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Name: Elizabeth
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Question:
My students tested the pH of snow the other day and discovered that the pH was about 7.8. Since the pH of "normal" rain is 5.6 I was expecting something similar or lower. Could someone please explain why our tests came out the way that it did? I have been researching for days and have not found one article about this.


Replies:
Elizabeth,

What you have found is not unusual.

When you say that the pH of normal rain is about 5.6, you are speaking of rain that has fallen in a more pristine area, such as the northern Rocky Mountains.

Rain that falls in most of the eastern part of the USA, such as in VA, normally has a pH of about 4.4.

Snow in the eastern USA generally has a pH of somewhere between 4.6 and 6.7 (this is also the common range in Illinois, where I live).

Rain drops tend to form on different, and more acidic nucleating particles (pollutants such as sulfates and nitrates) and tends to more efficiently scavenge pollutants, than snow does. Furthermore, snow often forms on smaller nucleating particles than rain does and so when the snow is melted to test for pH, the nucleating particle (assuming that it dissolves) contributes less to acidifying the water than usually happens with larger particles that rain tends to form on. Snow can also form on non-hygroscopic particles (such as soil, that do not easily dissolve in water), which do not contribute much to acidifying the melted snow. Therefore snow usually has a higher pH than rain.

The pH of snow can vary greatly (4.6 to 6.7 normally) depending on where the air came from that it formed in. If snow forms in relatively clean, unpolluted air during a cold arctic or polar outbreak coming down from Canada, the particulates on which the snow formed are likely to contain little for acidifying pollutant particulates, resulting in a higher pH than would result from snow formed in a cold front that has originated in the southern part or the plains of the USA.

A snow pH of 7.8 is unusually high, but is possible in very clean air. The highest snow pH that I remember measuring at our station here, west of Chicago, was 7.21, so you have bested that by quite a bit.

David R. Cook
Meteorologist
Argonne National Laboratory


Here is my hypothesis (actually I dislike that word because it is associated with the "scientific method", which seldom happens as presented in science texts). Here is what is going on, I think.

When snow (solid ice crystals) form from liquid water (or vapor) the dissolved gases are excluded from the solid phase. This includes CO2 -- an acidic gas. So, deprived of CO2, the melted snow has a higher pH than "normal" rain, which retains CO2, and hence has a lower pH. If you let the melted snow "hang around" for a while, CO2 will redissolve and the pH will drop. The complex crystal structure of ice tends to exclude both most soluble salts and gases. The exception is the formation of clathrates of molecules such as methane. Here water forms a "cage" around each methane molecule forming "flammable ice".

You also may notice that ice cubes from your freezer is cloudy and contains pockets of gases. This is because the atmospheric components as well as dissolved ions are "trapped" in the ice tray. In contrast, ice in your beverage at a restaurant is "crystal" clear. The reason for this is that commercial icemakers are designed so that dissolved gases and salts are washed away as the water freezes on the cooling cylinder.

Back to your results: The pH of 7.8 may be partially due to experimental error unless the pH meter was carefully calibrated, and appropriate temperature adjustments were made. But the big jump from pH from 5.61 is almost certainly due to the exclusion of CO2 from the snow flakes.

Vince Calder


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