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I am conducting research for a new drug that contains powdered sodium bicarbonate, and the drug will need to be sterilized after placement into it's container/closure system. Typically, this is performed with steam, at 122.5 °C for 15 minutes, but in the case of sodium bicarbonate, this would cause decomposition:

$$\ce{2 NaHCO3 → Na2CO3 + H2O + CO2}$$

The alternative is to sterilize with ionizing (gamma) radiation. Will this also cause decomposition? Any help or referral to another authority is much appreciated.

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    $\begingroup$ You may already be aware, but the term you're looking for is "radiolysis". A cursory Google search shows some results for bicarbonate radiolysis, but I haven't parsed anything. My instinct is that while it could theoretically cause some decomposition, almost all of the bicarbonate will remain intact in any reasonable sterilising gamma flux. Living matter is far more fragile than an inorganic salt. $\endgroup$ Dec 4, 2018 at 9:14
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    $\begingroup$ Long story short, don't worry: the drug will decompose way sooner than bicarbonate. $\endgroup$ Jan 29, 2020 at 13:13

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I think you are looking in the wrong direction.

If you subject oxoanions such as sulfate or nitrate to intense gamma ray fields then you will see decomposition of the anions to form oxygen and versions of the anions where they have fewer oxygen atoms per anion. So sulfate could be decomposed to sulfite or even sulfide by the radiation.

Carbonate when it is irradated does form formate, which is then transformed into acetate and then oxalate by further irradation. But the amount of formate, acetate and oxalate formed is very small.

A recent study considered this matter and an estimate was made of the formation of these things in dilute (0.2 mM) carbonate solution. In this medium using alpha particles the yields were

400 picomoles per joule of alpha radiation of formate, 1.7 nanomoles of acetate per joule and 1.0 nanomoles of oxalate per joule.

See Carboxylate anion generation in aqueous solution from carbonate radiolysis, a potential route for abiotic organic acid synthesis on Earth and beyond, Johan Vandenborre, Laurent Truche, Amaury Costagliola, Emeline Craff, Guillaume Blain, Véronique Baty, Ferid Haddad and Massoud Fattahi, Earth and Planetary Science Letters, Volume 564, 15 June 2021, 116892.

https://www.sciencedirect.com/science/article/abs/pii/S0012821X21001515

But the doses required to form these substances in large amounts will be very high. One of my academic interests is the radiation chemistry of organic materials, I happen to have access to a research irradator.

I also worry that at large doses of gamma radiation that the organic compound will degrade, the typcial dose required to make food and medical products sterile is 10 kGy (ten thousand joules of radiation per kilo of matter). This is at the bottom end of the dose which changes organic matter. It is hard to see changes in most plastics at 10 kGy.

When we get into the MGy region it is very easy to see changes in materials, I have seen wood and plastic change greatly after MGy doses of radiation. It would be unlikely that anyone would want to deliver MGy doses of gamma rays to medical products or food during the production process.

But back to your question, It is important to understand that gamma rays are "heat rays", they do increase the internal energy of objects. Thus by increasing the internal energy of objects the temperture of the objects increase. I have felt the warmth of the decay heat of a very strong gamma source. The source was in the range of about 1 PBq of cobalt-60, I touched the outside of a very thick lead cask containing the source. I could feel the warmth of the cask. I also have it on good authority (from the late John Peckett) that kilo lumps of plutonium are warm to the touch. If you pick one up with a gloved hand or keep it in a plastic bag then it feels warm.

It would be hard to imagine a situation where the gamma radiation level is sufficent to heat sodium hydrogen carbonate to the point at which it decomposes to sodium carbonate and carbon dioxide. The atomic bomb works by unleashing a vast flux of gamma and X-rays which then heat the air, rocks, water or whatever near it to an almighty temperture. This is the mechanism by which blast is generated by an atom bomb.

Consider water for a moment, the heat capacity of water is 4181 joules per kilo per kelvin, so if we were to deliver gamma rays to water which was kept in a perfect dewar. Then we would need 334480 joules per kilo to make the water boil. This would be a dose of 344.5 kGy. I predict that at these doses that many organic substances will degrade a lot.

I assume that the irradation machine will not be able to deliver the dose very quickly, so heat losses during the irradation will be significant. This will increase the dose required to boil the water. I am aware of a fatal accident in a commercial sized gamma irradation plant. In the irradation chamber the dose rate was about 10 Gy per minute. If we consider this as a "typcial" food irradation plant then it means that 33448 minutes (557.5 hours, 23.22 days) of irradation will be needed to make water in a dewar flask boil if the water enters the plant at 20 oC.

In real life a lot of the heat energy deposited in the water will be lost to the surroundings which will greatly increase the dose required to boil the water. This will greatly increase the degradation of the drug from the radiation.

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There is an equilibrium between decomposition and recombination, i.e. as $\ce{CO2}$ is given off, it joins back to the $\ce{Na2CO3}$ to form $\ce{NaHCO3}$.

$\ce{2NaHCO3 <-->Na2CO3 + CO2}$

So for moderate doses, given at moderate rates, it's unlikely great pressure would develop, but you'd need to test to see if the drug is degraded by decomposition products.

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The likely issue is the presence of any water (which can also be sourced as you noted by the heating of NaHCO3).

Here is a source to quote:

The radiolysis of water due to ionizing radiation results in the production of electrons, H● atoms, ●OH radicals, H3O+ ions and molecules (dihydrogen H2 and hydrogen peroxide H2O2).

The formation of any of such radicals would likely degrade your new drug.

So, does your irradiation, seemingly result in any visible signs of water? If yes, a possible quality issue.

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  • $\begingroup$ The drug is lidocaine (an injectable weak acid aqueous solution), and will be kept in a separate compartment from the bicarb powder until just prior to injection. I expect we would find some trace water in the bicarb just from being in the filling hopper. $\endgroup$
    – JSK28031
    Jan 30, 2020 at 15:55

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