Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol at 15 degrees celsius.
In that case, the question would be whether or not THC dissolves in ethanol.
In any case, the crux of the issue here appears to be whether or not THC is broken by citric acid.
The answer is yes. If you mix THC and citric in a solution of ethanol, the citric will break down the THC due to it's acidity - according to you LITM. The chemist in me would like more details on this please. A link to research that deals with THC breakdown by acids would be much appreciated.
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OP:
If you want to know whether or not cirtic acid would break down THC in the body, the story is a little different: THC and it's metabolic byproducts are absorbed by fat in fat cells, due to their lipophilic nature. These compounds leak slowly back into the bloodstream and then into the urine. In order to eliminate these smoking gun chemicals, the citric acid, or more specifically, the H+ ions from the citric acid, would have to get into the bloodstream to reach the fat cells. There are phisiological reasons why this cannot/must not happen.
Assuming that the H+ ions will be absorbed in the first place, and assuming that they will distribute evenly about the body, it can be inferred that in order for the THC to reach a low enough pH to be broken down, the rest of the body must also be brought to this pH. I know from biology that the the blood is maintained in a tight pH range, it must be held between 7.35 and 7.45, only .1 pH unit. A large deviation outside this range would kill you. Even a change to a pH of 6.5 would be far to large, that would be 6.3 times more protons in the blood, impossible to do without killing yourself. I assume that the pH of the solutions that LITM is referring to would be far lower than 6.5.
If you want to know if citric acid could be used to remove THC from a metal surface then the answer may be yes, if LITM is correct. A much easier way to remove THC from metal would be to use hot water and a detergent, washing up liquid would be effective.
If you mean spreading powdered or small crystals of solid citric acid on a metal surface which has THC on it, the citric will not react with the THC. To do so, they would need to be in solution together because they do not mix well on their own, as I have explained in my previous post.