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Stannous chloride, SnCl2 is a white or colorless salt of tin. It is used for testing for gold and PGM:s in solution. It is the eyes of the refiner and used to keep track on the precious metals when it can't be seen with the naked eye. Stannous chloride can't detect metallic substances so for instance solid objects or colloidal solutions doesn't give any reaction.
How it works
Stannous chloride reduces the dissolved metal and creates an environment where the metal clumps together into small aggregates, forming a colloidal solution. The color of the colloidal solution varies with different metals, by watching the color change the metal can be decided and an approximate concentration.
Reactions of the different metals
There are a number of metals that gives colored solutions when mixed with stannous chloride. If the solution is too concentrated the resulting color change can look black, in that case dilute with water and test again to see the correct reaction.
Platinum gives a golden yellow to brown stain with stannous. Iridium in solution could give the same color.
The color changes with palladium is special and it will go through several stages during a few minutes. First it will turn yellow, then after a few minutes it turns blue-green. This is the characteristic reaction of palladium.
According to Lazersteve this is a viable test for rhodium in solution.
- Add a drop of the solution to a test tube, dilute with three times the volume of water.
- Add a drop of stannous chloride solution to the sample.
- Heat the test tube for a few moments over a heat gun on low or a Bunsen burner. Be careful to not overheat it as the liquid may bump out of the test tube!
- For a rhodium only solution a golden yellow-orange color appear when the solution warms. If there are other precious metals present the solution will lighten towards golden yellow-orange when it warms. Typically 10-20 seconds of heating are required to reach this point.
- Remove the test tube from the heat source after the color has changed.
- After cooling the color of the solution will change to a deep red color, between rose red and blood red depending on the Rhodium concentration.
Silver in solution forms a white precipitate just as with any chloride. Stannous isn't usually used for tests of silver in solution. Mercury in solution also forms a white precipitate when mixed with a chloride. Also lead in solution can give a white precipitate.
It is well known that a solution with an excess of sulfur dioxide produces a brown stain. It also seems like molybdenum in solution can give a dark blue stain.
Solutions with iodine in will give a very dark purple to black reaction with stannous chloride.
Making stannous chloride
Stannous chloride is easy to make from tin and hydrochloric acid by dissolving the metal in the acid. The resulting stannous chloride solution will deteriorate over time but storage with a bit of tin seems to prolong this breakdown.
The most important part in storage of stannous chloride solution is to keep oxygen out of the solution. A beaker with stannous will go bad in days to weeks, while a closed bottle with a bit of metallic tin at the bottom can last for months or years.
An alternative to store with tin metal is to store it with an excess of hydrochloric acid which also seems to prevent the hydrolysis.
In crystal form the stannous is quite stable. Evaporation gives the hydrated salt SnCl2.2H2O.
Standard testing solution
To verify that the stannous chloride hasn't gone bad, a standard solution of gold chloride is used. It is easy to make by dissolving a bit of gold in aqua regia and dilute so the purple color is shown when tested. If there isn't a purple color the stannous have been oxidized and a new batch has to be made.
Some people tests their stannous chloride daily while other only after getting a negative test.
- Video of testing with and making stannous chloride
- Making stannous chloride from solder
- Using stannous chloride for testing