US Long-Term Ecological Research Network


  • Orion 4Star pH/ISE Meter
  • Radiometer Model GK2401C Combination Electrode
  • Rinse bottle 3 times with small volumes of the water to be sampled. Collect pH samples in 20-ml scintillation vials with displacement caps. Fill the vial with a minimum of splashing and air entrainment. Fill to overflowing and cap, excluding all air bubbles from the sample. It is important to ensure that no atmospheric gas exchange occurs between the time of sample collection and analysis.  Collect a second pH sample in a 125-ml wide mouth bottle.  The 125-ml bottles are for air equilibrated pH determination, and may have air in them. Keep all samples cold and dark to minimize biological activity.
pH samples should be analyzed as soon as possible after collection, and must be analyzed the same day they are collected.

Warm samples to room temperature in a dark container. The container may be filled with room temperature water to speed the warming.

Fill the electrode syringe barrel with an extra sample of the lake water to be analyzed. Leave the electrode to soak in this sample at least 15 minutes. The longer the electrode is "conditioned" before pH measurement, the faster it will respond during measurement.

Bubble air through the samples in 125-ml bottles for at least 15 minutes before analysis using a vacuum pump and the gang valve assembly. The intake hose should be pulling air from outside the building. After bubbling, cap samples tightly until analysis.

Uncover the electrode filling solution hole. Remove the conditioning solution from the electrode syringe barrel using the three way valve and aspiration system.

With a clean 5-ml syringe take an aliquot of the sample to be measured and inject about 2 ml into the electrode chamber. Drain chamber. Inject another 2 ml and flush in and out of the chamber several times. Drain.

Refill syringe with another aliquot of the sample and inject 2 ml into the electrode chamber. There should be enough sample in the chamber to cover the electrode junction.

Press the MEASURE button on the pH meter.

When the electrode response is stable the 'AR' and 'mV' indicators will stop flashing and the display will hold. Record the millivolt value on the data sheet.

Continue to run 2-ml aliquots of the same sample until two consecutive millivolt readings are within 1 millivolt. Record the last millivolt value in the FINAL MV column of the data sheet.

When all samples have been analyzed, obtain mV readings for the 3 buffer solutions: pH 10.00, 7.00, and 4.00. Only one 2-ml aliquot of each buffer need be run.

Rinse the electrode chamber with Trout Lake tap water and store the electrode in Trout Lake tap water. Cover the electrode filling solution hole securely with tape. Rinse all sample bottles and syringes three times with milliQ water and leave out to dry. Turn the meter off. 

Enter the buffer and sample mV values into the LTER 'pH Calc' data form to calculate pH values from the recorded mV values.

LTER Keywords
Protocol Format
Protocol ID
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