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Preparation of Potash Alum

Preparation of Potash Alum


Potash alum's chemical formula is

  • KAl(SO4)2
  • AlK(SO4)2

Potash alum contains potassium, aluminium, and sulphate ions in 1:1:2 ratio.


Chemical formula

  • The chemical formula for potash alum is KAl(SO4)2.
  • It is usually produced in its hydrate form, which has 12 molecules of water attached to it.
  • It is also known as the dodecahydrate form of potash alum. AlK(SO4)2X 12H2O is the chemical formula in this form.
  • The formula can also be written as K2SO4.Al2(SO4)3.24 H2O. in its double sulphate form.

Applications of Alum

  • Potash alum is quite useful as an antiseptic and astringent, especially after shaving.
  • It is used as a catalyst in laboratories.
  • It was used to purify water by settling dissolved sediments.It is widely used in India, particularly in small cities, villages, and towns.
  • It is also used in the dyeing and printing of fabrics.


  • Potash alum has been obtained from alum-K, a naturally occurring sulphate mineral.
  • However, due to increased production and usage, it is also produced on an industrial scale.

Raw materials required:

Following materials are required:

  • Aluminum sulphate
  • Sulphuric acid
  • Potassium sulphate
  • Distilled water

Apparatus Required

  • Bunsen burner
  • Watch glass
  • Funnel glass and stand
  • Beakers
  • Evaporating dish
  • Tripod stand
  • Stirring rod
  • Porcelain plate
  • Wire gauze
  • Measuring cylinder
  • Filter paper

Potash Alum Preparation

  • To begin, take 5g of potassium sulphate.
  • Then, after properly weighing it, powder it.
  • After powdering, transfer the potassium sulphate powder to a 150 ml beaker.
  • Add approximately 20ml of water to the beaker and continuously stir.
  • If the solution does not become clear, it can be slightly warmed.
  • Set aside the solution.
  • Add 20g of powdered aluminium sulphate to a second beaker of similar size.
  • Fill this beaker with 30ml of distilled water.
  • Add about 2ml of concentrated sulphuric acid.
  • To make a clear solution, continuously stir the solution with a glass rod.
  • If the aluminium sulphate does not dissolve properly in water, you can warm the solution and add a small amount of extra sulphuric acid.
  • We also require a clear solution in this beaker.
  • Pour the two prepared solutions into a china dish after passing them through a filter paper.
  • It aids in the separation of the solution's undissolved components.
  • Set up the Bunsen burner and a tripod stand.
  • Place the wire gauze over the evaporating china dish.
  • Warm the solution and stir it frequently.
  • Continue heating the solution until the crystallisation point is reached.
  • Take the stirring rod out of the china dish and blow some air on it to check for the crystallisation point.
  • The formation of a crystalline crust on the glass rod after blowing air indicates that the crystallisation point has been reached.
  • Remove the dish from the heat.
  • Place it over cold water after covering it with a watch glass.
  • Wait until alum crystals form in the solution.
  • After the crystals have formed, filter them from the mother liquor.
  • Wash them with 5 - 6 ml of ice cold distilled water.
  • Allow the washed crystals to dry by pressing them between filter papers.
  • Your alum is prepared.
  • Now, weigh the finished product.
  • Record your findings.

The procedure is based on the chemical equation shown below:


Equation 1: Potash Alum Yield Calculation


Equation 2: Percentage Yield Calculation

  • We know the experimental yield because we weighed the finished product at the end of the procedure.
  • To calculate theoretical yield, we must first determine how many moles of potash alum were prepared during the experiment.
  • Because potassium sulphate and aluminium sulphate combine in an equimolar ratio, moles of potash alum are equal to moles of reactants.
  • We now know that the number of moles is equal to the ratio of actual grams to the compound's gram molecular weight.


Equation 3: Number of Moles

As a result, we need to know the gram molecular weight of three compounds in this experiment.

We can calculate the number of moles using equation 3 as shown below:

  • We started with slightly more aluminium sulphate.
  • But because potassium sulphate will be the limiting reagent, we should get 0.029 moles of alum from this experiment.
  • We can convert the number of moles into grams using the formula below.

Potash alum weight = number of moles x gram molecular weight = 0.029 x 948.76 = 27.51 grams

We can now calculate the percentage yield of this experiment using the experimental yield to theoretical yield ratio.

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