Workers use citric acid to “degum” distillate or crude resin intended for making distillate. There are several ways to employ it, and all require a bit of water to disassociate the acid… i.e. make it acidic by letting it ionize into H+ (actually H3O+ “hydronium” cations in water) and negatively charged citric (citrate) conjugate base anions.
Before you begin working with any acid or base, you should measure the starting pH of your hydrated (water-containing) resin solution. To do this, use Hydrion pH paper (the thin roll of yellow paper sold packed in foil in a clear plastic “tape” dispenser with a pH 1.0 to 14.0 color key on it) to test the pH of your room temperature 190 proof ethanol:cannabinoid solution. Write down the starting pH!
In all likelihood, it will be somewhat acidic, but close to neutral. This is mainly due to a natural property of ethanol; it reads slightly acidic because of the strong hydrogen bonding with water. Depending on the resin/distillate content (decarbed or not, terpenols, etc.) and concentration, the pH of the ethanol:resin solution may read between 5 and 7.
The easiest way to use citric to degum is to dissolve the distillate or crude into 190 proof ethanol (95% ethanol with 5% water in an “azeotrope”), the same as is done for Winterization, then dissolve some citric acid into it, little by little with stirring at 25 to 30°C. You do not need or want any excess water in this mixture! The amount of citric acid used is small; since it is acting as a catalyst, it does not get used up, but keeps regenerating after each reaction (rxn) to break apart the phospholipids (aka: gums, phosphatides, lecithin triglycerides).
However, especially in crude, some of the citric does get used up in a complexation (aka chelation) rxn with metal atoms and/or the phosphorous compounds, so you can add small amounts of the solid citric acid to the liquid with stirring, waiting each time for it to dissolve/disappear, just until it stops dissolving, leaving a small amount of solid citric acid in the bottom of the mixing vessel or swirling about as you stir it.
In especially gummy material, you will likely see precipitating veg oil flocculant solid “clouds or puffs” swirling in the liquid… they come from the rxn of citric acid with the phosphatide triglyceride gum, as the triglycerides break apart into separate fatty acid and glycolated phospholipid chains. This just means the rxn is working, and that you have a visibly large amount of gum to react with. Do not confuse these with the solid citric acid crystals (xtals). They look like fluffy clouds, while the citric xtals look like grains of sand. You are done adding citric acid when you either see solid citric acid xtals that refuse to dissolve or if you see no more of the flocculant precipitate forming.
Caveat & Caution: If you are trying to retain all CBD or working with high CBD crude or distillate (like from hemp), it is best to avoid the combination of acid and heat (even just above 30°C) at all times. Otherwise you may end up isomerizing your CBD to Δ8-THC.
If there is visible solid flocculant in the room temperature liquid, go ahead and filter it out, now, using vacuum or pressure filtration as you would for Winterization. It is extremely fine sediment, so it really helps to use a 3-dimensional matrix of filter media over your standard 2D filter paper. This can be a cake of filter aid, like fine laboratory grade silica (e.g. 220 mesh sand) or calcined diatomaceous earth, such as Celite brand (make sure it is documented to have a neutral pH in water… or see ☆ below for this special case). You can also use glass wool or (my favorite) pure cotton.
The next, often mistakenly skipped, step is to fully precipitate the flocculant by chilling. This is done exactly the same way you would perform Winterization. After all, you just turned the gums mostly into the very same sort of vegetable oil you would normally Winterize to remove. Chill and filter in small aliquots (parts of the whole liquid body… in order to keep it all as cold as possible or practical during filtration) the same way you filtered above, using filter aid.
Now it is time to remove or neutralize the citric acid. Normally, I recommend a liquid/liquid partition extraction of the cannabinoids from the ethanol by adding excess water to louche (i.e. a milky microemulsion) the resin from the ethanol, then extracting the resin with a layer of n-alkane (hydrocarbon) like n-pentane, n-heptane, etc. Separate the now resin-laden alkane from the now depleted aqueous ethanol, and wash the alkane with some salty water. However, this is not always an option for my clients, nor is it always necessary or desirable, and I would like to introduce a new method…
☆This is speculation, as I have not tried it yet, but I am a pretty darn good “guesser”. I am fairly certain we can neutralize and possibly even sequester (lock down) the citric acid by using an alkaline Celite for filter media, but I do not know what makes them all alkaline; there are many types, so the salt products are unknown to me. If you want to use one, talk to Sambo Creeck (sic) Filtration @SamboCreeck.com about Celite filter aid options that are alkalinized with magnesium.
However, I have an idea that plays well with stuff we already use: We should be able to simultaneously neutralize, sequester, and filter out any cold-ethanol-insoluble-but-still-mobile magnesium citrate salt particles by simply adding a layer of the less expensive varieties of magnesium silicate on top of your filter aid of choice from before. Magnesium silicate, aka: MagSil, is now a very commonly available chromatography stationary phase medium known as MagSil PR for pesticide remediation in the cannabis industry… just ask @Shadownaught …but you don’t need the specially calcined PR type for this, so you may be able to get some less expensive back stock of non-PR magsil.
The best part about this is the magnesium citrate salt, even if some slips through the cold filtration, is already alkaline, not acidic, and has a high decomposition temperature of 200°C… and although that is certainly in the range of our distillation process, the decomposition products are magnesium carbonate and eventually magnesium oxide. I should also note that magnesium citrate is a complex salt that has a few different formulae, but most important, I believe, is that they are all hydrates… they require waters of hydration to exist (e.g. magnesium citrate • nonahydrate), so their formation will naturally attract and strongly bond to several water molecules, very likely drying your solution of any excess water, and possibly even breaking the ethanol:water azeotrope! Ionic complexation and requiring waters of hydration also makes the salt more likely to become part of the magnesium silicate, locking it down as a solid.
This all means that no matter what actually happens, the products of this procedure are not going to hurt and may even help your cannabinoid distillation and/or ethanol recovery process(es)!
Finally, use the Hydrion paper to test the pH of your degummed ethanol:cannabinoid solution. The pH should be the same as it was when you started, when the paper warms to room temperature after about 1 minute… unless some magnesium citrate salt made it through, which will make it more alkaline… hopefully a nice neutral 7.
However, just in case it is more acidic (i.e. ONLY if it has lower pH than it started with), let the liquid warm to room temperature in a covered vessel (to keep out water condensation from the air), then fully stir in small pinches of magnesium oxide (MgO) at a time to bring the pH to 7.
For the record, this is the first and only time I have ever recommended any pH adjustment outside of pesticide remediation… and I am really just hoping to neutralize the citric degumming acid. I am not shooting for acid or alkaline conditions at all.