Measuring & Analyzing GLOBE pH data Jean Morrill, Martha Conklin, Roger Bales, Jonathon Whittier & Alexander Leonard Hydrology and Water Resources University of Arizona Tucson, AZ

Background pH is one of the most-often measured surface water fields in GLOBE Over the past 6 years, more than 900 GLOBE schools in 45 countries have taken over 24,300 pH measurements

Background pH is a popular measurement because it is fairly easy and quick to take Three different instruments — paper, pens and meters — are used by GLOBE students

Motivation Some schools noticed large fluctuations in pH when they switched instruments

Questions Does instrument choice affect the pH measurement? How many schools use which instruments? Where are they located? What characteristics of the water body (conductivity, pH range, temperature) affect pH measurements ? What does this mean for data analysis?

GLOBE sites in this study In this study, we only looked at 201 sites –with at least 30 pH measurements (as of May 31, 2001) –24 countries –freshwater

Countries Collecting GLOBE pH data

pH averages at 201 GLOBE sites All Count22,104 Mean7.3 Low3 High12

pH averages at 201 GLOBE sites All pH meter Count22,1049,557 Mean Low33 High12

pH averages at 201 GLOBE sites All pH meter pH pen Count22,1049,5573,619 Mean Low332 High12

pH averages at 201 GLOBE sites All pH meter pH pen pH paper Count22,1049,5573,6195,639 Mean Low High12 9.5

pH averages at 201 GLOBE sites All pH meter pH pen pH paper unspecified Count22,1049,5573,6195,6393,289 Mean Low High

Methods used at the 201 Sites Percent of sites using each combination of methods

Most commonly used pH instrument pH paper pH pen pH meter

Charleston Lake, Arkansas, U.S. Low conductivity site

Opatowicki Weir, Wroclaw, Poland XI Liceum St. Konarskiego

pH example pH = N-1pH = NpH = N+1 An increase in pH of 2 increases the H + ion concentration 100 times.

What affects instrument readings of pH? Salinity / electrical conductivity Temperature difference between calibration buffers & samples in some instruments (pH pens) Instrument precision, instrument error, instrument drift User error – such as not letting the readings stabilize

Laboratory Tests Test 1: Examine the sensitivity of the GLOBE pH instruments to electrical conductivity. Method: Add NaCl incrementally to produce eleven solutions, with conductivities ranging from 3 S/cm (purified water) to 44,000 S/cm (sea water).

pH paper

Original pH pens Only one calibration point No temperature compensation New pH pens Two calibration points No temperature compensation pH pens

pH meters Multiple calibration points Automatic temperature compensation

pH Electrical Conductivity (  S/cm)

pH Electrical Conductivity (  S/cm)

pH Electrical Conductivity (  S/cm)

pH Electrical Conductivity (  S/cm)

Conclusions - 1 There is a range of conductivities ( S/cm) over which pH pens & pH meters give accurate pH readings. pH pens and meters are not accurate at conductivities less than 100 S/cm. Therefore, reporting water conductivity along with pH measurements is very important for those using the data! Useful for judging data accuracy & uncertainty.

Laboratory Tests Test 2: Compared pH instrument measurements of –Tap water pH near 8 low EC 360 S/cm –Salt water (0.5 M NaCl) pH near 6.5 high EC 44,000 S/cm –Sodium bicarbonate solution (10 g NaHCO 3 / 500 mL water) --- high pH solution pH near 8 high EC almost 15,500 S/cm

Salt Water Tap Water Bicarbonate Solution

Conclusions - 2 In waters of moderate pH ( ) & conductivity ( S/cm), the papers consistently read 0.5–1.5 pH lower than the pens & meters. In waters of high pH and high conductivity (> 8 and S/cm), all tested pH papers gave higher values than the pH pen & pH meters.

Conclusions – Data Analysis Most reliable data series are taken with one type of instrument. Do not recommend comparing data taken with different instruments (including different papers). Must assign higher uncertainty when comparing data taken with different methods. May miss or misinterpret trends.

Plea for Data We need schools to report: –TDS measurements –Manufacturer and model # of pH equipment – and we need to know whenever a new instrument is used Please do not continue to use the 1- calibration point pH pen