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Posts posted by UC

  1. so, you're sure that it's either NaOH or KOH? I'm only asking because Trisodium phosphate and Sodium carbonate are both frequently used for prepping and degreasing surfaces prior to painting. However, neither of the alternatives become rapidly "wet" from absorbed mositure when exposed to the air.


    My initial guess would be that you have NaOH since it's cheaper and just as effective for that kind of use. The problem with using a flame test is that the potassium emission line is very readily obscured by even traces of sodium. it's still worth a shot, because you can't get a false positive for potassium. If the flame is violet, it's definitely KOH. if yellow, it may or may not be NaOH, which brings me to this test...


    One of the quickest ways to get a definitive answer is going to be with a moderately concentrated sodium perchlorate solution. Take your mystery hydroxide and make a moderately strong solution. Mix the two. if it turns to the consistency of yogurt with gelatinous precipitate, you've got KOH. If nothing happens, you've got NaOH. This occurs because potassium perchlorate is soluble at a rate of 15g/L at room temperature. Sodium perchlorate is soluble on the order of two kilograms per liter of water at room temperature.


    If sodium perchlorate is unavailable, but you can get an authentic sample of NaOH, combine about equal weights of the mystery hydroxide and NaOH, melt, and take note of the freezing temperature, or remelt and take note of the melting temperature. Compare this to a plain sample of NaOH. KOH and NaOH form a eutectic, which melts at a significantly lower temperature than either one alone. If the unknown is simply NaOH, not much, if any difference should be noted.


    You could run a sample in solution (perhaps add an organic acid to lower the pH) through an AA spectrometer and look for K and Na. That's rather less accessible though.

  2. Hum ... limestone is to a large extent calcium carbonate, so I think mechanically removing the limestone would be the better solution.


    What he said. If acid worked, they'd have been doing it for years. The fossil and matrix are for the most part identical chemically so mechanical removal is the only way to not ruin what you're after.

  3. No common objects are made of tin (or were ever really made of tin). Tin cans used to be tinned steel, but they've been replaced by stainless steel and plastic-lined containers. Others are just made of aluminum.


    The closest you can get is pewter, which has antimony and copper added for hardness.


    It's relatively cheap to just buy bulk tin metal. Some places now offer lead-free fishing weights made of tin, but they are very expensive for how much metal you get. For larger quantities, I recommend this place:


    Knock yourself out: http://www.rotometals.com/Tin-Ingot-s/27.htm


    It does readily stick to steel and a lot of other metals when molten and it won't come off once cooled. Graphite is pretty standard for nonstick when working with metals though.


    Tin has a very high boiling point, unlike zinc, where the fumes are a hazard. I'd buy a ceramic crucible and use a bunsen and wire gauze for melting the stuff.

  4. be extremely careful, theo... this one's not forgiving at all... spitting liquid iron around the place and generally being extremely hot and uncontrollable.


    You know plain old metal sparklers? The cheap ones? Or do they not sell them up there? Just tie one to the end of a meterstick or a broom handle or similar and touch the thermite with the lit end. So far, I've had no problems lighting it like this.

  5. You're not going to make much dry ice that way, especially since that is a very small amount of CO2 and a lot of it is wasted cooling down the remainder to freezing temps. As a demo, you're in the clear though. You're probably better off with a small sewn felt bag than a pillowcase.

  6. We had a quiz and I got one of the questions wrong and it had to do with the Tollen's test. I missed the lecture on this topic and my book doesn't do a very good job of explaining it. I've posted the question followed by the general equation for the test, which I understand. It converts an aldehyde to a carboxylic acid. The problem I'm having is applying it to something that's in a chair conformation... there is no aldehyde.


    Can someone show me what the product would be and explain why?


    The pyranose (ring form) and chain form interconvert readily in water. Pay close attention to the attached image. I'm sure you can figure out the rest. On a technical note, those arrows should actually all be double-ended equilibrium arrows. The back-and-forth conversion allows for the interconversion of alpha and beta dextrose (which is what that molecule is).


    Reducing sugars are capable or ring opening to the chain form and have an aldehyde group. Do you see the ether-like portion of the ring and the hydroxide on the next carbon over? This hydroxide becomes the aldehyde group. The configuration is called a hemiacetal. You can look that up for more information. Nonreducing sugars have another ether-like bond formed with that hydroxy group (an acetal), preventing them from opening easily in this manner.


    As a side note, some ketoses (sugars with a ketone group in the chain form, instead of an aldehyde), which are intuitively not reducing sugars can interconvert into aldoses (ones with the aldehyde) via tautomerism. This is the case for fructose.

    glucose ring opening.jpg

  7. I was thinking of filling the holes with a clear epoxy. Although I don't know what kind or where to buy it from. It would need to be injected with a syringe, I think, and most epoxy would just clog that before I was able to fill the hole.

    Merged post follows:

    Consecutive posts merged



    You've got me curious, how would an alternating current and capacitor be set up? I'm an LED newbie.


    Go to a craft store and get clear (read the package! make sure the cured product is clear, many turn yellow) relatively quick setting epoxy, which might be referred to as jeweler's epoxy.

  8. I'm a science teacher buying supplies for this year and was wondering how much of a difference 95% ethanol would be from pure. I am planning on primarily using it for flame tests. I am also going to use it as a substitute for liquid nitrogen by putting it in a beaker with dry ice. I suppose it might not be quite as cold as pure ethanol, but that's the only drawback I can think of. I almost feel silly posting a question like this, but I thought it wouldn't hurt to ask- maybe I'd get some insight. If any of you have any suggestions on where to buy ethanol, that would be great too. The best price I've found so far is at carolina.com. Thanks.


    Use dry ice in acetone. It works quite well and is cheap. You may freeze out water from the azeotropic 95% by adding dry ice.


    Methanol is much better for flame tests as the flame is a consistent clear blue.

  9. a more specific question:


    Has anyone here ever added rubidium to water? Most chemistry instructors only go as far as potassium when demonstrating alkali metals with water. i want to try rubidium but want to stay safe. how much is safe to add at any one time? I know that it's not safe to store so i dont want to buy more than i can use at one time


    That's because Rb and Cs are generally very expensive in comparison and don't really offer much bang for your buck.


    This page has videos of everything: http://theodoregray.com/periodictable/AlkaliBangs/index.html


    ^^^ yes, I realize he asked over a year ago, but just for reference.


    As for the sodium, take a look at these videos and tell me if you think this is safe to do with a group of students: http://www.periodictable.com/Stories/011.2/index.html


    The explosions scatter both unreacted sodium and lye everywhere, so plan on killing off a large patch of grass and building something to release the Na from a distance.

  10. The chalcogen is sulfur.


    It is even on wikipedia, although it doesn't have it's own page.


    It is to the best of my knowledge not comercially available.


    I am looking at what I think is the first published paper on it: september 2006

  11. copper sulfate changes colour depending on how hydrated it is. [ce]CuSO4.5H2O[/ce] is blue, as you know, but copper sulfate with less water associated can be greenish, and completely anhydrous copper sulfate is a white powder, which slowly turns greenish blue as it absorbs water from the atmosphere.


    Of course... it could be impure, too :)


    What? I am unaware of any hemihdyrates.


    is there any chance that this solution contains a lot of chloride anion? Copper (II) ions form yellowish tetrachlorocuprate complexes in solutions with lots of chloride, which result in a green solution, since the water-coordinated copper ions are blue.

  12. i'm like a porphyrin, but with an extra pyrrole unit per ring. I am unbelievably difficult to make.




    Your first mistake was mentioning it in IRC a while back.


    I am a polycyclic aromatic molecule with a rather sunny name, consisting solely of carbon and a chalcogen. What am I?

  13. With UC's approval I will post the next riddle.


    I am an element named after Einstein's Marie Currie. The person I am named after took the first steps to Little Boy, but was never awarded The Prize for my nobel work.


    What is it?


    Meitnerium. Someone else post a riddle. I got nothin'.

  14. A saturated solution of NaOH in water consists of 111g of NaOH dissolved in 100mL of water. This solution is thick, attacks glass over time (forming silicates), and is air sensitive, precipitating Na2CO3 as it absorbs CO2 from the air. It will happily cause chemical burns and damage clothing.


    Instead of a saturated solution, you may consider a 50%wt/wt solution, which is simply 100g of NaOH in 100mL of water. This is close to saturated and is comercially available.


    Even if you start with crushed ice, adding this much NaOH rapidly will result in solution more than hot enough to burn you, which may boil. It will put off plenty of steam either way, which carries small droplets of caustic solution. If you dump it in all at once, it will solidify into a brick in the bottom of your container.


    Wear gloves, goggles, and a lab coat (or at least a crummy shirt).

    Start with ice water and have an ice bath prepared.

    Slowly add the NaOH in portions to the water in a large erlenmeyer, swirling constantly.

    Chill the solution between additions.

    Store in an airtight HDPE container. Nalgene makes these if you're looking for a supplier. They are reasonably priced.

  15. lovely.


    Thanks for the help. By the way, how'ld you make those images? Do you have some sort of custom program, to model structural formulas and whatnot? It could be damned useful to be honest (implying its taken on the free economy model in one way or another; I don't have much money to spend).




    I had been using it long before coming to this forum. To make forum-friendly images, you need to save as a bitmap and then convert to a JPEG in another program, which drastically reduces the file size.


    I actually find it more usable than chemdraw, which costs @$400 IIRC. Of course, it does lack some of the advanced stuff chemdraw has, but for anything you'll need to post here, it's just fine.

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