As regulations concerning PFAS continue to grow, one must realize the limitations of using refractive index to determine whether a solution could be contaminated with firefighting foam containing PFAS.

Resolution

Consider a 3% AFFF with 20,000 ppm (2%) PFAS and a refractive index of 1.3500, both of which are reasonable for such a product.  For the sake of this discussion, also assume the refractive index of uncontaminated water is 1.3330, a reasonable value for tap water.

Note that for both the foam concentrate and uncontaminated water, the refractive index values have only been given to four decimal places.   This is quite common in the firefighting foam industry and is typically the resolution for most portable refractometers, at least those recommended in various foam manufacturer technical datasheets.

In this hypothetical scenario, consider a solution with a true refractive index of 1.33304.  Given a refractometer with a resolution of 0.0001, it may display the refractive index as 1.3330 depending on the rounding rules it follows.  If this occurs, one could then incorrectly assume the solution is uncontaminated.  After all, the instrument would be displaying the same reading for both uncontaminated water as it would the solution in question (1.3330).

However, given the true result out to five decimal places in this hypothetical scenario, we know it must contain a small amount of the foam concentrate.  How much foam concentrate?  To determine this, a linear regression can be used to represent the relationship between the refractive index and the percent concentration of foam in solution: % Concentration = 5882.4 x Refractive Index – 7841.2

Therefore, if the solution’s true refractive index is 1.33304, it contains 0.27% of the specified foam concentrate.  Given the foam concentrate contained 20,000 ppm PFAS, one can then determine how much PFAS would be in a 0.27% solution of that foam: Without considering the limitations of the refractometer’s resolution, one could incorrectly assume the solution is uncontaminated in this example when, in fact, it contains 54 ppm PFAS (based on the variables specified in this example).

Accuracy

Along with the resolution, the accuracy of the refractometer is another limitation to consider when trying to use refractive index as an indicator of PFAS contamination.  Consider again the same foam concentrate and uncontaminated water as referenced before, but this time also consider the instrument has an accuracy of +/- 0.0001, a reasonable accuracy for today’s portable refractometers.

Given this accuracy, it is certainly possible the instrument displays a refractive index of 1.3330 for a solution that is 1.3331.  Again, based on the displayed reading, one could then incorrectly assume the solution is uncontaminated.  How much of the specified foam concentrate could be in this solution with the given true refractive index value (1.3331)?  Once more, the linear relationship between percent concentration and refractive index can be used to determine that a solution with a refractive index of 1.3331 would contain 0.63% of the specified foam concentrate and, therefore, 126 ppm PFAS (based on the variables specified in this example). Putting It All Together

Outside of these hypothetical scenarios, both resolution and accuracy will play a part in the refractive index reading.  If we put them together in our example, it is possible (albeit the worst case) the instrument could read 1.3330 when in fact the true result is 1.33314 given both the resolution and accuracy limitations.  This would mean a 1.3330 solution could contain as much as 0.86% of the specified foam concentrates and, therefore, 172 ppm PFAS (based on the variables specified in this example).

As just demonstrated, when trying to determine whether a solution could be contaminated with foam, using just the refractive index as an indicator may not be sufficient depending on the regulations in the area (always review local regulations when considering PFAS release).  If the limitations of refractive index are too much, more in-depth analysis, such as the total organic fluorine (TOF) analysis offered by Dyne Fire Protection Labs, may be necessary.  For example, consider a system which previously contained AFFF (a product formulated with PFAS) but will now contain SFFF (a product not formulated with PFAS).  According to UL 162 Foam Equipment and Liquid Concentrates, SFFF should contain < 1 ppm TOF.  If the system owner chooses to rinse (with only water) instead of replacing parts of the existing system, analyzing just the refractive index of that rinse water would not be sufficient to ensure the system contains no more PFAS than what could be in the SFFF.  The solution could have a refractive index of 1.3330, same as the incoming water, but could certainly still contain more than 1 ppm TOF as shown in the examples above.