Monday, January 3, 2011

Instrumental Finish in Gold Analysis

 One should first pick and flatten the beads and put them into 16 x 125mm disposable glass test tube. One should ensure water bath is a minimum of 75 0C. Another option is the use of fine sand. The sand gets far hotter than a water bath.

The assayer should add .5 ml of concentrated HNO3 from a calibrated dispenser and place rack in hot water bath for 15 minutes. After 15 minutes the assayer should confirm by visually inspecting the test tubes for the complete silver digestion. After the assayer confirms that the silver bead is digested, one can proceed to digest the gold by dispensing 1.5 mL of concentrated HCl from a calibrated dispenser into the test tube. 
Once both the acids are added one should again inspect each of the test tube for complete digestion of the gold. The final steps are to dilute to the final volume and shake the test tube to mix the solution thoroughly. The addition of the acids should be done inside of a fume hood because of the fumes that are produced.
Bead parting tips for readings done on an AAS or ICP OES/MS are as the follows. One should check that each tube has a bead in it. One should also check that the acids are dispensed correctly. The aqua regia should be an orange/yellow colour. One can look for the AgCl precipitate in each test tube. For the measurement of gold by AAS or ICP/OES measurement, the entire bead must be dissolved so the instrument can measure the solution concentration. The instruments determined the concentrations by aspiration the sample into the calibrated instrument. This instrumental method is much more common for finishing then measuring with the gravimetric technique. The instruments are capable of a much lower detection limits. Labs can measure a 1 or 2 ppb detection limits using AAS, ICP OES and ICP MS.

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