posted 08-16-2001 09:08 PM            

Effect of Methodology and Elapsed Time on pH Value Obtained for a Sample of Rainwater

Authors: 3T3L1 and toxdoc

Abstract:
The pH of sample of fresh rainwater was measured using three methods: (A) using a calibrated pH meter, (B) using pH paper, and (C) using an aquarium pH indicator solution. The pH paper and the aquarium pH indicator solution were rated for pH levels above that expected for fresh rainwater. The calibrated pH meter gave a value of 4.3 pH units, which corresponds exactly with the current published value for the same locality, 4.38 ± 0.08 (Source: National Atmospheric Deposition Bureau--NADP). The pH obtained by pH paper and by a pH indicator solution was limited in every case to the lowest sensitivity of the method used, and was incorrect by as much as 3.2 pH units.

It is therefore inadvisable to measure the pH of fresh rainwater using pH paper or a pH indicator solution outside its suggested range of accuracy. It is especially inadvisable to use such readings to infer that the pH of fresh rainwater in the United States may be considerably higher than that reported by the NADP.

Introduction:
One of the important characteristics of rainfall is its hydrogen ion concentration or pH. By definition, pH is the negative log of the hydrogen ion concentration, with a pH of 1 representing a 0.1 molar concentration of hydrogen ions (very acidic), and a pH of 14 representing a concentration of 0.00000000000001molar hydrogen ions (very basic). Distilled water has a pH of 7.0. (For a quick review of pH, see http://web.jjay.cuny.edu/~acarpi/NSC/7-ph.htm )

Because of the phenomenon of acid rain, the pH of rainfall has become important to environmentalists. More recently, the possibility that salts of barium are being sprayed into the atmosphere has raised the possibility that rainfall is becoming more basic.

The National Atmospheric Deposition Program (NADP) measures the pH of rainfall at approximately 282 sites within the United States and its territories. The first map shows the annual average pH of fresh rainfall in the continental United States in 1994. The second map, also from NADP, shows the same data for 1999. Comparison of values between individual sites on the two maps indicates some variation, but no particular trend toward a higher or a lower pH over the 5 year interval.

Source: National Atmospheric Deposition Program (NRSP-3)/National Trends Network. (2000). NADP Program Office, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820.
http://nadp.sws.uiuc.edu/isopleths/

The NADP obtains its pH values using electronic pH meters which are calibrated daily to buffer solutions of known pH ( http://nadp.sws.uiuc.edu/lib/opman.pdf ). Typical pH meters are accurate to ±0.02 pH unit and cost several hundreds of dollars. It is possible to obtain estimates of the pH of rainwater using commercially available pH paper and indicator solutions. In both cases, when a sample is measured, the resultant color of the paper or the solution is compared with a standard color strip, and the pH is estimated within ±0.2 or 0.3 pH units. Only a limited number of colored compounds can be used together in a particular indicator. Thus, indicator paper and indicator solutions are sold for specifically defined pH ranges.

Materials and Methods
On June 21, 2001 from 9:00 AM until 12:00 noon EST, approximately 40 ml of rainwater was collected in Butler County, Ohio. Rainwater was collected in sterile 50 ml disposable centrifuge tubes, with a Nalgene funnel fitted to each tube. The pooled sample was then brought into the lab and the temperature was measured (24°C). Aliquots were made of the sample for pH determination by each of six methods. pH determination was made by each method at 15 minutes, 2 hours, 4 hours and 24 hours after the sample collection was completed.
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Test method 1: Mettler-Toledo MP220 pH meter, standardized between pH 4.0 and 7.0 with buffer solutions calibrated according to the National Institute of Standards.

Test method 2: Mettler-Toledo MP220 pH meter, standardized between pH 7.0 and 10.0 with buffer solutions calibrated according to the National Institute of Standards.
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Test method 3: Baxter/SP pH Strips, pH range 5.1-7.2
Test method 4: Baxter/SP pH Strips, pH range 6.5-10.0
Test method 5: Baxter/SP pH Strips, pH range 7.5-9.8
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Test method 6: Freshwater Deluxe pH Test kit. pH range 6.0 -7.6, purchased at PetSmart
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Results:
As shown in the table below, and illustrated in its accompanying figure, the pH measured with the pH meter calibrated between 7.0 and 10.0 showed pH values between 4.8 and 5.1. These readings indicated that most accurate results would be obtained when the pH meter was calibrated between 4.0 and 7.0. When the pH meter was calibrated in the lower range, the value obtained was pH 4.3 for fresh rainwater and trended upward as dissolved CO2 was lost. The value at 24 hours was pH 4.6. The average annual pH of fresh rainwater for 1996-2000 in Butler County, Ohio and Washington County, Kentucky was pH 4.38 ± 0.08. (Source: National Atmospheric Deposition Program (NRSP-3)/National Trends Network. (2000). NADP Program Office, Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820.)

When the rainwater was tested with pH paper calibrated between 5.1 and 7.2, the pH obtained was that of the lowest pH color available, or pH 5.1. When each of the two pH papers calibrated for higher ranges were used, the pH obtained was the lowest color available in that range, i.e., pH 6.5 for paper in the range of 6.5-10.0 and pH 7.5 for paper in the range of 7.5-9.8.

Similarly, when the Freshwater Deluxe pH Test kit pH range 6.0-7.6 was used to test the pH of the rainwater, the pH obtained was that of the lowest pH color available, pH 6.0.

Conclusions:
The NADP reported that the annual average pH of fresh rainfall in this locality was pH 4.38 ± 0.08 in June of 2001. This is entirely consistent with the pH of 4.3 obtained in this study for fresh rainwater measured using a pH meter calibrated between 4.0 and 7.0. Values given by pH paper were incorrect by 0.8 to 3.2 pH units, and the value given by the aquarium pH indicator was incorrect by 1.7 pH units.

In this study, the pH of fresh rainwater was observed to increase over 24 hours when measured using the pH meter. By contrast, the pH obtained using indicators did not change over time, even though the final pH approached the range which could be measured by one of the indicator papers.

In light of these results, it is apparent that an investigator cannot obtain reliable values for the pH of fresh rainfall by using pH paper or an aquarium pH indicator solution which is not calibrated for the expected pH range of the samples to be measured. The inherent inaccuracy of pH measurement with such indicator paper and indicator solutions also makes it impossible to compare rising or falling trends in the pH of fresh rainwater.

We therefore conclude that it is inappropriate to compare pH values obtained by a pH meter with values obtained by pH paper or aquarium pH indicator solutions used outside their specified pH ranges.

posted 08-16-2001 10:11 PM            

1. because Homestead.com is showing signs of instability, and I would prefer not to lose the results of a study performed by toxdoc and written up by me.

2. because Clifford Carnicom suggests that it is valid to measure the pH of fresh rainwater using aquarium pH test kits, which are typically accurate in the range of pH 6.0 to 7.6.

so use a variety of paper ranges of high quality and fresh vintage and you can get very close to the proper value
I agree, Mark. I don't know why Mr. Carnicom recommends using aquarium pH test kits rather than pH paper of the appropriate range.

posted 08-26-2003 08:59 PM            

Here are some hints on how to collect samples

http://www.meteorology.org.hk/acid_guide.htm