Tuesday, 19 February 2013

08:50 – Sometimes I do really moronic things. Yesterday, I was making up two liters of the nitrogen-free Fertilizer C concentrate for the biology kits. That solution contains trace amounts of boron and cobalt, but the primary chemical is 194 grams of calcium acetate monohydrate.

So I went to the chemical stock shelves where I found a 500 g bottle of reagent-grade calcium acetate monohydrate. The only problem was that it had only a few grams of the chemical left in it. Rats. Normally, I reorder when I get short on something, but in this case I’d apparently forgotten. Either that, or I have a new bottle somewhere that I don’t remember. Oh, well.

But I had plenty of glacial acetic acid and calcium hydroxide on hand, and it’s easy enough to synthesize calcium acetate by reacting those two chemicals. One mole of calcium hydroxide reacts with two moles of acetic acid to form one mole of calcium acetate and one mole of water. Simple enough, and it doesn’t even foam as calcium carbonate would.

So I calculated the equivalent masses. The 193 grams of calcium acetate I needed was 1.10 moles, which corresponds to about 81.5 grams of calcium hydroxide. I then calculated the amount of glacial acetic acid I needed. The molar mass of acetic acid is 60.05 g/mol and its density is 1.049 g/mL, which means 1.10 moles would be 66.06 grams or 62.97 mL. But my reagent-grade acetic acid assays at 99%, so I divided those values by 0.99 and weighed out 66.73 g of acetic acid.

Calcium acetate is unusual in that it exhibits retrograde solubility. That is, it’s less soluble in hot water than in cold. I checked the solubility at room temperature and decided to dilute the acid aliquot to about 600 mL to provide enough water for the calcium acetate to dissolve. I then added the dilute acid to the calcium hydroxide, expecting the powder to disappear relatively quickly as it reacted with the acid.

Calcium hydroxide is only very slightly soluble in water, so as I swirled the slurry I wasn’t surprised to see a lot of suspended solids. I was surprised that the solids settled out on the bottom of the vessel, showing no sign of disappearing. Anhydrous calcium acetate is essentially odorless, but the more common monohydrate form and calcium acetate solutions have a very distinct odor. It’s not at all unpleasant, and has a slight undertone of vinegar that’s caused by the calcium acetate hydrolyzing to calcium hydroxide and acetic acid. So I sniffed the reaction vessel and detected a strong odor of calcium acetate but no vinegar odor at all. You’d think that’d have given me a clue, but no. I decided that the reaction might be slower than I’d expected, so I decided to give it a while to finish reacting.

It finally hit me. Duh. One mole of calcium hydroxide reacts stoichiometricly with two moles of acetic acid, not the one mole I’d added. So I added another 67 g of glacial acetic acid and swirled the reaction vessel. Sure enough, nearly all the solids disappeared, but after standing there was still no odor of vinegar. So I added a few extra mL of acetic acid, just in case. It’s not like a bit of extra acetic acid will harm anything. We supply this concentrate in 15 mL bottles. Diluting 125 mL of part A, 30 mL of part B, and 15 mL of part C yields 12.5 liters of nitrogen-free fertilizer working solution, so this concentrate ends up being diluted with more than 800 parts water.

13:50 – I now have all of the solutions made up for biology kits. Except for the ones for which I was short of chemicals–I have enough for at least 60 bottles of each of those–I made up enough for 120 or more kits, which should hold us for a while. Now to start filling bottles.