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John Cuthber

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Posts posted by John Cuthber

  1. The idea of using beta emitters directly as a source of electricity does work. The currents involved are tiny but the voltage can be huge.

    As for pointing an electron beam at an isolated metal sphere it sort of works but, once the sphere has the same voltage as the accelerator potential of the electron gun the electrons are repelled but the negative charge and the no more current flows onto the sphere.

    You would do just as well to connect a wire from the high voltage source to the sphere directly.

  2. The reactants get turned into products so, at any given time the rate of change of ammount of reactant with time is negative.

    Imagine we start with a gram of reactant and that after 5 minutes there is half a gram left. The change in amount of reactant is minus half a gram and the change in time is 5 minutes so the rate of change is -0.5/5 ie -0.1g/min

    Since we prefer our constants to be positive we chose to use the expression

    dx/dt=-k[A] (with a minus sign in it.)

    It's a matter of convention; we could use rate negative constants but they would look odd.

  3. It's not exactly a well kept secret what goes into the tablets.


    Active Ingredients: Each Tablet Contains: Aspirin (325 mg), Citric Acid (1000 mg), Heat Treated Sodium Bicarbonate (1916 mg). Alka-Selzer in Water Contains Principally the Antacid Sodium Citrate and the Analgesic Sodium Acetylsalicylate. Inactive Ingredients: None.

    BTW, the equation in the first post doesn't reflect what is going on here; in fact that reaction goes the other way.

  4. Nitric acid reacts with HCl which pretty much rules it out.

    Often the scheme is to add an acid to a salt then distill out the acid. There needs to be a reason why the reaction goes in the right direction ie the way you want. With sulphuric acid (boils about 300C) you can remove the acid you want eg HCl, acetic, nitric by distilling it. If you were to mix HCl with sodium acetate you would certainly get acetic acid produced. However if you were to try to distill out the acetic acid at least some of the (volatile) HCl would also come over. With H2SO4 that's not going to happen.

    H2SO4 is also cheap (industrially, rather than from the shop up the road) and a strong acid.

    Phosphoric acid might work for some of these reactions too- it's available and cheap. It's not as strong an acid but that doesn't matter so much if you can remove the product by distillation.

    Unfortunately hot H3PO4 kills glassware.

  5. There are about a trillion hits on Google for "amino acid analysis" HPLC. Most of them involve reacting the amino acids with something to make them easier to detect and less polar.

    This one seems pretty typical.


    I can't say I've tried it but it looks like a good place to start.

    Of course, that's OK for a UV or fluorescence detector. LC/MS can be a bit fussy. You will need a volatile buffer rather than sodium acetate and you might need some clean-up stage.

  6. "How can I tell if a particular bored is made of Gold or something less valuable like copper?"

    Gold is golden in colour whereas copper is copper coloured.

    The gold plating is very thin If you are setting up a "collection of the elements" then old circuit boards are one of the few places you will get gold for free.

  7. Part of the problm for making a hexanitrobenzene is that each nitro group you add removes electrons from the ring and makes it more difficult for the next substitution to happen.

    It's possible to add methyl groups to a benzene ring using chloromethane (or, more practically iodomethane) and a lewis acid catalyst like AlCl3.

    If you do this you tend to get more than one methyl on each ring because the methyl benzene is more reactive than benzene itself and, similarly the dimethyl benzenes are more reactive still. Getting all 6 to react isn't as easy as it looks because of steric effects.

  8. Most of the alphas just crash into the Bi and are slowed down, pick up a couple of electrons and are converted to He gas.

    A tiny percentage are caught by the Bi nuclei and form 210At.

    That At will, in time decay largely by electron capture (and give off a gamma ray) Partly by alpha emmission.

    Of course, that all depends on having 206Bi to start with and, since it has a rather short half life (about 15 days), you almost certainly don't have any.

    I doubt that the relatively stable 209Bi traps alphas to any great extent - not least because it's a (very weak) alpha emmiter.

    Overall, I'm not sure what budullewraagh is talking about.

  9. I guess he wants to know why, unlike most indicators, starch is added near the end of the titration. The fact that this is true for thousands of titrations doesn't really matter.


    Anyway there are several reasons.

    Starch is a reducing agent (especially if it hydrolyses to glucose) so you don't want to add it too soon or it may react with the oxidant and upset the result.

    More importantly when the starch reacts with a large excess of iodine it tends to "trap" some of the iodine so, even after enough thiosulphate has been addded, the mixture stays blue for a while and you tend to add too much thiosulphate. This spoils the accuracy of the titration.

  10. If I get a piece of glass rod and clamp it vertically, (held at the top) then heat it in the middle eventually, when the glass melts, it will snap in 2 under its own weight.

    I bet that's not what "hello help100" had in mind, but it does show that the temperature may have an effect.


    My guess is that he's on about shattering glass by heating it quickly. The problem is that taking ordinary glass and quencing it in liquid nitrogen will shatter it too. You could say that it breaks at roughly minus two hundred degrees.

    With some types of glass like fused quartz it's practically impossible to shatter them this way. With other glasses it's quite easy.

  11. I thought the body's acetate pool was turned over pretty fast.

    Anyway, measuring traces of formaldehyde in beer would be easy enough in a lab but (at least as far as I can see) rather difficult at home.

    I can't see why on earth anyone would add something to their product that would make people like it less. Beer doesn't go off so long as you keep the dirt out of it so I can't see HCHO as a preservative- you would need quite a lot anyway and the stuff would be undrinkable.

    What's your friend's evidence for the addition of formaldehyde to beer?

  12. I don't see what the big deal is.

    Mathematically 0.001 is the same as 1/1000,

    so 0.001 M is the same as 1/1000 M

    Since multiplication gives the same answer whichever way round you do it 1/1000 M is the same as M X 1/1000.

    Multiplication by 1 does nothing so you can leave it out and get M/1000

  13. Al is relatively transparent to X rays.

    Thin layers of Al are transparent to ordinary light (and are used as beamsplitting mirrors).

    Al2O3 (corundum, btw; carborundum is SiC) is transparent to, not just visible light, but quite a long way into the UV and IR as well.

    I will be impressed by the plasic with a better strength to weight ratio at say, 400C.

    What was the original question meant to mean?

  14. Gallium and tin are probably the 2 commonest (after water) things that expand on freezing. The answer involves the lattice formed from water, being held together by relatively weak hydrogen bonds, has quite big gaps between the molecules. When it melts the molecules can slip into those gaps.(And I'm afraid I haven't a clue why Ga and Sn do this, it may be related to the fact that they have unusually long liquid ranges)

  15. What are you on about?

    Ever heard of a high pressure mercury lamp?

    They have relatively large amounts of mercury in them and they work quite well at producing UV light.

    Anyway, as has been said, the little Hg in an ordinary lamp isn't a problem. If it's an old enough lamp then the Be in the phosphor might be.

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