The finishing.com Hotline -- Serious Education ... plus the most fun you can have in metal finishing
Dissolving gold deposits for analysis
(thread begins in 1999 and continues through 2012)
Q. Hi. I'm trying to determine the quantity of gold deposited on our product. I'm using an ICP to do so. I've played around with two sample prep techniques. The first is to semi-selectively dissolve the gold from our product with cyanide stripper. I then take the gold laden stripper solution and put it into a 20% HCl, 5% HNO3 matrix since I would prefer to deal with an acid matrix to ensure compatibility with the other samples run on the instrument. The dilution of the stripper solution in the acid matrix is of course performed in a hood to contain any hydrogen cyanide liberated in the process. My problem with this technique is that it seems to result in the formation of a suspended solid. The numbers I'm getting indicate to me that the precipitate is a problem (either it's a gold salt or it absorbs gold). The second procedure I've tried is to dissolve the entire part in aqua regia. This procedure seems to work ok but I don't like the fact that I have to dissolve all that extra metal and complicate the matrix.JD Kronicz
- Hanover, Pennsylvania
A. 1. This will work with copper alloys: Cut open the part so that the base metal is exposed. Use nitric acid to dissolve away the part. What you have left (after careful rinsing) is the gold shell. Then dissolve this in aqua regia. This is a very thin foil so be careful you don't lose any while rinsing.
2. This works best when the deposit is particularly thick: It is the usual procedure of one gold bath vendor that I know to have their customers plate a copper or brass coupon (small disk or rectangular piece) with over 1000 µinch, just the gold, not the underplate. Then they cut the coupon in half and do a cross section and Knoop hardness on one half. The other half goes as I detailed above, first nitric acid, then rinse, then aqua regia. The test is for impurities. You don't actually look for the amount of gold deposited using this technique. The vendor will do one free test a month for the customer.
3. Just measure the thickness using cross section or XRF. It's an easier method than anything else I know of for estimating the gold deposit.
microwave & cable assemblies
Mesa (what a place-a), Arizona
Q. Thanks for your response Bill. I am familiar with the technique you referred to. The problem is (as you alluded to) when you're talking about actual product and much thinner gold the foil is very hard to handle. I am interested in quantitative gold and not impurities. With impurities it is ok to lose a little gold as all you are interested in is the ratio of impurity to gold. The x-ray technique is fine for thickness but I'm looking for usage on the part. We do routinely perform thickness by XRF to check functional areas of the part. JDKJD Kronicz
- Hanover, Pennsylvania
WARNING! The internet is a giant one-room schoolhouse where people will read or overhear details about operations that they are not equipped or qualified to attempt. Specifics you read here may be intended for qualified chemists with haz-mat training, and working in fully equipped labs.
A. JD, If you can strip the part using aqua regia, using care to minimize attack on the base metal, the gold can be precipitated from the aqua regia as follows: 1) Boil aqua regia down to incipient dryness- till only a few mls. of liquid remain. 2) Reconstitute using 100-200 mls. deionized water. 3) Adjust pH to 2.5 and bring solution to boil. 4) Add 10% sodium nitrite a few mls. at a time. The gold will precipitate first as a fine powder and with further boiling will begin to agglomerate. When adding sodium nitrite no longer produces any precipitate, filter out the gold, wash with HCl then DI water and oven dry. Determine the gold content gravimetrically. The precipitate will be 99.99% pure, so if your deposit is a Type 1 or 2, the alloying elements will not be present in your precipitate. Good luck.Keith Wicklund, CEF-SE
avionics - Minneapolis, Minnesota
Q. Dear Keith W.
I read your suggestion for determining the gravimetrically in reply to JD. You have not mentioned what will have happened to silver chloride which get settled in gold chloride solution (aqua regia solution)?
Does silver chloride remain as it is after boiling the aqua regia? Kindly reply for this.
- Vellore, TamilNadu, India
Langford and Parker, "Analysis of Electroplating and Related Solutions" [linked by editor to info about book at Amazon] indicates that sulfuric acid, with careful additions of Hydrogen Peroxide [linked by editor to product info at Amazon], will precipitate the gold from a cyanide solution. Other reducing agents listed are ferrous sulphate, Oxalic Acid [linked by editor to product info at Rockler], and potassium nitrite. Regards, Tom
Falls Township, Pennsylvania
A. Another method for precipitating the gold from aqua regia which doesn't involve boiling away the nitric acid and generating toxic nitrogen oxides is as follows:
After dissolving your gold or base metal/gold into the aqua regia, add granulated or prilled urea to the acid/gold complex until the fizzing stops. This will destroy the nitric acid without boiling. The fizzing isn't violent nor are any disagreeable fumes released. A slight excess of urea at this point is ok. Next, slowly add a solution of sodium bisulfite (sodium metabisulfite in water) to the HCl/gold/urea mixture. When you start to smell sulfur dioxide being released, that is a good indication that all of the gold is reduced. Any further addition of sodium bisulfite will begin to reduce copper chloride to cupric chloride, which will precipitate along with your gold. This can discolor the gold, release clouds of noxious fumes when the gold is melted and generally screw up your assay. The gold will precipitate over the course of several hours as a chocolate brown powder which melts to the yellow color we all know and love. Add a few ml of Isopropyl Alcohol [linked by editor to product info at Amazon] of other non-acid labile surfactant to free the finer particles which are floating atop the surface tension. After the solution becomes a transparent green, decant off the remaining HCl/copper chloride solution, cover the gold with concentrated HCl and swirl. This will dissolve out any precipitated cupric chloride. Wash the gold several times in distilled water until the water shows neutral to litmus. You can substitute concentrated ammonium hydroxide solution for the HCl wash since it too will dissolve cupric chloride with the added feature that cupric chloride will turn aqua ammonia blue. Repeat the ammonia rinse until no more blue is seen, and then do it once more. One caution here: If there is any possibility that silver salts have been precipitated along with the gold, do not use ammonia. Ammonia will form explosive acetylides such as silver fulminate, especially with insoluble silver chloride or silver oxide. When these dry, they will violently detonate if you even look at them too hard. This hazard can be completely avoided if you filter your acid after the urea step since this would trap any insoluble silver chloride and keep it out of your gold. I know this post is long, but the beauty of this process is in the details. I will post a second, even easier method ...
- Austin, Texas
A. As promised, here is another simpler method for precipitation of gold from aqua regia:
After dissolving your gold/base metal in the aqua regia, allow the acid to cool and then add an excess of sulfurous (not sulfuric) acid to the pot. This both destroys the excess nitric acid and reduces the gold in one step. Unlike the urea/sodium bisulfite method, no cupric chloride will be precipitated, only your gold. Since sulfurous acid is a water solution of sulfur dioxide gas, you can also bubble SO2 gas through your acid, although this method takes a few hours of slow bubbling to precipitate all of the dissolved gold.
- Austin, Texas
Q. Thank you Frank Roberts.
Your method of Recovering Gold from Aqua Regia worked well for me. However, I found at first with high quantity of copper from circuit boards I had problems with precipitation. I solved this problem by first dissolving most of the copper etc with nitric acid and pouring it off before Aqua Regia process. I now have this nitric copper etc solution. Can you suggest the best way to recover silver and copper from this?Tony Plummer
- Harare, Zimbabwe
Q. HI TONY PLUMMER. I WAS READING YOUR ANSWER TO FRANKS ROBERTS THAT YOU SOLVED YOUR GOLD PRECIPITATION PROBLEM BY USING NITRIC ACID TO DISSOLVE THE COPPER. MY QUESTION TO YOU, HOW DO YOU SEPARATE YOUR GOLD FROM THE NITRIC ACID? BEFORE USING AQUA REGIA? THANK YOU.Chris Miller
- Palm Beach Gardens, Florida, USA
A. Chris Miller,
I would like to answer your query. Gold only dissolves in Aqua Regia. Copper dissolves in Nitric acid. Therefore if you want to spend the time and additional Nitric acid you can pretreat your gold bearing copper material with a solution of nitric acid until all the copper dissolves, decant fluid only and rinse well. Then you would treat your remaining solids with AR to dissolve the gold separately from the copper.
Alternately, you can start with aqua regia and once all the gold is dissolved filter the solution into a clean container leaving the solid copper in the first container. Rinse the filter well and save it. Then you would need to neutralize the nitric acid before gold precipitation. After the gold is precipitated, 'cleaned' and settled to the bottom you can clean it again with either HCl or nitric, but not both at the same time, to attain a higher purity gold.
Refining Precious Metals Wastes by CM. Hoke is an excellent book, though a little dated, and can be found online for free.
Q. How to make luster gold powder form real gold?Mahendra Bir Singh
- Kathmandu, Nepal
Q. What to do if the gold came from electronic scrap (from contacts) and the concentration is too poor? Something to do first?Eric Tordeur
Q. Dear Sir,
I want to refine gold from Scrap and I use aqua regia reaction to dissolved gold. now for precipitation I use SODIUM METABISULFITE solution (300 Grams per Liter) and added as 1 Gram Gold = 3 ml of SODIUM METABISULFITE solution.
I find some amount of precipitation but it dissolved again with the solution.
How I can recover my gold from this solution?
GOLD SMITH - INDIA
A. Talk about precipitating gold from the solution but nowhere I read the easy way to determine if any or all gold is precipitated out. I use a little tin (from lead-free tin solder for water pipes) dissolved in Hydrochloric acid. Put a drop of this on absorbent paper and then add a drop of the solution where the gold is from precipitated. If any gold remains in the solution the drop turns black. When all gold is precipitated the color stays neutral. This detects 4 parts in a million.Alexander Baldal
- Philipsburg, Saint Maarten, Netherlands Antilles
Q. I AM IN THE MIDDLE OF CONFUSION WHEN I MAKE MIXTURE OF AQUA REGIA, I REALIZE THAT OUR NITRIC ACID SUPPLIER SUPPLY ME WITH ONLY 68% CONCENTRATED NITRIC ACID SO WHAT WILL BE THE RATIO OF MIXING IT WITH HCl I WONDER AND I HAVE BEEN GETTING PINK COLOUR MIXTURE WHEN I MIX THE NITRIC AND HCl WITH RATIO OF 3 TO 1 (3 HCl TO 1 NITRIC) NEEDED ADVISE AND ALSO THE WAY TO NEUTRALIZE NITRIC BEFORE EXTRACTING THEM,
MY VERY MUCH THANK TO WHOEVER CAN HELP AND ADVISE ME ON THIS MATTER.
April 1, 2008
Q. Hi I am working in gold refining plant. We dissolve 12.5 kilos of gold in approx. 50 lt. aqua regia. After filtration of silver chloride we bring the pH to 7 to destroy the excess nitric acid by adding soda ash solution. We precipitate the gold using sodium metabisulfite solution. Can anybody give me full chemical reactions taking place in the whole process? Thanks,Vijay Pillai
chemist - UAE
June 16, 2008
I'm from Indonesia need help.
Note : Nitric acid 68%, HCl 30% (sold in the store)
I use Aqua regia (450 ml. HCl, 150 ml. Nitric Acid, 200 grams scraps of metals).
I wait one night (the color of solution is yellow-brown), then tomorrow I added 450 grams sodium bicarbonate and some of lime to neutralize the acid. The reaction is :
Bottom precipitate is metals, middle precipitate is orange-brown mud.
I pour off the container with filter. I get the clear solution. The orange-brown mud in the filter when dried is change to brown. The metals color is cloudy brown-grey.
1. How to neutralize the solution (HCL, Nitric Acid, Sodium Bicarbonate) pH 1.
2. How to find out the brown mud if is indicate of gold.
3. How to cleans the metals cloudy brown-grey.
I'm very thankful for any response because I'm really confused.
hobbyist - Jakarta, Indonesia
August 9, 2008
Q. I've yellowish gold chloride solution. In it I've added half the quantity very strong HCl acid. Then I dissolve melted gold alloy as an anode while the cathode is pure gold strip. Expecting pure yellow gold to sponge around the cathode, instead only a coffee brown sponge is formed. No reaction to HCl but fizzes to HNO3. Result is pure gold. But expected gold weight to karatage ratio is below calculation. Where am I wrong?
For example if a kilo of gold scrap has the karat fineness of 875/1000 then after refinery shouldn't the gold recovered weigh 875 grams since the remainder 125 grams of base metals, other than Pt, Pl, etc, should be gone as chlorides. I always seem to accumulate a few grams lesser than expected. I don't even transfer any liquids or solids to other containers. I'm stuck me mateys.Balasaravanan Palanirajh
employee - Colombo, western province, Sri Lanka
September 24, 2008
Q. How can we dissolve gold in HCl using hydrogen peroxide and in what amount should we add these solution, and how to recover dissolved gold in the solution the method should be easy and without fumes?
Can anyone help me please?
university student - Australia