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Remove used acid
No reaction will occur once the acid is depleted. Used acid can be removed by means of siphoning. Basically, this involves the use of pressure difference to draw fluids from a higher level container to another container places below
the former. Diagram below shows a safe way to siphon used acid.
The setting consists of a glass jar to contain the acid. Instead of a sealed cap, a removable air-tight cap or cork must be used so that the acid can be safely disposed once it is siphoned out. Two rubber tubings is inserted into the cork. The longer tubing (see diagram above) is to connect to the siphoning glass tubing from the gas generator. The whole jar must be placed at a level BELOW the gas generator jar such that the end of the long rubber tubing inside the siphoning jar is BELOW the end of the
long siphoning glass tubing inside the generator jar.
To initiate siphoning suck out the air at one end of the shorter rubber tubing (you can use your mouth). The lower pressure inside the siphoning jar causes the acid to flow. Stop sucking once the acid is in the rubber tubing and almost reaches the siphoning jar. The acid should continue to flow into the jar. To stop the flow half way, simply lift the siphoning jar above the generator jar.
Don't worry if you over suck, the siphoning jar is meant to use as a acid reservoir and will prevent it from sucking into your mouth!
Precaution
Below lists some of the points you need to take care when constructing the gas generator:
(1). The size of the funnel must be large, at least enough to contain the amount of acid solution when the gas tap is closed.
(2). Instead of using an elaborate tap, simple tap can be constructed by attaching a glass tubing to a rubber tubing. A clip is placed over a section of the rubber tubing to control the flow.
(3). When constructing the siphoning jar, take care that the two rubber tubing is far apart to prevent the acid from getting into the sucking tubing.
(4). The size of the siphoning jar must be large enough to contain all the acid from the gas generator jar.
(5). The end of the sucking tubing inside the siphoning jar must not touch the acid and should be at least a few centimeters above the acid level at any times.
(6). Nitric acid, instead of the hydrochloric acid can be used as a reacting agent. However, sulfuric acid must be avoided because it tends to form a protective layer of calcium sulfate (CaSO4) that does not easily dissolve in water and will slow down the reaction.
Okay, you may not wish to produce green house gas and I take your point. You can produce hydrogen gas by using zinc metal instead of marble slabs. In this case, either sulfuric acid or hydrochloric acid can used.
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