DCHAS-L Archives, Nov 12, 2025 - Re: [DCHAS-L] Phenolphthalein or Replacement?

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Subject: Re: [DCHAS-L] Phenolphthalein or Replacement?

Date: Nov 11, 2025 22:13 UTC

Author: Pemberton, Drew (apemberton**At_Symbol_Here**uidaho.edu) <0000231dd2f469c4-dmarc-request**At_Symbol_Here**LISTS.PRINCETON.EDU>

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Subject: Re: [DCHAS-L] Phenolphthalein or Replacement?

Date: Nov 12, 2025 23:50 UTC

Author: David EldrEdge <Dave.EldrEdge**At_Symbol_Here**NALTIC.COM>

I am a complete neophyte with respect to waste regulations. Can someone please explain why this conversation is even happening? I can’t see any statistically possible way pouring this down the drain poses a risk to anyone or anything. To wit:

Phenolphthalein is normally used as a 1% by mass solution at most - usually 0.1% if titrating a dilute or weak acid. And one uses one or two drops per titration; maybe three if 0.1%.

So even if I had a full lab class of 24 students and each did 3 titrations that afternoon, that’s maybe 150 or even 200 drops - less than half an ounce.

Half an ounce is about 15 mL. At 1% by mass that’s 0.15 g of phenolphthalein, 150 mg. Figure with glassware washing and whatnot, each student is going to put at least 4 gallons down the sink so call it 100 gallons total, or 400 liters in round numbers.

Phenolphthalein has a molar mass of 318 g/mol, so we’re talking 0.0004717 moles. In 400 liters of water. So it’s 0.00000118 molar. Which is 0.375 ppm in the waste stream. And that’s before being diluted by the rest of the building and campus which will take that down another couple orders of magnitude.

1. I doubt any wastewater authority would be looking for, let alone be able to detect, what will ppb levels of the stuff where it hits the monitoring point.

2. Sure, phenolphthalein is a suspected human carcinogen but according to the 15th NTP Rerpot on Carcinogens “The data available from epidemiological studies are inadequate to evaluate the relationship between human cancer and exposure specifically to phenolphthalein.” And that’s from looking at people that ingested 5-10 mg (or more) orally over long periods of time (to get a 5 mg dose from our 0.375 ppm waste stream would require drinking 13 liters of it). Even in the rodent models, it took fantastically high dosages to produce effects (hundreds or thousands of ppm daily).

3. So this clearly comes down to Regulations somewhere.

So, what specific Regulation is being applied here, is it bound by hazard (regardless of risk) vs the risk (which is based on the actual amount) or some absolute ppm level, and is this something local wastewater authorities can mandate on their own?

Thanks,

Rob Toreki

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On Nov 11, 2025, at 4:28 PM, Jeffrey Lewin <jclewin**At_Symbol_Here**MTU.EDU> wrote:

When I checked a couple of SDSs for phenolphthalein solution they indicated they were mixed with ethanol and had a flashpoint below 60c, which makes the stock solution ignitable and a characteristic hazardous waste. Presumably you are diluting it as part of a reaction. If the alcohol is diluted to less than 24% it is no longer a characteristic hazardous waste (by definition) but may still be ignitable and prohibited by the sewer authority.

I found two different SDSs for bromothymol blue solution, one of which used IPA and, again, a flashpoint less than 60C.

But, even if you use an aqueous based dye you might still be not allowed to sewer it; some sewer authorities have prohibitions from dyes being sewered.

Hence, it should be collected for waste disposal, the question is does it meet the RCRA definition of a hazardous waste (EPA code D001) or is it collected and disposed of as a nonRCRA (Liquid Industrial Byproduct in MI) waste?

Jeff

On Tue, Nov 11, 2025 at 9:01 AM David EldrEdge <Dave.EldrEdge**At_Symbol_Here**naltic.com> wrote:
Our School's EHS responded to a question that was brought up about phenolphthalein:

"I verified this with the wastewater district, and they confirmed that it [phenolphthalein] cannot be disposed of down the drain. I want to be cautious here if we start discharging materials improperly, it could trigger a wastewater audit and potentially open Pandora’s box for the college."

What do you all do? Are you still using phenolphthalein or have you replaced phenolphthalein with something like bromthymol blue or something similar? Do you get the same results or better?

or...

Are you still using phenolphthalein, if so how do you deal with the waste? If collecting the waste, is it worth the cost of disposal?

Warm regards,

David EldrEdge
Co-Owner
NALTIC Industrials, LLC
de**At_Symbol_Here**naltic.com
www.naltic.com
888.891.0077 Main
435.503.4972 Cell
435.767.7714 Google Voice & Text
435.654.2727 Fax

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Jeff Lewin
Chemical Safety
Environmental Health and Safety
Office of the General Counsel
Michigan Technological University

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