Day: September 5, 2014

Friday, 5 September 2014

07:55 – We’re all caught up on shipping kits. The only orders outstanding are the ones that came in overnight and this morning, which we’ll ship this afternoon. Meanwhile, I need to make up a bunch of solutions today and get started on bottling them.

One of those solutions is 4 liters of 6M sodium hydroxide, which has gotten me thinking about chemical storage. I’m down to my last three 500 g bottles of sodium hydroxide. When I finish those, I have to open a new container of sodium hydroxide, which in this case is a 10 kilo bucket rather than a 500 g bottle. Right now, that 10 kilo bucket is sitting on the floor because it won’t fit my storage shelves.

When we got started building science kits a few years ago, I put up shelves for chemical storage. Most of them are 4″ (10 cm) wide with vertical separation of 6″ (15 cm). Those worked fine when I was buying chemicals in 25 g, 100 g, and 500 g bottles. They’re not wide enough now that I’m buying a lot of chemicals in 1-kilo, 2- or 2.5-kilo, 5-kilo, and 10-kilo containers. That’s why there are still a couple of cartons of chemicals from Fisher Scientific sitting on the floor where UPS delivered them. I thought about repackaging them into 500 g and one kilo bottles, but that’s just too much work. Instead, I think I’ll remove some of the smaller shelves and replace them with wider shelves with more vertical separation. But that’ll have to wait for things to calm down a bit around here.

11:02 – I’d forgotten how obnoxious lead acetate is. We provide a 0.1 M solution of lead acetate in many of our kits, and I was just making up four liters of the stuff. I weighed out the appropriate mass of reagent-grade lead acetate and added it to distilled water. One might expect a nice, clear water-like solution to result. Instead, one gets a solution that looks like milk, literally.

The problem is that most common lead salts, with the exceptions of the acetate and the nitrate, are extremely insoluble in water. And water exposed to air just loves to suck up carbon dioxide. At room temperature, a liter of water dissolves about 1.6 grams of carbon dioxide. That doesn’t sound like much, but with the molar mass of carbon dioxide about 44 g/mol, that means that plain water exposed to air is actually about 0.036 molar with respect to carbon dioxide. That carbon dioxide reacts with water in a reversible reaction to form carbonic acid, the acid whose salts are carbonates. And lead carbonate is extremely insoluble in water, which is why my solution looks like milk. That 0.036 molar carbonic acid reacts 1:1 with my 0.1 molar lead acetate solution precipitating out nearly a third of the lead ions as insoluble carbonate. What’s worse is that that reaction removes the carbon dioxide from the solution, so it promptly sucks more carbon dioxide out of the air, until all the lead is precipitated and the solution reaches equilibrium with about 1.6 g/L of dissolved carbon dioxide. Basically, my dilute solution of lead acetate eventually turns into a dilute solution of acetic acid with most of the lead precipitated out as lead carbonate.

Fortunately, one can use Le Chatelier’s principle to shift the equilibrium by dissolving the lead acetate in a dilute solution of acetic acid rather than plain water. Although it’s a weak acid in absolute terms, acetic acid is a much stronger acid than carbonic acid. That forces the equilibrium of the reversible carbon dioxide <-> carbonic acid reaction to the left, keeping the dissolved carbon dioxide in the form of the molecular gas rather than the carbonate ion. And the lead acetate remains in solution as lead acetate.

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