Why Do I Need ph/ORP Data, and What Do I Do with It?
Several years ago, Massachusetts Department of Environmental Protection (MassDEP) added a new analytical protocol to the Compendium of Analytical Methods (CAM) for proper sampling for hexavalent chromium (Cr VI) in soil, one that included a requirement to sample for pH and oxidation/reduction potential (ORP). It appears that the reasons for these additional requirements may not be well understood by everyone in the regulated community, nor the appropriate application of the pH/ORP data to the chromium data. This article attempts to clarify these issues.
The necessity of the new Cr VI sampling protocol really arises only in one fairly unique situation: when the matrix spike / spike duplicate for a Licensed Site Professional’s (LSP’s) Cr VI samples is below the acceptable recovery range. In this situation, the LSP would be required to distrust, and perhaps reject, the Cr VI analytical results. However, if the LSP’s pH/ORP data indicate that the sampled soil is from reducing conditions, he or she may be able to justify relying on the analytical results despite failing the QA/QC requirements of the analytical method.
Chromium typically occurs in the environment either as Chromium III (more prevalent) or Chromium VI. Chromium can be found in other valence states, but they are typically inconsequential for work that LSPs do. Chromium VI (hexavalent chrome) is significantly more toxic than Chrome III, and therefore has a lower notification threshold (100 mg/Kg in S-1 soil, versus 1,000 mg/Kg for Cr III). However, the standard analysis for chromium is for total chromium (without regard to valence state), so MassDEP requires that notification decisions based solely on total chromium analyses assume that all chromium is the more toxic Cr VI variety and use the lower reporting threshold. To avoid having to report total chromium concentrations between 100 mg/Kg and 1,000 mg/Kg, LSPs can re-analyze their samples specifically for Cr VI, in hopes of demonstrating that the fraction of total chromium that is hexavalent is low or nil.
It is also important to remember the portion of standard QA/QC procedures associated with matrix spikes and spike duplicates (MS/MSD). Labs run metals analyses in batches generally larger than the group of samples one LSP submits, and the lab will choose a sample from the batch at random to spike with a small, known quantity of Cr VI (and other metals), to check the ability of the analytical instrument to accurately detect the presence of Cr VI (and other metals). The CAM stipulates that when an LSP requests metals analyses, he or she request that one or more of his or her own samples be selected as the MS/MSD – and if the instrument’s recovery of the spike in the LSP’s sample is above or below the range stipulated as acceptable by the analytical method (70%-120%), the LSP must question, or even reject, the analytical results of his or her samples.
If you are testing for Cr VI, and none of your QA/QC parameters are questionable (especially recovery of Cr VI in your MS/MSD), then you probably have no need to read further here. If, however, your MS/MSD recoveries are outside (especially below) the acceptable range, read on about the quirks of chromium chemistry.
In oxidizing conditions, which are typical at most sites, both Cr III and Cr VI are stable; both species tend to maintain their existing valence state. In reducing conditions, however, Cr VI is unstable, and fairly quickly reduces to Cr III. Thus, a release of Cr VI into reducing site conditions will tend to convert to the less toxic Cr III, unless the release is large enough to overwhelm local environmental conditions. If the release does not change prevailing conditions, an analysis for Cr VI of soil from reducing conditions is likely to (rightly) indicate that little or no Cr VI is present, because it has been reduced.
Keep in mind, however, that a sample that maintains the reducing condition in the sample jar, and that is spiked in the lab with Cr VI, is likely to also reduce the spike (MS/MSD). This could potentially result in a very low spike recovery (below the acceptable 70-120% range), which would suggest that the analyses were unreliable. This, in turn, could lead the LSP to conclude that the non-detect (ND) results for Cr VI should be rejected, even though they might actually be correct. It is therefore important to demonstrate that reducing conditions exist, so that the poor spike recovery can be explained, and the LSP can justify not rejecting the analytical data. This is where pH/ORP data come in.
ORP versus Eh, and pH
Appended to the CAM chromium protocol is an Eh/pH diagram (Eh being another expression of oxidation/reduction) that shows the range of conditions in which Cr VI and Cr III are each stable. If the LSP’s pH/ORP data plot clearly and consistently within the reducing field on the diagram, where Cr VI is unstable, then the LSP can make the argument that he need not reject his Cr VI data due to poor MS/MSD recoveries, because the poor recoveries are probably due to reduction of the spike by the sample matrix.
Most LSPs are aware that the holding time on pH/ORP samples is 24 hours. As a result, the LSP cannot hold his pH/ORP samples pending the results of his total chromium analyses; they should be run concurrently. The CAM protocol also stipulates that Cr VI samples be in a separate container, so that reducing conditions that would be important to identify are not compromised, as they might be when a single container is opened for the total chromium analysis (and potentially oxidized), and opened again later to test for Cr VI.
There are some further wrinkles, however, with respect to measuring Eh/ORP. First, although the units (millivolts) of Eh (the oxidation/reduction parameter used in the stability diagram) are the same as for ORP (the parameter typically measured in the field, and often by labs), they will not have the same value for the same conditions. Eh is converted to ORP or back by subtracting or adding a certain number of millivolts. However, the number of millivolts to add or subtract is a function of the instrument used to measure ORP (if your field or lab results are reported as Eh, no conversion is required before using the diagram). Such instruments typically have either a mercury chloride (“calomel”) or a silver chloride electrode. Conversion of ORP measurements to Eh requires addition of 244 millivolts when measured with a calomel electrode, and 199 millivolts when measured with a silver chloride electrode.
Although this issue is alluded to in a footnote on the Eh/pH diagram in the CAM (as an oblique reference to the difference between Eh and ORP), the method does not provide explicit guidance about the need to convert ORP data to Eh data, or how to do so. While using unadjusted ORP data may not change the conclusion concerning whether sample conditions were oxidizing or reducing (the change of value is not that large), it is nonetheless important to be aware of the issue for the sake of accuracy, or when near boundary conditions. Thus, the LSP should identify the type of electrode installed in his field instrument or ask the laboratory what type of electrode the lab employed. As an aside, it is also worth noting that several laboratory representatives have commented that there is no scientifically-rigorous way to measure the ORP of soil – the consistent comment is that ORP values measured in a soil/de-ionized water slurry (the most typical field method) have as much to do with the water as they do the soil.
The check list below is provided to assist the non-chemist LSP with complying with the CAM method. The following bullets highlight some useful reminders, although it is not intended to be exhaustive, and there is no substitute for familiarity with the CAM protocol. In addition, compliance with this checklist does not, of itself, guarantee presumptive certainty:
- Check with your lab in advance of sampling to be sure that the lab’s instruments and procedures are set up to complete a CAM-compliant Cr VI analysis.
- Ask your lab if it can provide redox measurements. If yes, ask whether the lab reports redox data as Eh, or as ORP. If the latter, ask the lab what type of electrode its instrument uses to measure ORP. If the lab cannot provide Eh/ORP analyses, you may wish to find another lab, or measure pH/ORP in the field. Remember that the holding time on pH/ORP samples is 24 hours.
- If you are testing ORP in the field, ask the instrument rental agency what type of electrode their instrument uses, or consult the instrument manual or manufacturer.
- Read the ORP meter manual in advance of field work for information about how to measure ORP. The procedure will probably require making a slurry of the soil sample using de-ionized water, which you will need to purchase ahead of time (most supermarkets now carry DI water).
- During sampling, collect Cr VI, pH, and ORP samples each in a separate jar.
- Collect pH/ORP samples at the same time as your initial, total chromium samples, and submit them for analysis (or analyze yourself) even before you know that you will need pH/ORP data. The holding time on pH/ORP samples is typically too short (24 hours) to permit your waiting for total chrome results to decide whether to run pH/ORP.
- When completing your Chain of Custody for Cr VI analyses, instruct the lab to use your sample(s) for the matrix spike/matrix spike duplicate.
- Upon receipt of Cr VI data, review the MS/MSD recoveries for acceptability. If the recoveries are within the acceptable range, then your Cr VI results are probably usable without additional concerns or arguments.
- If the MS/MSD recoveries are outside (especially below) the acceptable range, convert your ORP data to Eh data.
- Plot your ORP data (converted to Eh), along with pH measurements, on the Eh/pH diagram appended to the method; if your samples fall consistently in the reducing field, then you may be justified in concluding that the poor MS/MSD recoveries are attributable to reduction of spiked Cr VI to Cr III, and that the sample results need not be rejected simply because the MS/MSD recoveries fall outside the acceptable range.
It should be understood that the scenario the Cr VI protocol anticipates is seldom encountered and will seldom require an LSP’s additional attention. In more typical, oxidizing conditions, Cr VI is stable, and analytical results are not generally subject to the QA/QC concerns described above. Nonetheless, at the point that the presence of Cr VI becomes an issue, and at the point that the LSP seeks presumptive certainty for his chromium analytical results, understanding all the small details of the CAM protocol becomes important.