Science Buddies: "Ask an Expert"

For my science project I am measuring the pH of water samples from a) precipitation, b) ponds & lakes, and c) creeks & rivers. I stored all samples in plastic bottles and glass jars until the collection phase was complete. Then several weeks later, I measured the pH of all samples using a pH meter calibrated using pH 7.0 and 4.0 Buffer Solutions. To my surprise the average pH values were from 6.5 to 7.4. Where is the acidity? Could waiting several weeks have affected the pH value? The USGS shows a pH value of 4.6 for this part of Virginia. Any suggestions as to what may have affected my readings? I have completed all aspects of the project and must submit a draft report very soon.

Hi Warren,
Welcome to the Ask an Expert forums. I'm glad to hear that you are working on this important and interesting issue.
The length of time between sampling and analysis is known as the "holding time". If you run a search for "pH holding time" you'll see that pH analysis are generally recommended to be run immediately (within minutes) of sampling. For example:
http://h2o.enr.state.nc.us/lab/qa/pHpre ... 00.pdf.pdf
The pH can be affected during storage by microbial reactions, chemical reactions with solid phase particles and by gas exchange with the atmosphere (or "headspace" in the bottle).
It's very hard to say how these factors might have affected the pH of your samples. "Several weeks" does sound like a very long time to store these sorts of samples. Can you go back and collect a few fresh samples for comparison?
When you say "The USGS shows a pH value of 4.6" do you mean in the rain, or in some particular surface water? Can you give a link or citation for that information? pH values can vary considerably in natural waters with time and location, so it is possible (though I don't know if it is feasible) that you happened to sample some locations with pH ~7.
Looking forward to hearing more.
Chris

ChrisG,

Thank you very much for the information. We ( me and my dad) did search for pH holding time and related subjects as you suggested and found quite a bit of information. I am including the excessive holding time of my samples (when compared to "test immediately" and "test within 15 minutes") as the most probable reason for the non-acidic measurements I measured. My draft is due this Monday and there is no available opportunity for my parents to take me back out to the water bodies to retest the values before then. However, if time and opportunity exists within the coming weeks, my parents said they will take a trip to at least 2 or 3 of the local ponds or lakes to remeasure the pH values.

I did learn quite a bit doing this science project with my dad: discussing the problem, planning the process, researching background information, going out into the field with a bucket and rope and collecting samples (we even lowered a bucket over a bridge into The James River), measuring the pH, and cataloging the samples.

By the way, we didn't actually plan to collect all of the samples first and store them before measuring the pH, - it just happened that way. The reason was because we wanted something with more resolution and accuracy than pH paper and we ordered an inexpensive (when compared to a professional lab unit) pH meter and it was back-ordered and took quite a few weeks to arrive. So, in the meantime we started collecting samples and storing them in bottles and jars. When the meter did arrive, we just continued to collect samples not realizing the error of our process. I may not have gotten the results I expected, but at least I know how NOT to measure the pH value.

Again, thank you very much for your guidance and quick reply to my inquiry.

I'm glad to help. It sounds like the project was a great learning experience. Sometimes "mistakes" in procedures can lead to unexpected and interesting discoveries. For example, you can possibly test the hypothesis of whether the storage time resulted in elevated pH by examining whether older samples tend to have a higher pH. Do you see any trend in a graph of pH versus storage time? Also, it would be interesting to discuss what could possibly cause a rise in pH during storage. Dissolution of carbonate minerals is one possibility, but it's unclear to me whether you would have enough of that those minerals in your samples to cause a substantial change. Growth of algae is another possibility, because algae consumes CO2 in the water and reduces the concentration of carbonic acid. Were the storage conditions warm enough and well lit enough to promote growth of algae? If some samples were stored in different conditions, do you see any differences in the pH? I don't know if any of these angles will work, but they might be fun to consider.
Also, please keep in mind, the recommended holding times are useful as general guidelines but they are not set in stone. Some samples can yield useful data even when they are stored for much longer than the holding time.
Good luck with the writeup.

Chris