Entertaining, and, as suggested in the first three words, that’s *clearly* an urban legend with respect to the laxative effect.
One uses only a *trace* amount of indicator in a titration so as not to change the concentration of the solution and because the indicator is being co-titrated with your solution. If one added a significant quantity of indicator it would throw off the results of your titration. The commonly used indicators work so well because they have relatively high molar extinction coefficients that allow the unaided eye to see when the tiny amount of indicator has changed between protonated and unprotonated forms.
A typical titration of the type described below would use 2-3 drops of a 0.5% ethanol solution of phenolphthalein. See, for example: http://www.titrations.info/acid-base-titration-indicators That’s 0.5% by weight. I have no idea what a drop weighs offhand, but let’s call it 5 milligrams for kicks and allow some excess - let’s say one used 25 milligrams of this solution. 0.5% is 1/200th, so there are 25/200 = 0.125 mg of phenolphthalein added to this solution.
Further, let’s assume the final volume at the end of the demonstration was 50 mL of solution. If you take a 1 mL taste, that’s an intake of (0.125 mg)/50 = 0.0025 mg.
The usual therapeutic dose for laxative effects in an adult is 30-195 mg:
https://datasheets.scbt.com/sc-206050.pdf And it is by no means has an instantaneous effect. It can take 6-8 hours for a standard dose. So
0.0025 mg or even the full 0.125 mg is going to do absolutely nothing. Nada. Zip. Zero.
Perhaps a cheeky lecturer could adulterate the sample with grams of the stuff. Even so, I doubt you could get more than a therapeutic dose from taking a taste. And even if one took an epic dose, the mechanism of action of phenolphthalein requires absorption from the intestines, so an instant effect, other than perhaps vomiting and epigastric distress, is simply implausible.
On a related note, apparently even “respected” sources for health information are now contaminated with drivel. I stumbled across this last night:
https://www.webmd.com/balance/grounding-benefits I sent their web team a a well-deserved withering criticism; we’ll see if come around.
Best wishes,
Rob Toreki
LabLocks™ - The first device that can lock out standard laboratory ball valves: https://www.safetyemporium.com/07400
US-Made NIOSH and FDA-approved N95 respirators: https://www.safetyemporium.com/safety-items/respirators/
Safety Emporium - Laboratory and Safety Supplies https://www.SafetyEmporium.comesales**At_Symbol_Here**safetyemporium.com or toll-free: (866) 326-5412Fax: (856) 553-6154, PO Box 1003, Blackwood, NJ 08012
On Dec 4, 2024, at 4:43 PM, Clark, Eric J <CLARKEJ**At_Symbol_Here**lattc.edu> wrote:
Laboratory Urban Legend describes mid-late-1800s General Chemistry titration lectures where the pink endpoint of phenolphthalein was demonstrated using HCl and NaOH. In that century a student was asked to taste the pink neutralized solution and describe the salty taste. And the lecturer knowingly and for entertainment purposes encouraged others to try – so that they would almost immediately have an extreme case of diarrhea. Later phenolphthalein was used as the main ingredient of ExLax for most of the 20th century until it was reformulated and removed from the product in 1997 after a study found that it increased the risk of tumors in rodents.
Eric
Los Angeles Trade Technical College
400 West Washington Blvd.
Los Angeles CA 90015
