Photographic fixer

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Photographic fixer or fixer for short is the solution used in chemical photography used to remove unexposed silver from film and paper during developing. Fixer is a classical source of silver and has been since the dawn of photography.

Contents

Testing

There are specially made silver testing papers but a low tech simple testing is to cement the silver onto a copper wire.

Silver test papers

There exists test papers for determining pH and silver concentration in fixer baths. For example the AG-fix brand.

Copper wire

By measuring the time it takes for a copper wire to be covered by silver the concentration can be approximated from the following table. (Table data originally by Juan Manuel Arcos Frank)

Take an isolated copper wire and remove the isolation, the surface needs to be fresh and without oxidation to work. Dip it in the solution and measure the time it takes to get covered by silver. Then compare it to the table below to decide the silver content of the scrap.

Time (s) grams / liter
1 12
2 10
3 6
4 3
5 1.5

Recovery

The silver from fixer can be recovered in several ways.

Cementation

The silver in the fixer can easily be cemented with zinc (or other base metals).

  • Measure the amount of silver in solution.
  • Adjust the pH to 5 by additions of 5% sulfuric acid solution.
  • Add equal weights of zinc to the amount of silver. Ie if the silver contents were measured to 3 g/liter and there is 5 liters of solution then use 3*5 = 15 g of zinc powder. Keep stirring the liquid up to 5 minutes.
  • After five minutes measure the amount of silver left in solution and add more zinc if needed. Stop adding zinc when a copper wire doesn't get covered by silver after 20 seconds.
  • Let the black sludge of silver and zinc settle for a while.
  • Decant and filter off the barren solution.
  • Put the black mud in a beaker and add hydrochloric acid or diluted sulfuric acid to dissolve any left over zinc. When the bubbling stops all of the zinc is dissolved.
  • Let the powder settle and decant or filter off the liquid.
  • Wash the black mud with water.
  • Melt the black mud with a mixture of borax and some sodium or potassium nitrate as flux.

One way used to remove silver from the wash water in a photo shop is to use an iron wool canister. The canister is put inline with the water and any dissolved silver ends up cemented onto the wool while the iron is dissolved. When the iron wool is used up it is filled with a black sludge of silver and silver sulfide. The traditional way to process the content of the canisters is by smelting with iron rebars.

Precipitating with sodium sulphide

Adding sodium sulfide (Na2S) to the fixer is a cheap and fast way to treat it. But there is a risk associated with it, when adding too much sodium sulfide the deadly gas hydrogen sulfide is produced.

  • Measure the amount of silver in solution.
  • Add equal weights of Na2S to the amount of silver. Ie if the silver contents were measured to 3 g/liter and there is 5 liters of solution then use 3*5 = 15 g of Na2S.
  • The black mud that is created is made up of silver sulfide, Ag2S
  • Measure the amount of silver left in solution by dipping a copper wire. Add more Na2S until all silver is precipitated, the copper wire stays clear after 20 seconds in solution.
  • Let the black sludge of silver sulfide settle and then decant and filter off the barren solution.
  • Wash the black silver sulfide mud with water. If it doesn't settle by it self, try boiling it to make it clump together and precipitate.
  • The resulting silver sulphide is melted with sodium or potassium nitrate to reduce it back into metallic silver.

Electrolysis

An Electrolysis Silver Recovery Unit, (ESRU) is made up of two electrodes with a voltage added between the electrodes. Silver is electrolytically deposited onto the cathode. If too high current is used then the silver is turned into black silver sulfide.


References

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