Introduction: Crystallization of Homemade Sodium Acetate
(Updated - see end, below)
If you've ever wanted to play around with sodium acetate, but you're too much of a nerd to simply go online and buy some from a chemical supply house - noooo, that'd be too easy, you want to make it, from scratch - then this instructable is for you.
Inside, I show the whole process, from baking soda and vinegar, through concentration and filtering, to final crystallization.
(Please note that you, not I, explicitly assume all risk associated with playing with chemicals, fire, or hot things. Use common sense. If you're not an adult, enlist the help of a parent. If you're an idiot, close your browser now before you burn yourself. And regardless, by reading any of the suggestions contained herein, you implicitly assume full responsibility for any and all accidents, burns, lacerations, ruptured spleens, loss of consciousness, death, shin splints, hangovers, spurned advances, or insolvency that may result. Seriously... use your brain.)
Update: When I first wrote this Instructable, I somehow got it in my mind that acetic acid had a boiling point that was slightly lower than water - this is incorrect! Acetic acid (ethanoic acid) has a boiling point of 118.1 C. This may change some steps and I may amend the instructions after I've had a chance to play with a few things; for now, though, when I talk about acetic acid boiling off in some of the steps, take it with a grain of salt (or at least sodium bicarbonate). :-)
Step 1: Preparation
We'll be making approximately 330 mL (or 11 fl oz) of supersaturated solution.
You will need the following items:
- One 16 oz box of baking soda (sodium bicarbonate)
- One gallon jug of distilled white vinegar (acetic acid)
- Clean pot for boiling (5.5 Qt or larger)
- Another clean pot for filtering (4 Qt or larger)
- Pack of coffee filters (basket style, not funnel shaped)
- Small wire mesh strainer (big enough to hold a coffee filter)
- One cup measuring cup for pouring hot solution through filter
- Large clean cooking spoon for removing samples while boiling
- Small clean dark dish (e.g. custard cup) for holding samples
- Clean jar to hold final solution
- Distilled water (in case you over-boil the final solution - see step 14)
Optional Items - this is for a purification step that I found I needed (see step 6 for an explanation). If you decide to do this too, you'll need:
- Two cups granulated activated charcoal (from drug store or pet supply)
- Lab stand with burette clamp (if you have this)
- Funnel (if you have a lab stand; should be big enough to hold folded coffee filters - see step 10)
Step 2: Combining Baking Soda With Vinegar
The first step is the fun part!
Start by pouring all but one cup of the white vinegar into the boiling container (5.5 Qt or larger). (See note about acids below...)
Then, carefully add baking soda to the vinegar, small amounts at a time (no more than a tablespoon). Sprinkle it over the vinegar - don't just dump it in. If you add too much baking soda too quickly, the foam from the reaction may overflow your container.
Stir gently after each addition of baking soda to ensure no unreacted bicarbonate remains.
After adding about a half box of baking soda, you should notice the reaction starting to slow down. At this point you may want to reduce the amount of soda you're adding each time to a teaspoon. When a teaspoon (or less) of baking soda sprinkled over the solution no longer bubbles instantly but bubbles very sluggishly as it sinks into the liquid, it's time to stop. In my experience, it's taken roughly 12 oz of baking soda to get to this point (about 3/4 of a 16 oz box).
Finally, add the retained cup of vinegar to the liquid. Since judging the stopping point can be difficult to eyeball, I've found it helpful to withhold a small amount of vinegar and add it after the baking soda reaction almost stops. This way, we err on the side of excess unreacted acetic acid (which will largely boil off) rather than excess unreacted sodium bicarbonate (which will not, and can interfere with crystallization).
(Note on acids: yes yes, I know, we're all taught in science class that you never add X to acid, you add acid to X... Good! Brownie points for remembering that! This stuff, however, is a 5% solution of acetic acid, and personally, I'm not too concerned about burning my skin off. If you're too uncoordinated to keep it out of your eyes, wear goggles.)
Step 3: What's Going on Here?
When you mix baking soda with vinegar, what you're really doing is reacting sodium hydrogen carbonate (more commonly, sodium bicarbonate) with acetic acid. Those components react, producing sodium acetate and carbonic acid; the carbonic acid immediately decomposes into carbon dioxide gas and water.
Step 4: Boiling Down the Solution
The reaction is finished, and we're left with sodium acetate dissolved in water (with a little excess acetic acid). Now, theoretically, all we have to do is boil off the excess water until we get to a metastable supersaturated state! Ideally, anyway. I've had difficulty with this part, but let's continue...
Put the solution over medium-high heat and...
Yes, watch it boil. This isn't terribly fun.
You don't want a violent boil, but you are trying to reduce the volume of the solution, so a happy medium rolling boil should be fine.
Take note of the color of the solution when you start your boil... it should be colorless (more on that later).
The observant reader will correctly infer from the pictures that I'm making a double batch.
Step 5: Boiling, Continued...
By the time you've reduced the solution to about half of its original volume, you will have noticed a color change. The solution will turn a distinct straw color, and will deepen to gold and amber the more it's reduced.
I've never found a satisfactory explanation for the color change - almost certainly it's due to organic impurities in the vinegar, owing to the natural source of the acetic acid.
My early attempts at getting the solution to a metastable supersaturated point all failed... I would get crystals if I let the solution cool overnight (with or without crystals having formed immediately on the surface of the liquid), but it seemed as though I could never get a supersaturated solution that, when cooled, would immediately crystallize when a seed crystal was introduced.
The next few steps attempt to remove as much of the impurities as possible, and it is up to you whether you want to follow them or continue boiling. (If you decide to skip them, go to step 11.)
(Note about the liquid volume in the picture - recall that I'm making a double batch. There's about a gallon of liquid in that picture, which is half of what I started with.)
Step 6: Purification With Activated Charcoal
In this next series of steps, we'll use granulated activated charcoal to clean up any large molecules that could interfere with our desired crystallization later.
Stop boiling the solution and let it cool to at least close to room temperature. If you're impatient, you could cover the pot with a lid and set it in a sink full of cold water and ice (or if it's cold enough outside, cover it and set it outside for a while).
Step 7: Adding Activated Charcoal
After the solution has cooled to near room temperature (or lower), add two cups of activated charcoal directly to the solution and stir gently for several minutes. Let it sit for a while, maybe ten minutes or so, before moving to the next step.
(In the first picture, I'm using a 1/4 c. measure - while this should be more than enough for the liquid in the pot, after I took the picture I decided to trade charcoal for speed, and ended up adding two cups, rather than setting up a funnel with a coffee filter to hold the charcoal, through which the solution could be poured.)
Step 8: Removing Activated Charcoal Granules
We'll remove the charcoal granules and residual carbon particles in several steps.
First, pour the solution through a small wire strainer into your second pot. This will remove the granules and large particles. If you don't wish to save the charcoal for another project, you could skip this step (the next filtering step will remove them just as well, and it won't significantly slow down that step).
Step 9: First Filtering to Remove Most Carbon Particles
Now place one coffee filter in the strainer and begin pouring your solution into it. Be careful not to overfill the filter - if some of the solution overflows the filter, you'll have that much more residual carbon to remove later. (It's not terribly important for this step, but will become extremely important in the next two filtering steps.)
After two or three fills of the filter, you'll notice the solution taking longer and longer to flow through the filter. When it slows noticeably, stop adding solution, pour the solution remaining in the filter back into the unfiltered container if you wish, and discard and replace the filter.
Continue until all solution has been filtered once.
Step 10: Second Filtering to Remove (most) Remaining Carbon Particles
From this step forward, cleanliness will be of utmost importance to avoid getting dust or other contaminants into the solution you're working so hard to filter.
First, heat up the solution. It doesn't have to be boiling, but a hot solution will give a higher flow rate through the filter. While it's heating, wash out the empty container you're going to use to receive the filtered solution; if it's the one you poured the solution out of in the previous step, don't just rinse it - wash it with a little soap and a cloth or rag, to make sure all carbon particles are removed. When you finish rinsing it out, do not dry it with a towel or anything else! Shake it out, but leave it wet.
This filtering step will use six coffee filters at a time. If you're lucky enough to have a lab stand with clamps, you can use it to hold a funnel over your receiving container as shown in the picture (get 'em cheap at American Science & Surplus). If not, no worries - you can use your strainer instead. If you use the strainer, just be certain to wash it carefully beforehand (again, wash - not just rinse), so that no carbon particles remain that could contaminate the filtered solution.
Place six filters, nested together, into your clean strainer and set it in position on your receiving
container; or, if you're using a funnel, fold the stack of filters the way you were taught in science class - flatten them into a circle (all together, not individually), fold the circle in half, fold that in half again, and open one "pocket", forming the folded filters into a conical shape that fits into the funnel.
Using a one cup measuring cup (or similar small container with an appropriate lip for pouring), fill the filter with hot solution and let it drain. Keep transferring hot solution into the filter until all solution has been filtered. If flow through the filter slows down too much, you can replace the filters. Be careful not to overfill the filter.
While you're waiting for the solution to filter, blanch one head of cabbage... (oh wait, nevermind, that was for dinner...)
Step 11: Final Filtering Step
If you've gone through the purification steps, this will be the final filtering step (if not, this is the only filtering step). The solution should be hot (if you skipped purification, turn down the heat so the solution isn't actively boiling but is just being kept hot, and go read step 10 which talks about setting up filtering with either a strainer or a funnel).
Clean the strainer and the receiving container and place the strainer in position (or, if using a funnel and lab stand, prepare the funnel). This filtering step will use ten coffee filters at a time. Place ten coffee filters, nested together, into the strainer (or, folded properly, into the funnel).
Using a one cup measuring cup (or similar small container with an appropriate lip for pouring), fill the filter with hot solution and let it drain. Keep transferring hot solution into the filter until all solution has been filtered, taking care not to overfill the filter.
Step 12: Final Boiling - Evaporating Excess Water
We're almost done! We've got sodium acetate dissolved in excess water; now we just have to evaporate off excess water until we get to the right ratio of sodium acetate to water.
This part can be tricky, and it takes a fair amount of patience.
Start by bringing the solution back up to a gentle boil. Take note of how the solution "sounds" while it's boiling - specifically, how the bubbles sound. When you start to reach saturation, you'll notice that the sound of the bubbles has changed - crinkly and crackly is the best way I can describe it.
By the time you've reduced your solution by about half (see pictures), it will have turned a golden color, and you'll see a spray of fine white powder all over the inside of your pot (and, unfortunately, on your stovetop around the pot - fear not, this stuff is extremely easy to clean up).
Step 13: Final Boiling - When to Stop
When your solution has been reduced by a little less than half (less than that if you're not sure), we'll begin removing very small samples to see if they crystallize.
Place several ice cubes or a quarter cup or so of crushed ice in a small bowl.
Using a clean cooking spoon (or small ladle), transfer a very small amount of solution (one to two teaspoons) into a clean custard dish or other small bowl; to watch for crystal formation, it's best to use a dark bowl.
Cool the custard dish containing the sample by setting it on (or holding it on) the ice (first picture). After a minute or so of cooling, blow gently on the solution to see if you can trigger crystallization. If nothing happens, you can either discard the solution or return it to the boiling solution. If you're not absolutely certain it hasn't been contaminated with dust or anything else, it's better to discard it.
Rinse the custard dish, shake excess water off (don't dry it with a towel or cloth), and take another sample after 5-10 minutes.
Eventually, you should be rewarded by seeing long, thin crystals bloom across your sample as shown in pictures 2 and 3, below (taken about 2 seconds apart).
As soon as you see this happen, remove the solution from heat and put a lid on it!
Set the crystallized sample aside and cover it with another bowl or with plastic wrap for the moment - you'll want to save these crystals to use them as "seed" crystals later, to trigger crystallization in the cool solution when you're done.
Step 14: Cooling Solution
Gently pour the still-hot solution into the container you've chosen to hold the solution for cooling and crystallization. You can use a glass jar, or a flask if you have one handy. Make sure the container is absolutely clean before pouring in the solution, and loosely cover the container (with a lid, plastic wrap, or plastic baggie).
Allow the solution to cool to room temperature. You can refrigerate it or put it in an ice water bath if you wish.
There's a small chance that some of the crystals on the side of the boiling pot could have gotten washed into your container when you transferred the solution, in which case your solution may spontaneously crystallize while it's being cooled.
If this happens, recharge the solution while still in the same container (see the next step for instructions).
If you find that the solution still appears to contain crystals after double boiling for 20 minutes or so, it's possible that you've boiled off too much water. In this case, try adding two teaspoons of distilled water to your solution, stirring gently with a clean spoon after adding it, and then continue double boiling for an additional 10-15 minutes. If crystals are still visible, repeat the process - they should dissolve eventually.
Step 15: Try It Out!
Gently set your container down on a hard surface and remove the lid or covering. Using tweezers (or clean, dry fingers), pick up one or two small crystals from the test dish you set aside in step 13, and drop them into the solution.
You should see a mesmerizing "bloom" of long, thin crystals growing outward from the crystals you dropped into the solution, growing at maybe 1 cm / sec.
Pick up the container after crystallization finishes - note how warm it is. That's excess energy, effectively stored by the supersaturated solution, and released when the sodium acetate crystallizes.
Note also that the crystals you see in the container are actually sodium acetate trihydrate. Once crystallization begins, one sodium ion and one acetate ion will join together with three water molecules. If you were to gently heat these crystals, as the water is driven off, they'd turn pure white and powdery (instead of milky white and translucent), and you'd be left with anhydrous sodium acetate.
After crystallization, recharge the solution while still in the same container by double boiling it - place the container into a pot, fill the pot with water to or just below the level of the solution in its container, and bring the water to a boil. If your container is sealed (with a lid or stopper), you'll want to break the seal first to allow for air expansion and contraction during heating and cooling.
Here's a video showing about 525 mL of my solution doing its thing:
Step 16: Additional Purification
As an addendum, I've discovered a simple way of further purifying the crystals, should you wish to do so. I've no pictures, but I wanted to at least add this description for those of you obsessive enough to consider it.
I knew that sodium acetate is not very soluble at all in isopropanol (isopropyl alcohol), and it occurred to me that i could essentially "wash" away most of the impurities from the crystals after crystallization. There's no real chemistry here - it's just the mechanical action of the liquid isopropanol that does the trick.
Crystallize your solution in a beaker or a bowl; after it's completed, use a clean glass rod or a clean fork to break up the crystals as thoroughly as possible, yielding an amber colored mush of small crystals and remaining liquid and impurities.
Then, a couple of tablespoons at a time, "wash" the crystals with isopropanol in a coffee filter in a small strainer over an appropriate collection container. It's best if you can use a wash bottle to hold the alcohol; otherwise, pour it in small amounts at a time. You can agitate the crystals with a glass rod or small spoon. To help with drainage, periodically sweep away the crystals from the center area of the filter (be careful not to rip the filter doing this).
I went through approximately two liters of isopropanol to wash my entire batch of crystals. It's not very cost-effective, but it's worth the fun of learning.
After cleaning the crystals, you'll want to eliminate residual isopropanol. The only practical way to do this is to spread the crystals evenly inside a large baking dish and heat them, driving off both the isopropanol and the water. I did mine in several small batches in my oven, at 230 degrees F or so, holding the oven open slightly by closing it on a heat-proof trivet, which allowed some air circulation (to avoid igniting the isopropanol vapor).
Check the crystals after a half hour or so. You may need to break up still-moist areas with a spoon. As the water is driven off, the trihydrate will turn into plain sodium acetate (anhydrous). When it's finished, it will probably be a light, puffy mass.
Once the crystals are dry, you will need to redissolve them in fresh distilled water. Use about 50% more water than the volume of the solution you started with. (That is, if your supersaturated solution was 200 mLs, add 300 mLs.) Heat the solution to almost boiling, filter it to remove any dust or other contaminants (using 4-5 coffee filters - see step 10), then boil off the excess water again as you did in step 12. You'll need to boil off at least the 50% extra volume of water that was added. A small amount of sodium acetate will have been lost in the washing step, so the resulting volume of supersaturated solution will be somewhat less than what you started with.