Hello Joris

Thank you for sharing your question with the RG community.

Alcohols ROH react slowly with potassium hydroxide KOH to form the corresponding alcoholate ROK. These are usually yellowish in color and very likely the explanation for the coloration of your solution. The same happens, for example, with alcoholic potassium hydroxide used as a 0.5M solution to determine the saponification value.

The process cannot be stopped as far as I know. You should therefore consider whether or not the alcoholate is disturbing for the further planned use of the solution.

I wish you much success for your research.

Florian Morsbach ,

Thank you for your quick response. I did not know any of these things regarding alcoholates and their color. I will look into this.

The thing is that I know of a company whom are able to prevent this and actually deliver a product that contains this much glycol as wel as this high of alkalinity. However, they do not disclose how they do this. I've looked into APG's and though im currently trying them to see if they can inhibit this, they are also yellowish. I'm not sure whether any surfactant is able to inhibit this process, and since there are many surfactants i would not even know where to start.

Thanks anyways,

Alkali-metal alcoholates are air-oxidized to peroxides and free-radicals and then to aldehydes. The aldehydes in basic media at ambient temperature undergo aldol condensation to give beta-hydroxy aldehydes, which eliminate (Claisen condensation) to give alpha-beta-unsaturated aldehydes. These can react further via many different pathways to make cyclic systems (e.g., furans and phenols), which are further oxidized. Unsaturated aldehydes are yellowish owing to an absorption in the near-ultraviolet to violet part of the spectrum; carefully prepared alcoholates are colorless (I've prepared these many times). The color deepens through orange to red and finally black as the condensation products and oxidation products further condense into rings and chains, with concomitant oxidation by atmospheric oxygen to other species with more extended pi-electron systems. There are two ways to prevent the discoloration of the alcoholate solutions if you don't want to decrease the concentration of base. The first is to work under an inert gas in preparing and using these solutions, and to keep them in tightly-closed containers under a blanket of (for example) argon gas, in the dark when not in use. The second is to tolerate a small concentration of anti-oxidant like BHT (butylated hydroxytoluene) in your alkali-metal alcoholate solution; this second route is probably what commercial suppliers would call adding a "stabilizer." The needed concentration of antioxidant depends on the length of time the solution will be exposed to oxygen from the air. 10-100 ppm is a typical concentration. Either way the solution has a limited shelf-life because it functions as an oxygen scavenger, irreversibly reducing trace amounts of oxygen all the way to water over time. Best wishes in your chemistry.

Hi Louis Whaley ,

Thank you for this stack of information. It has really helped me confirming a lot of hypotheses I laid out.

The problem is that the decoloration is still occuring even with abnormally high amounts of Radical Scavenger (1-2%) (BHT in this case). So my question remains in the stability of the mixture and the stability of BHT in highly alkaline solutions.

You can substitute BHA as an antioxidant; it eliminates the formation of colored stilbene-like products. BHA is cheap and easier to get than more hindered phenols.

Also, if I didn't mention it before, you should keep your alkaline glycol solutions in plastic containers, not glass. Glass gradually releases iron, which contributes to discoloration as a catalyst for oxidation. You didn't mention the type of container in your reply, and your picture shows glass containers.

If you can spend some $$ then try an idea inspired by the new "non-yellowing" antioxidants for polyurethane foams such as "Omnistab AN 1135" : https://www.partinchem.com/products/omnistab-an-1135/

These were demonstrated to stop a similar yellowing problem in polyurethane foam that was formerly thought unsolvable. The principle is to use a much more hindered aminophenol than BHT. 2,4,6-tri-tert-butyl phenol would be the closest commercially available reagent like this.

Phosphites are another antioxidant used to prevent discoloration. Phosphites are slower to react with oxygen and peroxides, but are complete in the sense that they can lower oxygen levels to below detectable limits. A similar phosphite-based antioxidant from the same source above is omnistab-an-6136. Both of these "omnistab" products have side-chains that consist of branched, high-molecular weight alkanes. You don't need these necessarily, as they are intended to act like plasticizers in the yellowing polymers. Instead, you may want to test something commercially available in small amounts like tri-isopropyl phosphite or di-tert-butylphosphite at a concentration of 0.5% to see if that works better than BHT. If it does, lower the phosphite concentration until you reach the minimum amount of time for which you want the solution to remain colorless. Phosphites scavenge oxygen, becoming phosphates in the process; hindered phosphates and similar hindered phosphite esters are generally non-nucleophilic and being hindered, should have little effect on reactions. Hindered tri-alkyl phosphites are also higher-boiling than ethanol, but can be distilled off at reduced pressure. Some are checked-for as known impurities in pharmaceutical drugs.

Depending on your chemistry, it may be advantageous to know that tri-aryl phosphines also scavenge oxygen; triphenyl phosphine is a commercially-available example. Triphenyl phosphine reacts slowly with air at ambient temperature, so it must be handled quickly in air. Both triphenylphosphine and triphenylphospine oxide can be difficult to remove from reaction products. This makes them disadvantageous with respect to phosphites, which generally can be distilled under reduced pressure or extracted by water washes during workup of a reaction. Triphenylphosphine immobilized on resins might also help scavenge oxygen from your alkaline ethanol solutions, and the resin beads should be easy to remove when exhausted.

I would not recommend tri-alkyl phosphines as antioxidants as these are typically too reactive toward oxygen; they can be pyrophoric (burst into flame in contact with air), even in dilute solutions. They also participate in reactions with ketones and aldehydes to make alkenes (Wittig reaction).

It's little comfort but the best minds in industry and academia also find this problem vexing.

That's all I can contribute on the subject. Hopefully another contributor will have better ideas or more experience.