Day: January 22, 2012

Sunday, 22 January 2012

11:39 – While she was watching basketball yesterday, Barbara finished packing and labeling 60 each of five groups of solids: amoxicillin capsules, ascorbic acid tablets, dextrose, lima bean seeds, and carrot seeds. She’ll finish up agar and Rhizobium innoculum today.

I took a break from writing to do some stuff in the lab. I wanted to have several solutions ready for packaging after deadline. Unfortunately, some stuff doesn’t go into solution very quickly. For example, even with constant agitation and warming, copper(II) sulfate takes a long time to dissolve. I make up one molar solutions of that for the chemistry kits, which is about 500 grams of the pentahydrate dissolved and made up to two liters of solution. I finally gave up trying to get the stuff to dissolve faster. Now, I just transfer 500 grams of the salt to a clean two-liter soft drink bottle, fill it up most of the way with distilled water, and just invert the bottle a few times any time I think about it. It takes several days to a week for the salt to dissolve completely.

Some stuff that goes into the biology kit worries me a bit in terms of dissolution speed. One of those is Sudan III stain. Some vendors sell what is supposedly a 1% solution of Sudan III in isopropanol. I’m not sure how they do that. The best figures I can find say that Sudan III is soluble only to about 0.2% in alcohol, and nearly insoluble in water. (That’s certainly true; adding a tiny amount of Sudan III powder to some water yields a barely pink solution with undissolved solid on the bottom of the container.)

Anyway, some of these vendors include images of the bottle, which makes it very clear that the concentration is nowhere near 1%. Here, for example, is the Home Science Tools page for Sudan III. The stuff is pinkish orange in this image. According to the MSDS, the solution is 1% Sudan III in 99% isopropanol. I don’t see how that’s possible. The best I can do is about 0.2% in 99% IPA.

And isopropanol isn’t the best choice of solvent for Sudan III. A better choice is 50% acetone, 35% IPA, and 15% water, by volume. Conveniently, that means a 1:1 mix of acetone to 70% IPA. So I just went down to the lab, measured 500 mL of acetone into a clean one-liter soda bottle, and added 2.0 grams of Sudan III powder. The solution was immediately intensely colored blood-red, and opaque even when I held the bottle up to the light. I made up the solution to one liter with 70% IPA, at which point it was still so intensely colored that it was opaque to direct light. And, not all of the Sudan III had dissolved, although I had to invert the bottle and look at the powder still in the bottom to see that dissolution was not complete. It’s possible the remainder will go into solution over time, but it doesn’t really matter to me. I’m labeling the Sudan III bottles in the kit as “saturated” rather than with a percentage. That means all I need to do is keep some Sudan III powder visible in the bottom of the stock bottle. When I need more stain solution, I can just add more 1:1 acetone:IPA to the bottle and make sure there’s still undissolved solid present.

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