wet process that creates droplets, the tiny liquid droplets can have a pH, but I think they would probably be the same pH as the wet process tank. Can you clarify your situation for us a bit, so we have some idea whether you are studying an industrial process in a plant, or acid rain, or humidified air in a building or whatever? Thanks!

A. There has to be a change in the acidity of air when other gases such as ammonia are dissolved in it. Perhaps this is a way to determine the health risks of pollution!

Dr. Smith

A. It is possible to test air pH. Museums need to keep an eye on the pH of their air.

See this paper:

sciencelinks.jp/j-east/article/200005/000020000599A0753344.php

Air Pollutants Trapped in a "pH Test Paper for Indoor Air/Henshoku Shiken-shi".
Accession number;99A0753344
Title;Air Pollutants Trapped in a "pH Test Paper for Indoor Air/Henshoku Shiken-shi".
Author; SANO CHIE (Tokyo Natl. Res. Inst. of Cult. Prop.)
Journal Title;Science for Conservation
Journal Code:Y0449A
ISSN:0287-0606
VOL.;NO.38;PAGE.15-22(1999)
Figure&Table&Reference;FIG.6, TBL.4, REF.4
Pub. Country;Japan
Language;Japanese
Abstract: Indoor air pollution has become a menacing problem in many museums, but the cost for analyzing the air pollutants is so expensive that all museums cannot afford it. To evaluate the acidity of indoor air in museums, a specific test by a "pH Test Paper for Indoor Air/Henshoku Shiken-shi" is used in Japan, which has been used over twenty years. A "pH Test Paper for Indoor Air" is made by dipping in a glycerin/ethanol mixed solution dissolving four chromatophores: chlorophenol red, bromothymol blue, phenol red, and bromocresol green. The making procedure was also given in this paper. The concentration of inorganic ions by ion chromatography and of carboxylic acids by liquid chromatography were done to know the concentration of air pollutants trapped in a test paper. Ammonium ion was observed in every sample and its amount changed as the color of the pH test paper showing the "alkalinity" of an indoor air of a museum. Formic acid and acetic acid were observed only in the newly built museum. Author suggests that the usage of a "pH Test Paper for Indoor Air" is still useful, and also shows the possibility to estimate the quantity level of indoor air pollutants when measuring the quantity of trapped pollutants in a test paper. (author abst.)

Ed. note: Thanks for a great response, John; and Dr. Smith's reference to ammonia sounds important as well.

A. Acid may exist as a fume or a mist. Either one can be measured down to the ppm level. I can't give you a brand name due to editorial policy ( huh? why?), but there are simple, manually operated air pumps and tubes that will collect and quantify the concentration of acid.

You should be aware that there is also an OSHA PEL for acid fumes and mist.