Thursday, 28 August 2014

07:57 – So, I was down in the lab yesterday making up a new batch of Kastle-Meyer reagent, which is used in forensic science as a presumptive test for blood. It’s made by dissolving phenolphthalein powder in a concentrated solution of potassium hydroxide and then refluxing it over powdered zinc until the intense pink color of phenolphthalein in basic solution fades to colorless as the phenolphthalein is reduced to phenolphthalin.

Even cold, concentrated solutions of strong bases like potassium hydroxide etch/dissolve glass, and if they’re boiling they do so very quickly. Within a couple of minutes, the glass starts to turn cloudy with chalky white streaks. Once a flask is used to make up KM reagent, it’s too ugly to even consider using for anything else. So, the first time I made up a big batch of KM reagent a couple of years ago, I devoted a 2 L Erlenmeyer flask to the job, and that’s all I’ve used it for ever since. For the first batch, I put a kilo or so of zinc powder in the flask, made up the KM reagent, and then washed the flask out with several changes of water, leaving the unreacted zinc powder in the bottom of the flask. I store the flask full of water and stoppered, because damp zinc powder is pyrophoric (catches fire spontaneously when exposed to air). The next time I need to make up a batch, I drain the water, rinse the zinc several times, and use it again for that batch. I’ve done that several times over the last couple of years, and it’s always worked as expected.

Normally, I just add a liter of water to the flask along with the appropriate amounts of potassium hydroxide and phenolphthalein powder, put it on the hot plate, bring it to a boil, and then let it reflux for a few minutes. As it simmers, the bright pink color starts to fade and after five or ten minutes the solution turns colorless. But yesterday it didn’t work. After sitting there refluxing for half an hour or more, the solution was as pink as ever. Hmmm. Obviously, the zinc wasn’t reducing the phenolphthalein to phenolphthalin. It looked like there was still plenty of zinc in the flask, but instead of powder it looked more like a zinc coral reef. So I transferred another couple hundred grams of zinc powder to the flask. Sure enough, within five minutes the solution had turned colorless. The moral here is that just because it looks like there’s plenty of zinc remaining doesn’t mean there is.

10:45 – I get a surprising amount of private email from preppers, many of which ask me science-related questions. Sometimes they link to threads on various prepper forums. For example, one topic that I’m frequently asked about is storing antibiotics. The usual questions have to do with how long various antibiotics can be stored and the suitability of veterinary antibiotics for human use. I’m always surprised by how bad the information is on many of these threads, including quite a few comments by physicians, who should know better.

With regard to shelf life, the real answer is that most antibiotics if stored in the freezer will still be usable 20 or more years from now. Their potency may decline a bit, but long-term tests have shown that most antibiotics lose 10% or less (often, much less) of their potency after being stored frozen for 10 years. Just as important, any degradation that does occur does not create toxic byproducts. The one exception is the tetracyclines, which should not be stored long term. Tetracyclines do in fact produce hepatotoxic and nephrotoxic degradation products. Administering old tetracycline or its derivatives can kill the patient from liver or kidney failure.

With regard to human use of veterinary antibiotics, that’s generally not a problem. It’s not like pharmaceutical companies produce amoxicillin for humans in one plant and amoxicillin for veterinary use in another. It all comes from the same vats, and veterinary medications are packaged as carefully as human medications. One problem arises because people are not dogs or cows or chickens. The mechanisms are very similar in any of these animals, including humans, but our internal organs and processes may differ, sometimes significantly.

For example, on one forum thread someone asked if erythromycin packaged for oral veterinary use was suitable for oral human use. A physician responded that it was fine. It’s not. Veterinary erythromycin for oral use is often in the form of the phosphate salt. That’s fine if you’re treating chickens or turkeys. In humans (or other mammals), not so good. The problem is that the phosphate salt is quickly broken down by human gastric juices and the erythromycin is destroyed before it can be absorbed. Erythromycin for oral use in mammals is compounded with a different anion that renders the salt much less subject to being broken down by the hydrochloric acid in mammalian stomachs.

I keep a pretty good stock of veterinary antibiotics. For example, I order penicillin G potassium and sulfadimethoxine literally by the kilo for use in biology kits. Neither is intended for human use, but both are usable. The penicillin G potassium is not ideal for oral human use because it’s also degraded by stomach acids, but it can be used orally by increasing the dose and administering it when stomach acid is minimal, such as an hour or so before meals. One can also administer sodium bicarbonate (baking soda) a few minutes before the antibiotic to reduce stomach acidity even further. The sulfadimethoxine has never been approved for human use in the US, but it’s widely used in other countries, particularly Russia, and has been for decades. It’s as effective as the other sulfas on organisms susceptible to sulfas, and it has the added advantage of a very long biological half-time. That means it needs to be administered only once per day rather than the every four hours typical for short-acting sulfas.