Would a standard US 8 fluid oz glass of ultrapure water kill? If it would, this seems like a good mystery novel gimmick.
Does drinking water contain any nutrients not adequately available from a normal diet?
I am extremely skeptical of the osmosis story told in the last two paragraphs, as I am doubtful that there is that much difference between clean soft drinking water and ultrapure water.
If you injected the pure water directly into your bloodstream, you'd have an osmosis problem, thus the need for saline in IVs.
[1] https://www.happyscribe.com/public/the-joe-rogan-experience/...
If the pure water gimmick doesn't work out, then maybe for your mystery novel switch over to heavy water, which is known to be toxic iirc. Eccentric, recluse, rich person decides to drink nothing but heavy water for the exotic allure and then dies on the eve of a corporate merger or something. Bonus points, heavy water ice cubes will sink in regular water but over time they will float because they melt off the heavy ice and refreeze regular water, changing their buoyancy. Something the eagle eyed investigator can notice in the 11th hour.
“If ultrapure water was consumed continually, epithelial cells would be steadily leached (or perhaps instantly in the case of the goldfish’s respiratory system).”
Basically it would scour cells from you digestive system, roughly equivalent to corroding it away. It would probably take a few liters or more, but it would also probably hurt like hell.
Yes, just drinking some will do damage. I've heard of a non-tech seeing 'water' and ignoring the rest of labels and warning and drinking very pure water. Had to be sent to the hospital.
It happens with any water, but IIRC it happens faster with pure water by washing away electrolytes and disturbing signaling.
And the teachers warned against it, and someone told about a child dying from drinking just one liter of tapwater