Risks from overheated phenolphthalein in green/orange silica gel drying

Question

Recently I tried to dry out some indicating silica gel. When I googled, I found advice on line saying that it should be heated to 150 C and that such was a standard procedure, so I popped it into the toaster oven set it to 300 F which is just over 150 C and it started to make a smell and turn black. I stopped of course, cooled it down, and aired out my kitchen. Futher research indicates that the temperature I used is applicable to blue indicating or non-incdicating gel, but the newer Orange/Green gels contains an organic indicator which turns out to be phenolphthalein, rather than Cobalt Chloride and as such should not be heated above 120 C. Basically I followed outdated advice for the older type of product, a classic internet fail.

So now the question is, what have I done? What did I breath? I was cautious, watched the process stopped it early and don't have any outward symptoms, but now I'm curious how dangerous this was.

I've done a lot of searching and found nothing beyond "Toxic oxides of carbon, acrid and irritating fumes" in MSDS sheets regarding what happens when phenolphthalein combusts let alone what might happen as it degrades inside a silca gel bead. MSDS for the beads are just as vague. The lack of info is slightly alarming considering the ease with which a mistake such as mine could be made. Does anyone know what happens when heating breaks down phenolphthalein? There was a smell so something's definitely been produced other than water vapor, and the blackening of the beads might indicate that some carbon probably was left behind.

I understand that a variety of chemicals might be released, but I wonder if anyone is aware of a characterization of the types and relative quantities, or if there are any specific concerns.

Answer 1

I believe you are trying to reactivate moisture absorbing silica gel with a indicator dye. However, you are blaming the internet for the mishap you have faced. Yet, the website you have directed is ScinceDirect, which provides subscription-based access to a large database of scientific and medical research (Producer: Elsevier). Therefore, I find they give wrongful information on scientific research (considering almost all of these papers are peer-reviewed). I think you made a mistake where considering the indicator dye in green/orange silica gel desiccants is phenolphthalein. My conclusion is based on the inventors' remark in their patent application (Ref.1), which states that color change is from orange (when the gel is dry) to creamy white (when the gel is saturated with moisture). My best guess is it contains something different. Even with something else as an indicator dye, it should be able to stand at $\pu{150 ^\circ C}$ temperature in order to be able to reactivated. What that suggest is, your silica gel might be absorbed unknown chemical without your knowledge, which must be thermally unstable.

According to the patent, there are three kind of materials they are suggesting as indicator dyes: (1) phenolphthalein (color change Orange-red to creamy white); (2) thymolphthalein (color change dark-brown to creamy white); and (3) mixture of phenolphthalein/thymolphthalein (color change Orange-is not provided). All three types are able to get reactivated under $\pu{150-250 ^\circ C}$ temperature range (Ref.1).

References:

1. Waclaw Lanowy, Krystian Lukaszczyk, Przemyslaw Kasprzycki, “Silica gel with moisture indicator, a method of obtaining them, and use of phthalein dyes,” International Patent 2006, WO2006/130029 A2 (PDF).