I have checked with EPA’s Emission Measurement Center (EMC) involving the addition of a pH indicator to the last NaOH impinger to show the visual change in color if the normality of the NaOH changes below 0.1 N (pH ~ 10). EPA has not received request for approval of the methodology by adding a color indicator to the impinger.
However, thymolphthalein is blue at a pH of > 11, but becomes colorless at a pH < 10. I would see nothing wrong with adding a few drops of thymolphthalein in the last impinger as an indicator of pH change during sampling rather than stopping the test and having to leak check before checking the pH of the last impinger. Of course, you must receive prior approval from the Administrator before implementation.
However, don’t forget our other options to maintaining proper strengths of our absorbing solutions:
- Use stronger base (e.g., 0.5 N NaOH);
- Add additional volume to the last impinger ( e.g., 200 mL); and
- Recharge impinger during sampling.