I have the following observations:

  • When I put a solution of glucose (80 g/ℓ) and ammonium chloride (20 g/ℓ) in a drying cabinet at 37°C (maybe a bit warmer) a brown substance forms when all (or most) of the water has evaporated.
  • When I add water to the reaction product, the substance dissolves within minutes (under shaking), but the resulting solution is brownish. From this I conclude that a reaction happened and the brown colour isn’t just the colour of some salt crystal.
  • If I perform the same procedure on glucose solution or ammonium chloride solution alone, nothing similar happens, which indicates that it’s not a reaction with the air or the container. PS: If I redissolve the product of drying ammonium chloride alone, the result is acidic (pH of 5 or lower for 1 g/ℓ).
  • No browning happens if I replace ammonium chloride with sodium chloride, which indicates that chloride is not responsible for the reaction.
  • The same thing doesn’t happen in the presence of water. Even when the solution has already been dried to a highly viscous sludge, I still don’t observe any browning. It only happens in final stage of drying.

My questions are: What reaction is this, why does it only happen when drying, and is there some way to prevent this, e.g., with a lower temperature? I ideally want to get a dried residue containing glucose, ammonium, and chloride in my container. Just filling in the powders doesn’t help, because I cannot do this efficiently and precisely.

What I found so far is this:

  • Typical browning reactions of sugar (Maillard reaction, caramelisation) require much higher temperatures or additional ingredients.
  • According to the paper Reactions of Free Sugars with Aqueous Ammonia, some reactions between sugar and ammonia happen at this temperature, but with a low yield and in the presence of water.

When a sugar solution turns brown (or black), it's generally a sign of polymerization of the sugar molecules triggered by dehydration reactions that create more reactive molecules. A classic example of this is caramelization, in which sucrose is dehydrated by heating and forms many types of furans and pyrans and the oligomers caramelan, caramelen and caramelin. The latter three all contribute to the brown color.

Although caramelization typically requires heat, a similar effect can also be achieved by use of an acid catalyst. A simple example is treatment of sucrose with concentrated sulfuric acid, which very quickly results in a solid black mess.

In your case, you've started with a nearly neutral solution of glucose in ammonium chloride. The solution is nearly neutral because it is a 1:1 mix of a moderate base (ammonia) with a strong acid (HCl). As you evaporate off the water, however, some ammonia evaporates as well, resulting in a slight excess of HCl and a lowering of pH. The combination of mild heat and acid is likely causing a caramelization-type reaction at a lower temp than is usually required. The ammonia might also enhance the color via a Maillard-type reaction.

A good review of carmelization is: Sengar, G.; Sharma, H. K. Food caramels: a review. J. Food Sci. Technol. 2014, 51 (9), 1686–1696. DOI: 10.1007/s13197-012-0633-z

  • $\begingroup$ The pH of 0.1 M NH4Cl is given as 4.6 in scrippslabs.com/ph-of-common-reagents-at-room-temperature. Altho this is lower than most values given on the internet, all agree that NH4Cl is acidic. $\endgroup$ May 21 '21 at 12:57
  • $\begingroup$ @JamesGaidis - I also would expect a solution of NH4Cl to be somewhat acidic, but OP said in a now deleted comment that it was pH 7 and went down to 5 during evaporation. I assume he adjusted the pH with a bit of base when he made the original solution or something like that. I wrote "nearly neutral" as a hedge to just keep it simple and because I wasn't sure exactly how he made the solution. I should have mentioned that the relative weakness of ammonia as a base compared to the acidity of HCl means net acidic. $\endgroup$
    – Andrew
    May 21 '21 at 13:35
  • $\begingroup$ OP said in a now deleted comment that it was pH 7 and went down to 5 during evaporation – It turns out that my pH measurement did not work on NH₄Cl for some reason. I now repeated it with another method and it is indeed acidic (pH of 5). Presumably my dried and redissolved NH₄Cl solution has a much lower pH than 5 as the same problems should apply. However, I cannot easily measure that one with the other method. CC@JamesGaidis $\endgroup$
    – Wrzlprmft
    May 26 '21 at 10:02

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