Wikipedia:Reference desk/Archives/Science/2020 October 25

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October 25

Which foods becomes to hydrogen ions in stomach?

Are there foods that dissolve into hydrogen protons in the stomach? --ThePupil (talk) 06:12, 25 October 2020 (UTC)

By the definition of acid, these would be acids. Several organic acids occur in food stuff – prime examples being citric acid, malic acid and acetic acid –, but these are already in solution when being consumed.  --Lambiam 08:17, 25 October 2020 (UTC)

To be pedantic, they would form hydronium ions, not free protons. Fgf10 (talk) 10:20, 25 October 2020 (UTC)

The human stomach is already very acidic with gastric acid and the amount of any food consumed is unlikely to alter the pH there, which is maintained at between 1.5 to 3.5 by a proton pump. Mike Turnbull (talk) 11:47, 25 October 2020 (UTC)

To follow on, blood's pH is maintained tightly between 7.35 to 7.45 (slightly alkaline). If it varies from this range it can endanger health. Highly metabolic active cells (including, but not limited to cancerous) may go into anaerobic respiration, producing lactic acid. However, this is a sign, not cause (or cure) of cancers. Trying to change blood pH will have no affect on the cancer. LongHairedFop (talk) 09:08, 26 October 2020 (UTC)

Fusion reactors and the helium crisis

I had an interesting conversation recently that left me wondering how much helium "exhaust" a fully functional fusion reactor might actually produce. AIUI current experimental reactors produce minuscule quantities. If fusion reactors became "mainstream" could they solve the helium shortage? Roger (Dodger67) (talk) 16:52, 25 October 2020 (UTC)

Not likely. Remember e=mc² and the amount of energy it means you get from fusion. According to [1], the amount of fuel they estimate is only 250 kg per year. I'm not sure what sort of power they're expecting from such a reactor they compare it to is a 1000 MW coal fired power plant but given the amount of fuel, and it being a fusion plant, I sort of expect it's a lot more. But anyway, even if it all becomes helium, this will only mean under 250 kg per year of helium. This paper [2] estimates that because of helium used in various parts of the plant some of which will be lost during operations, such power plants will be significant net users of helium. They also estimate 555 kg production from one plant and 2760 plants for a 30% base load for the world. That said, they also estimate it may be possible to simply recover enough helium from the atmosphere using energy from the plant to make up for the helium lost from the power plant. Nil Einne (talk) 11:21, 26 October 2020 (UTC) 13:00, 26 October 2020 (UTC)

To be clear, we're not talking about annihilation, so we're not talking about all 250 kg being released as energy which would also mean you're not getting any helium. However e=mc² still helps explain why we need so little fuel for fusion. I'm not sure what the mass difference actually is hence why I just said under 250 kg. Nil Einne (talk) 11:40, 26 October 2020 (UTC)

I hope I get this right, a kind of a back-of-the-envelope estimate of an upper bound on the helium production. In deuterium–tritium fusion, for each atom of helium produced, the energy gain is about 17.6 MeV, of which, however, about 80% is that of the emitted neutron, which is hard to use; so conservatively 20%, or 3.5 MeV, is usable energy; which is about 156×10⁻¹⁸ Wh. The mass of a helium atom is about 4 Da, or 6.64×10⁻²⁷ kg. Taking a world energy consumption of 140×10¹⁵ Wh per year, and assuming, extremely (and unrealistically), that 100% is supplied by deuterium–tritium fusion with 3.5 MeV usable energy per reaction, the amount of helium produced in a year is (140×10¹⁵ Wh/156×10⁻¹⁸ Wh)×6.64×10⁻²⁷ kg, which comes out at about 6×10⁶ kg.  --Lambiam 23:53, 26 October 2020 (UTC)

Two notes on that: First, most of the energy of the neutron is absorbed by the walls of the reaction chamber, heating the walls. This is where the energy from the reaction is usually expected to be extracted (some alternatives, more efficient and more difficult, are being considered). Most neutrons never leave the walls of the reaction chamber. The energy of the neutron is not completely lost, although it is lost from the plasma, which is more of a problem. Second, the tritium in the deuterium-tritium reaction must be produced first. This is done by using those neutrons and having them react with lithium in the walls of the reactor, producing another helium atom as a byproduct. The net reaction in a deuterium-tritium fusion reactor is therefore deuterium plus lithium to two helium atoms. But neither of that changes the order of magnitude of your back-of-the-envelope estimate (which is correct, I checked). PiusImpavidus (talk) 10:33, 27 October 2020 (UTC)

For comparison, Helium production in the United States says the US produced 73×10⁶ m³ in 2014. Assuming that's at room temperature and standard atmospheric pressure, where He has a density of 4 g per 24.5 L, I ballpark the production as 12×10⁶ kg. DMacks (talk) 14:48, 27 October 2020 (UTC)