Potash alum is a chemical compound widely used as the potassium sulfate dodecahydrate. It is double salt that is used commonly in medicine and the water treatment process. Potash alum is also known as potassium alum or potassium aluminium sulfate. The chemical formula of potash alum is K2SO4.Al2(SO4)3.24H2O. It is also commonly referred to as ‘fitkari.’
Potash alum is an important member of alum which is one of the most popular coagulants used in water treatment, especially in low-coloured waters. It has numerous highly complex species, including aluminium polymers in conditions that affect the product type which further depends on different conditions such as dosage, pH and hydroxide supplement rate and mixing. The versatility also means that alum is very flexible and various coagulation mechanisms can be manipulated.
| IUPAC Name | Aluminium potassium sulfate dodecahydrate |
| Chemical Formula | KAl(SO4)2·12H2O |
| Molar Mass | 258.192 g/mol (anhydrous)
474.37 g/mol (dodecahydrate) |
| Density | 1.725 g/cm3 |
| Melting Point | 92 to 95 °C (198 to 203 °F; 365 to 368 K) |
| Boiling Point | 200 °C (392 °F; 473 K) |
Read More: Alums
Potash Alum Preparation
Potash alum is usually extracted from a mineral called alunite. However, potassium alum is also industrially produced today. One of the most common methods of preparation includes adding potassium sulfate in aluminium sulfate solution, which is in concentrated form. If the sulfate contains a higher amount of iron instead of potassium sulfate, potassium chloride can be used.
Minerals such as alum schist, bauxite and cryolite treated with sulfuric acid are often used to obtain aluminium sulfate. The condensed solution containing equimolar quantities of potassium sulphate and aluminium sulphate is usually formed by crystallization. The crystalline solid that is formed is acidic and colourless.
Al2(SO4)3(aq) + K2SO4(aq) → K2SO4.Al2(SO4)3.24H2O(s)
A small amount of diluted sulfuric acid is used to dissolve aluminium sulfate in warm water to prevent the hydrolysis of the salt. In the preparation of potash alum, when a two-salt solution is allowed to crystallize to some degree, a double layer is formed. The word alum is given a special sequence of twin salts. Meanwhile, “neutral aluminium” is obtained by adding sodium carbonate to an aluminium solution.
In a lab preparation, we will need two beakers (250 ml), funnel, funnel kiosk, glass-rod and wire-gauzes, washing bowl, tripod stand, china-dish, sulfate of aluminium, potassium and dilute sulfuric acid.
Procedure
- Pick up a 250 ml beaker. Wash and add 2.5 g potassium sulphate crystals with water. Add about 20 ml of water. Remove the dissolving crystal and let it dry.
- Take the other 250 ml of sand, wash and apply 10 g sulfate crystals of aluminium to the beaker in water. To prevent aluminium sulphate hydrolysis, add approximately 20 ml of water and 1 ml of distilled sulfuric. Heat for 5 minutes. When milkiness continues, the solution will be filtered.
- In a china-dish, combine the two solutions and put the china-dish on a wire-gauze on top of a burner. Stir the glass-rod solution. Concentrate the solution until it reaches the point of crystallization. Place the dish over a cold water beaker.
- Separate the potash crystals. Wash and decant mother’s crystals with ice-cold water in small quantities.
- Dry the crystals or scatter them on the pore plate by filtering paper pads.
Observations
| The shape of the crystal | Octahedral |
| Colour of the crystal | Colourless |
| The action of blue litmus paper | Blue litmus turns red |
| Solubility in water | Soluble |
Precautions
- Do not mess with the solution when it is cooled.
- Slowly refresh the solution to get stable crystals.
Properties of Potash Alum
The form or structure of potassium alum crystals is octahedral. It is almost colourless solid and is highly soluble in water. Obtains a red colour when heated to a certain degree and forms a porous, cold weight called ‘burnt alum’. When it is exposed to very high temperatures, hot molecules of water evaporate and sulfuric acid is released leaving behind a mixture of alumina and potash sulfate.
Potassium aluminium sulfate fuses in its own crystallization water at 92 °C. It is acidic in nature and changes the colour of the blue litmus.
Uses of Potash Alum
Fire Retardant
The use of potassium alum for textiles, wood and paperless flame resistance is a fire-retardant.
Tanning
For leather tanning, potassium alum is used to extract moisture from the hide and avoid rotting. Alum is not covered and can be washed out, as compared to tannic acid.
Iron and Steel Dissolving
This aluminium solution has the property that steels are dissolved without affecting aluminium or base metals. For machined castings of steel parts of machinery, alum solutions can be used.
Gourmet Food
Potassium alum may be an acidic component in baking powder to provide a second leavening step at high temperatures (although sodium alum is more widely used for this purpose). Bakers in England made bread whiter with alum during the 1800s.
Used for Dyeing
Alum was used to form a permanent link between natural textile fibres like wool and dye, as mordant.
Pigmentation of the Lake
Aluminium hydroxide from alum acts as a base for most lake pigments.
Blocking Chemicals
Since remote antiquity, potassium alum was used for purifying turbid liquids. The drinking water and industrial water systems, effluent treatment, and post-storm lake procedures continue to be commonly used for the treatment of pollutants in precipitation.
Water is applied to the domestic wastewaters roughly 30 to 40 ppm of alum, but more commonly to the industrial wastewater to clump negative particles into the flocs and float them up to the liquid bottom, or more conveniently filter them out of liquid before further filtration and disinfection of the water.
Alum Concept Video
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