TM94-02 NWQL Policy on pH Analytical Determinations

National Water Quality Laboratory (NWQL) Technical Memorandum 93-07 presented a brief history of pH determinations at the NWQL and asked for input to determine if the current procedures are meeting user needs. This memorandum presents a new policy statement for pH determinations based on user responses and technical discussions within the NWQL. The method for pH determinations (Lab Code 68) at the NWQL is Method I-2587-85 from the Techniques for Water Resources Investigations (TWRI) Book 5, Chapter A-1. This method specifies an automated system, applicable only for samples which had a specific conductance exceeding 70 microsiemens per centimeter. For low-conductivity samples, the automated system may not provide reliable pH results because of slow electrode response. The pH for many of these low conductivity samples was determined using a manual variation incorporating longer equilibration times. Alternatively, low-conductivity samples can be submitted for a pH determination by Lab Code 1268. Therefore, manual variations to the method will no longer be 
conducted on samples submitted for pH determinations by Lab Code 68. Projects needing more reliable pH for low-conductivity waters should use Lab Code 1268 performed in the Low Ionic Strength Unit. This is a more laborious and more costly procedure. Keep in mind, even this more precise measurement may be inaccurate when compared to field measurements of pH due to changes in the sample. For all laboratory determinations of pH, whether by Lab Codes 68 or 1268, NWQL will include the remark code of "E" (estimated). This acknowledges that laboratory measurements of pH, no matter how reproducible, may not accurately reflect the pH of a sample at the time of collection. Finally, because pH is not a stable parameter and because NWQL results are only estimates, we will no longer accept normal rerun requests for pH.