Q. We have been checking the water in our steam coils in our cleaners for alkalinity by adding phenolphthalein indicator to the solution from the coil. If it turns red, we know caustic is present. Is there another way to check for this? M.S.
A. The use of an indicator such as phenolphthalein is a very simple and quick method for a semi-quantitative evaluation of pH. A more accurate (but not necessarily quicker) method, would be to use a pH meter. That way you could record the exact pH value.
The color change of an indicator simply informs you that the pH is above or below the transition point of that indicator. In the case of phenolphthalein, the transition to pink occurs in the pH range of about 8.5–10. A quick change to a strong pink color indicates the pH of the water in the steam coils is greater than 10, but no more than that. If you are simply testing to ensure there is still some amount of rust inhibitor in the steam system, that may be adequate.
However, the use of the pH meter would allow you to track actual pH values and understand the potential range of values that the system produces. The pH probes can also be integrated directly into the process stream for either a continuous display, dump to a data acquisition system or run to a controller that could be used to add inhibitor automatically. It all depends on how automated you want to make it and how much you are willing to pay for the automation.