pH of Rain and Snow
Location:* New England
Date: Winter 2012-2013
If air pressure increases with temperature, how come the poles are regions of high pressure? Should they not have low pressure?
Your results are unusual.
Snow commonly has a higher pH than rainwater, as it does not absorb acidic pollutants as well as water droplets do as they fall through the air to the ground. Rainwater in your area has a pH of around 4.3, and thus snow will often have a pH around 5.0.
Your pH meter should be calibrated with two buffers, of pH 4.0 and 7.0
(two buffers that bracket the usual range of pH). Using only one buffer will not result in proper calibration of the pH meter. Distilled water is not a
good pH 7.0 buffer.
Buffers can be obtained from chemical companies such as Fisher Scientific.
David R. Cook
Atmospheric and Climate Research Program
Environmental Science Division
Argonne National Laboratory
Thanks for the question. You are correct in that the pH of rain should be lower than 6.8 due to dissolved carbon dioxide as well as sulfuric acid and nitric acid. The sulfuric acid and nitric acid cause what is considered "acid rain."
First, let us assume that there is no contamination of the pH electrode with ammonia or other bases. The distilled water you are using to calibrate the pH meter does absorb carbon dioxide from the air. This absorbed carbon dioxide causes the pH to be actually lower than 7 (neutral). So, when you make a measurement of rain or snow pH, there is not much difference because both the sample and distilled water have the same concentration of dissolved carbon dioxide. To remove the dissolved carbon dioxide in the distilled water, I suggest you boil the water and keep it covered so as to prevent carbon dioxide from dissolving in the water. Now, use this carbon dioxide-free solution to calibrate the pH meter.
I hope this helps. I can go into greater detail, if necessary, but I do not want to drop lots of math on you. Please let me know if you have any further questions.
With the amount of rain the NE has received recently, that does not surprise me. We are getting similar results, a mean of 6.3, here in New Hampshire. We can expect that to drop as the pollutants re-establish after the scub washing from Sandy.
The true difficulty with acid rain is not what the pH is as it is raining, but what it is when it collects on leaves and concentrates as it dries. Especially the repeated drizzles we have here. The acids, metal ions and particulates concentrate greatly. At pH 6.8, you are only three orders of magnitude away from 3.8! pH of 3.8 will begin to burn the edges of leaves. There is also a concentration effect on metals, particularly Fe+++, which can be devastating to a leaf.
In the classroom this is easily demonstrated by collecting rain water and checking pH. Simply cover the beaker with a towel. Let our dry Winters and ambient room heat do the work of concentrating. Monitor the pH daily, we use a sign up sheet, and plot the day vs pH. The pH will fall. If you also track the volume, you may make suggestions as to the droplet volume and local pH effect on the plant matter.
Have you considered using a buffered pH standard to calibrate against? The concern is that distilled water can be incredibly unstable in terms of pH with dissolved gasses and temperature flux.
Hoping this helps! Peter E. Hughes, Ph.D. Milford, NH
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Update: November 2011