07:41 – You’ve probably seen phenolphthalein used as an acid-base indicator. In solutions ranging from acidic to moderately basic, phenolphthalein is colorless; at pH 10 and higher, phenolphthalein is fuchsia. (Okay, “fuchsia” is a girl-color, and I annoy Barbara by pronouncing it as German. Guys would call the color “pink”, or, if they’re into photography and imaging, perhaps “magenta”.)
At any rate, the forensics kit includes a 30 mL bottle of Kastle-Meyer reagent, a presumptive color test for blood. I had planned to buy the stuff premade, figuring it might cost me $30/liter or thereabouts. Instead, I found several suppliers, the least expensive of which was charging $230/liter. Geez. Obviously, taxpayer-funded law enforcement agencies aren’t too concerned about price.
It’s easy enough to make the stuff up, so that’s what I’ll do. All it requires per liter is 20 g of phenolphthalein, 200 g of sodium or potassium hydroxide, and about the same amount of zinc. You start by dissolving the hydroxide in about 900 mL of distilled or deionized water. Phenolphthalein powder is extraordinary insoluble in water, but freely soluble in a strong aqueous base solution. (And a 20% w/v solution of sodium or potassium hydroxide counts as a “strong aqueous base solution” in spades…)
Of course, as the phenolphthalein powder dissolves, the solution turns an incredibly intense fuchsia color. That’s where the zinc comes in. It reduces the phenolphthalein to pale yellow phenolphthalin. The instructions for making up KM reagent always talk about refluxing the solution until the pink color changes to pale yellow. I was hoping to avoid refluxing large volumes of a 20% hydroxide solution–the stuff is fearsome enough at room temperature, let alone boiling–so I decided to make up a small sample batch at room temperature. I made up 100 mL and let the stuff sit at room temperature for two or three weeks, with occasional swirling. Over that time, it turned from pure pink to an orangish pink.
So yesterday, I decided to speed things up a bit by refluxing the solution. The problem with that is that strong hydroxide solutions literally dissolve glass, even at room temperature. At boiling, they simply eat glass. After two or three weeks of contact, the 250 mL Erlenmeyer flask that I’d made up the solution in originally was already etched, so I decided to see what would happen at boiling. The image below shows the grisly details. What look like white chemical deposits on the sides of the flask is actual where the glass has been etched. But the good news is that after only 30 minutes or so of refluxing the solution has assumed its final light yellow color.
This little experiment did lead to a change in plans, however. I was expecting to have to reflux the solution for at least two or three hours. To do that refluxing in volume, I planned to buy a large round-bottom flask (or several) and a Graham condenser with ground-glass joints, and dedicate them to making up KM reagent. If you’ve ever priced glassware with ground-glass joints, you know how costly it is. Even the Chinese stuff ain’t cheap, and I’m not at all sure how many iterations the flask and condenser would survive. So I decided instead to order some large (2 L to 5 L) Erlenmeyer flasks, which aren’t cheap, but they’re a lot cheaper than ground-glass glassware. And I won’t need a reflux condenser at all. For the sample batch, I simply placed a one-hole stopper loosely in the neck of the 250 mL Erlenmeyer, brought the solution to a boil and then turned it down to simmer. There was a small amount of steam exiting the hole in the stopper, but not enough to noticeably reduce the volume of the solution over the whole reflux period.
12 Comments and discussion on "Wednesday, 30 May 2012"
Eat your heart out, all you German (and other nation’s) brewers:
Hey, are these water well self tester kits any good?
I bought one of these last week and we passed everything except for the hardness with flying colors. Now I am kinda wondering about the 48 hour bacteria test. This is a water well at my office (we do not have city water here). I have no idea how deep the well is, etc.
Okay, I see that purple indicates no bacteria and yellow indicates bacteria are present. I’m guessing that the test vial includes lactose fermentation broth and bromophenol blue. E. coli and other gut bacteria are typically lactose fermenters, so they grow in the broth. Their waste products cause the BPB to change color from purple to yellow.
IIRC, this is a standard FDA-approved method of screening for coliform bacteria, so if your results are negative you should be fine.
“IIRC, this is a standard FDA-approved method of screening for coliform bacteria, so if your results are negative you should be fine.”
Thanks ! I now understand a little more about the test. BTW, the test vial had yellowish solids in it to start but turned purple immediately when I filled it with tap water.
If you start with a solution of pure whatever-it-is in a glass flask, and it eats away at the glass, don’t you now have an impure solution of whatever-it-is plus what used to be glass?
Yep. Glass, whether ordinary soda-lime or borosilicate, is made up mostly (70% to 80%+) of silicon dioxide (sand). Sodium or potassium hydroxide converts that to sodium or potassium silicate, both of which are pretty inert.
You can think of the potassium silicate in my KM reagent as an “unofficial” ingredient. That is, KM reagent has been made in glass containers since it was first used more than 100 years ago, so it’s always contained silicates. In fact, it’s even possible that the silicates contribute to the reaction. If I made it up in a container that was impervious to hydroxide, it’s possible the reagent wouldn’t work or wouldn’t work as well.
As odd as it sounds, there are many examples of chemists and chemical engineers coming to grief because the reactants they used were too pure. Sometimes, an impurity present in tiny amounts is actually necessary to the reaction.
I was reading of one example a couple of years ago. Iron(III) chloride (ferric chloride) has been used as a catalyst in many large-scale reactions for 150 years or more. Someone was trying to reproduce one of those reactions on a lab scale, and used very pure iron(III) chloride. The reaction had a very low yield. So he tried again, using even purer iron(III) chloride, and the yield was lower still. So he used technical grade (crude) iron(III) chloride, and the yield was excellent. He investigated, and found that what everyone had believed–that the iron was the catalyst–was wrong. It turned out that the iron had no catalytic activity whatsoever. It was actually the tiny amount of copper present as an impurity–at the parts per billion level–that was acting as the catalyst. So now we all know that that reaction (and probably many others) was actually being catalyzed by copper rather than iron.
Just put our German exchange student on the plane to send her home. It was a good year and I think she had a good time. It is hard to say goodbye as they become like your own children.
No more kids for the next year. We will travel to Germany in the spring of 2013 to meet with the family of our next exchange student, whom we have yet to pick. The trip in 2013 will be to visit with as many of our exchange students as possible. It will be a three week trip and even at that it will be tough to schedule the visits.
What I want to do on one of these trips is to rent a meeting room at a hotel in Frankfurt. Have a dinner catered and invite all the families of all the exchange students. I think it would be nice to have them all together and to let them talk with each other. May trip to do that in 2016. Frankfurt is about as centrally located for a large city and can be easily reached by train from anywhere in Germany.
Meanwhile, two of our exchange students from Germany are returning this summer for a month long visit. They will overlap their stays by 10 days and we will use that opportunity to travel to Niagara Falls.
Our daughter’s German friend went back to Germany for the summer, and in the fall she resumes her senior year at the University of Glasgow, very sharp young woman, and cute, too. She’s invited us to her graduation next year; we shall see. Some of my ancestors came over from Caledonia in the 18th-C so it might be fun to tool around the countryside, maybe get some nice videos of Nessie, shoot the breeze with Glaswegian heroin addicts, and go piss on the grave of Reverend John Knox.
“…and go piss on the grave of Reverend John Knox.”
Why? Wasn’t he Calvinist enough for you? 🙂
No, he could have been a bit more Calvinist and of course more hateful of women and Catholic scum. A typical Jock slacker. A golf course drunkard, most like.
Ahhh yes, the author of the notorious The First Blast of the Trumpet against the monstrous regiment of Women.
Well, as someone who’s seen the likes of Bella Abzug, Barbara Boxer and many others I have a certain sympathy for him.
True, that, homes. Not to mention Diane Feinstein, Madeline Not-Too-Bright, Donna Shalala, Maxine Waters, and the list just goes on and on, and note well, sir, that they are all from the hard Left. Boxer, though, in her younger years, was kinda hot. She got nabbed getting it on with an African-American Panther defendant inside a janitor’s closet at the Marin County Courthouse way back in the day.
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