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Potassium Bromide: Iron Bromide and Potash
Potassium bromide is the bromide of photography. Thorpe (1913): iron bromide solution run into hot potash solution until it is just neutral throws down the iron as oxide and leaves potassium bromide in solution, which is filtered, evaporated and crystallised. 100 parts of iron bromide take 56 to 60 parts of potash.
The picture: potassium bromide crystals (Wikimedia Commons).
Intermediate
2 days
Instructions
1
1
Make the two solutions
Make the two solutions
Dissolve the iron bromide in water and filter it. Dissolve 56 to 60 g of potash, according to its purity, in a little water and heat it.
Materials for this step:
Iron Bromide100 g
Potassium Carbonate (Potash)60 gTools needed:
Water
Coarse Cotton Filter Cloth
Cast Iron Pot
Balance Scale2
2
Run the iron bromide into the potash
Run the iron bromide into the potash
Pour the iron bromide solution into the hot potash until the liquid is exactly neutral or very faintly alkaline. Iron oxide falls out.
Tools needed:
Cast Iron Pot
Litmus Paper
Rubber Gloves3
3
Boil and filter
Boil and filter
Boil for some time so the iron oxide settles dense, then filter it off.
Tools needed:
Cast Iron PotCoarse Cotton Filter Cloth4
4
Dry, then dissolve again
Dry, then dissolve again
Evaporate to dryness. Dissolve the residue again in water: potassium sulphate and other dirt stay behind. Filter.
Tools needed:
Cast Iron PotWaterCoarse Cotton Filter Cloth5
5
Crystallise
Crystallise
Boil down to specific gravity 1.53, then evaporate further at a gentle heat in a stoneware or enamelled dish, for hard crystals.
Tools needed:
Cast Iron Pot
Hydrometer
Evaporating Dish6
6
Dry at 40 to 50 °C
Dry at 40 to 50 °C
Dry the crystals at 40 to 50 °C, not above, or they crackle and burst.
Tools needed:
Evaporating Dish
ThermometerMaterials
2- 100 gPlaceholder
- Placeholder
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