on the following day, all the junk is in thebottom of the flask and the gold solution is ready to be filtered. it is highly recommend using a proper laboratoryfilters and not store bought coffee filters. i'm using medium-slow speed filters, whichwill strain any solid particle over 5 microns. the fluted filter paper sets in the funnelas soon as it gets wet. first, the bulk of the gold solution whichis free and clear of solid particles is poured to the filter. this portion will run downvery fast. here in the flask you can see the remainingjunk, i'll just wash it several times with water and pour it down the filter. the bulkof the solids are left in the flask.
please remember to wash the filter with plentyof water so that all of the yellow gold bearing solution will run down to the receiving flask. when you are done, the filter should be whiteagain. our next step is to remove the gold from thesolution. this is done by using sodium meta-bisulfiteor in short s.m.b., the s.m.b is first dissolved in water and then added to a stirred goldsolution. the s.m.b will react with the gold ions inthe solution and reduce it to metallic gold powder. as a result, some sulfur dioxide gas will release... it'spretty toxic so the reaction must be done
under a fume-hood. btw, you don't really have to use a magneticstirrer like in this video, it is just my personal favorite. the solution is again left to settle overnight here you can see the gold powder all settledto the bottom of the flask and the supernatant is practically colorless. this is a good indicationfor purity. the bulk of the solution is slowly decantedand the washing cycles begin. the washing cycles are extremely importantif commercially pure gold is wanted. starting with simple tap water wash... thegold powder is covered with boiling hot water,
swirled for a while and left cool back down. this water wash is repeated two more times. take note of the color of the gold powderprior to the acid wash. now for the acid wash, the powder is simplycovered with hydrochloric acid 32% and brought to a gentle boil. take note of the color of the solution...the acid dissolve traces of copper, nickel and silver chloride and it will also dissolvesome gold. the acid wash is left to cool down and laterdecanted. the process is repeated until the acid wash doesn't gain any color anymore.
it usually takes three to five washes. in between each acid wash, it will be a goodidea to do a quick water wash so that a better indication of color change can be seen onthe next wash. i like to keep the acid washes in a separatecontainer and use it later to dissolve more dirty gold. take a close look on the powder after theacid washes... the color is clearly lighter. which is another good indication for purity. the powder is later washed several times withdistilled water and placed on a hot plated to slowly dry out.
and here's the result. i hope you found this demonstration usefuland interesting... please comment, share and subscribe. oh... by the way, click here to jump overto the melting video...
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