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Member Since 27 Dec 2003
Offline Last Active Aug 14 2008 10:12 AM

Topics I've Started

Peltier Drying Cabinet

04 January 2005 - 10:41 PM

I just finished building a drying cupboard this morning, and so far it seems to be working pretty well. I searched for a while to find a small dehumidifier, to save space, money and energy, since a normal 8 litre one is way overkill for the size of my cupboard.

For a long time the only type I could find that was the size I wanted was basically a box of desiccant with a fan on it, which you plug in to dry out every time it becomes saturated. Something about having to charge it for 10 hours every couple of batches of stars just put me off these. I did eventually find a miniature dehumidifier, but it cost ?90.00.

However, after a while, I came across a "Peltier" module, which I'd never heard of before. What is it? A thermocouple in reverse. Run a current through it, one end gets hot, the other gets cold. A Peltier dehumidifier is free of any compressor or refrigerant, and is therefore much smaller and cheaper than a conventional one. The one I got is rated at 65 Watts, removes 380ml per day, and would fit comfortably in a shoe box. Cost: ?40.00. Perfect.

My drying cabinet is about 45x45x50cm. Since I usually make stars in <500g batches, this is plenty big enough. The dehumidifier itself keeps this sized cupboard warm without any need for light bulbs or other additional heating. I suspect that Peltier dehumidifiers are somewhat less energy efficient than regular ones (ie convert a greater percentage of the supplied energy directly to heat, rather than using it to dry the air) which would explain why they are not more commonly used, but in the case of a drying cupboard, where heating is desired anyway, this is not a problem.

I've had it running this afternoon with a few comets in it, which seem to be making very good progress towards dryness, but I haven't really had the chance to fully evaluate whether or not it works yet. Seems to though...

Barium Carbonate --> Barium Nitrate

13 September 2004 - 05:07 PM

I've been looking into making barium nitrate, and I have found a method that seems to show quite a lot of promise, and I am quite surprised that more people don?t use it already. It has already made me several grams of real barium nitrate, which gave a bright, unmistakable green when I tried it in a couple of published compositions.

The basic outline of the method is to react barium carbonate with acetic acid - ordinary vinegar - to produce barium acetate, which is soluble in water. This is then mixed with potassium nitrate solution. The "ion soup" is then left to evaporate. Fortunately, barium nitrate is the least soluble of the various combinations of ions possible from the ingredients (potassium nitrate, barium acetate, potassium acetate, and barium nitrate) so is the first substance to begin crystallising out. Potassium acetate is also very soluble in water, so will remain in solution whilst this is going on. The barium nitrate crystals can then be filtered out, and purified by redissolving and crystallising.

I think that the reaction is something like this:

BaO4C4H6 + 2KNO3 --> Ba(NO3)2 + 2KO2C2H3

Therefore one needs 255g of barium acetate and 202 g of potassium nitrate to start with, giving a final yield of 196g of barium nitrate. Since crystallising out the barium acetate is unnecessary, it is simpler just to weigh out the barium carbonate, then react it with sufficient vinegar (add until reaction ceases, then add a little more barium carbonate to neutralise excess), and mix this solution with the potassium nitrate solution. When doing this, for every 202g of potassium nitrate, you should start with 197g of barium carbonate (which will react to give 255g of barium acetate). Using typical distilled vinegar, just less than two litres was required to react with barium carbonate. As I said, this is currently evaporating, so I do not know how much product I will get yet. I actually added a slight excess of potassium nitrate to the solution, in the interest of making sure that it doesn?t ?run out? before the barium acetate, to avoid having too much left behind afterwards.

I did a small trial run of the method a few weeks ago (reduced quantities), which has so far yielded a couple of spoonfuls of barium nitrate, and is still evaporating. Unfortunately, in my eagerness to experiment with this initial harvest, I did not weigh it, so cannot comment on the efficiency of the method. However, I have just started a ?production sized? batch, which will give an ideal yield of 392g of barium nitrate. I will let you know how much I actually get in a couple of weeks.

Finally, make sure that you have some potassium or magnesium sulphate solution on hand whilst doing this, so that any spilled barium compounds can be rendered safe with it, and since there is bound to be some barium left in the potassium acetate solution, this should be made safe with a sulphate too. Since the resultant barium sulphate and potassium acetate are not particularly dangerous, this solution/suspension can probably be safely disposed of down the drain. (For anyone who didn?t know, barium sulphate is almost totally insoluble, and not terribly reactive, and therefore does not pose a serious danger. Adding a soluble sulphate to a soluble barium salt will cause the barium to be precipitated out as barium sulphate).

I may well have added a few things up wrong or made other mistakes, so I welcome any suggestions people have for improvement, and maybe I have missed some issue that causes it not to be generally used (I can?t really believe that I am the first person to try it), but I have made several grams of barium nitrate in this way, which did definitely work as it was supposed to. Whilst this method may be a little less efficient than reacting barium carbonate with nitric acid or ammonium nitrate, it does seem to work, and uses readily available chemicals, so may be of use to some people.