Frequent Questions

Can you provide more definitive guidance regarding the interpretation of “as soon as possible” in SW-846 Methods 9040 and 9045?

Methods 9040 and 9045 for pH both say that samples should be analyzed as soon as possible. 40 CFR 136 Table II defines the maximum holding time for Hydrogen ion (pH) as 15 minutes.

Can you provide more definitive guidance regarding the interpretation of “as soon as possible”?


Methods 9045D and 9040C can be done in the field, but often field conditions cannot be controlled and may be unfavorable for accurate analyses.  The methods are generally meant to be used in a laboratory setting under controlled conditions.

Because of potential changes from microbial activity, or shifts in chemical equilibrium, sample pH should be taken as soon as possible upon receipt at the laboratory.  It is expected pH of collected samples would be analyzed within a few hours of laboratory receipt, and not the next day.  The method is written to be followed sequentially with no undue delay between steps, except for the one hour settling time for soils in Section 7.2.2 or 15 minutes settling time for wastes in Section 7.3.2 of 9045D.   Also, keep in mind that Methods 9045D and 9040C are Method Defined Parameter (MDP) methods that must be followed exactly as written with no modifications.

The holding time of "Analyze as soon as possible" is not clearly defined for SW-846, but is only recommended as a qualitative goal.  In 2007, the definition for “immediately” was established as 15 minutes for the NPDES Program (40 CFR 136, Table II), however, that does not apply to the RCRA program and SW-846.  Even though SW-846 Method 9040C (and Method 9045D) are MDP methods and must be followed prescriptively, Section 6.0 of Method 9040C on holding time is not prescriptive.  The method authors recognized changes in pH begin as soon as a sample is put into a container and specified that samples should be analyzed as soon as possible.  If they meant to analyze by an exact time, the method authors would have put that time into the method.  Instead, they left it up to the user to do what is feasible as soon as possible.  SW-846 also makes a distinction between "must" or "shall" language that is mandatory, as opposed to guidance language like "should" or "may".  Note that Section 6.0 of Method 9040C uses the word "should".

The actual holding time for pH analysis should be specified in the quality assurance plan under which the sample was collected and agreed‐upon by all stakeholders involved, including those who have to measure the pH (be it in the laboratory or in the field).  In the laboratory, we recommend performing the analysis within a few hours of receipt, and not the next day.

Other Categories: Holding Time & Preservation, Hazardous Waste Characteristics

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