Gravimetric Finish in Gold Sample Analysis Technique

Proper Gravimetric  Technique starts with ensuring parting cups are clean before using. The assayer should put a flatten bend in hot 6:1 nitric to start, and watch the bead for its reactivity, add stronger acid if it is needed. This should be done on a hot plate and inside a fume hood. If stronger acid is needed it should be hot. One should carefully add and remove the acid with a disposable pipette. To make sure that floaters are fully parted, use glass rod or tap cup to sink to sink them. One can add ammonia rinse solution and to remove any AgCl that is present. There are microbalances that can go down to 0.1mircograms which allows the gravimetric detection limit to improve over the gold balances in the pasts. Handling the very small beads can be very difficult, so gravimetric finish is reserved for higher grade samples.  Analysis of gold was by fire assay, with a gravimetric finish on samples exceeding 3 g / t gold

Rinsing.
After parting is completed, one should rinse the bead. One can use rinse with 14:1 NH4OH to remove any residual AgCl or use distilled water.

 

Drying & Annealing.
One should allow any moisture to dry because the Au can be lost due to spiting. One can do this by placing the parting cup on a hot plate to dry. Annealing the Au should be done over a flame. This turns the Au from black to gold and hardens it and causes the particles to stick together.
Annealing Tips
Inspect and remove foreign particles. When using a flame for the purposes of annealing, one should heat the bottom of the parting cup until it is red.
 
Problems
The bead should not have a gold/ brass color, if it does, it means the bead is not fully parted because of the low Ag:Au ratio. For gravimetric finish one should inspect the parting cup to make sure that it is clean and weigh the sample for the same length of time as it takes to tare the balance. Flouring is when the bead breaks up during parting due because of several reasons because of incorrect Ag:Au ratio, cold parting acid, and impurities.