Uncertainty in pH measurements
November 2, 2022
Data quality checks have revealed that grab sample pH measurements in Chemical properties of surface water (DP1.20093.001) do not always align with contemporaneous in-situ sensor pH measurements in Water quality (DP1.20288.001). In addition to the values reported by the external lab in the table swc_externalLabDataByAnalyte, the initial pH of the grab samples is also recorded by the NEON Domain support facility lab as part of the alkalinity and acid neutralization capacity measurements and reported in the table swc_domainLabData. These are typically performed within 24 hours of sample collection (max 72 hours), versus the external lab measurements which are made several days (up to a week or more) later due to shipping. While these domain measurements are almost universally better correlated with the sensor measurements than the external lab measurements are, in many sites the relationship is still very poor.
In response we are taking several actions. External lab measurements prior to July 2021 were made using unfiltered samples. Users can determine whether a raw (unfiltered) or filtered sample was analyzed from the suffix .RAW or .FIL in the sampleID field. Measurements made using unfiltered samples are in the process of being redacted. Moving forward, we are evaluating several potential changes to sampling protocol to minimize potential changes in pH during sample collection, transport and analysis resulting from temperature changes and head space equilibration.
An update will be provided by the end of May 2023 once we are able to determine what impact these changes have on the data. 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.