Update: Uncertainty in pH measurements

It was previously reported that grab sample pH measurements in Chemical properties of surface water (DP1.20093.001) did not always align with contemporaneous in-situ sensor pH measurements in Water quality (DP1.20288.001). In response, NEON has taken several corrective actions.

First, NEON continues to recommend avoiding use of the pH measurements made by the external lab in the table swc_externalLabDataByAnalyte because of the length of time between sample collection and analysis. Instead, we recommend using either the in-situ pH measurements included as part of the Water quality data product, or the measurements made in the NEON Domain support facilities, which are measured within 24 hours of collection and reported in the table swc_domainLabData. We have updated the sampling protocol and trained field staff to limit degassing or equilibration with the atmosphere during collection and transport, and to allow samples sufficient time to return to room temperature before analysis. We have instituted a system for tracking the age of pH probes, purchased replacements, and stopped using the same probes for measuring soil pH to prevent possible contamination. We have also instituted more frequent checks against standards. Finally, we have added field measurements made at the time of sample collection using a handheld pH meter, which are reported in the table swc_fieldSuperParent.     

Over the six months since implementing these changes, we saw a significant improvement in the correlation between Domain pH measurements and sensor pH measurements when pooled across the Observatory. There were still varying degrees of correlation within sites, but this is likely because of the limited range of pH variation within a particular site, which is often of similar magnitude to the measurement uncertainties.

For more detailed information on the problem please Contact Us. Select DP1.20093.001 in the Data Product dropdown list and include the term “PRB0041000” in the body of your request.