Preparation of Potash Alum

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What is Potash Alum?

Potash alum is a double metal sulphate of potassium and aluminium metals. It is an inorganic sulphate salt and is also known as potassium aluminium sulphate or aluminium potassium sulphate. It is a commonly known substance since ages. In India, it is also known as phitkari. 


What is the Formula of Potash Alum?

The chemical formula of potash alum is KAl(SO4)2 ,but it is also written as AlK(SO4)2. The aluminum, potassium and sulphate ions are present in the ratio 1:1:2 in potash alum. The chemical structure of potash alum is as follow:

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Although the chemical formula of potash alum is KAl(SO4)2, but it is usually produced in its hydrate form where it has 12 water molecules attached to it. It is also known as dodecahydrate form of potash alum. In this form, the chemical formula is AlK(SO4)2 * 12H2O. Another variation of the formula is in its double sulphate form, where the formula is written as K2SO4.Al2(SO4)3.24H2O.


Uses of Potash Alum

Potash alum is used commonly as an astringent and antiseptic, especially after shaving. Its use is pretty common in India, especially in small cities, towns and villages. Historically, it has been used to purify water as it settles the dissolved sediments. It is also used in the fabric printing and dyeing industry.  In chemical laboratories, it is used as a catalyst.


Preparation of Potash Alum

Historically, potash alum has been obtained naturally from alum-K, which is a naturally occurring sulphate mineral. But because of increase in usage and production, it is now produced at an industrial scale as well. In this section, we will discuss the same procedure in a laboratory setting.


Material Required

This preparation requires following materials:

  • Potassium sulphate

  • Aluminum sulphate

  • Sulphuric acid

  • Distilled water


Apparatus Required

  • Bunsen burner

  • Evaporating dish

  • Beakers

  • Funnel glass and stand

  • Tripod stand

  • Measuring cylinder

  • Stirring rod

  • Wire gauze

  • Porcelain plate

  • Watch glass

  • Filter paper


Procedure for Preparation of Potash Alum

To start with, we first need to take 5g of potassium sulphate.  Then, powder it after weighing it properly. After powdering, transfer the potassium sulphate powder to a beaker of about 150 ml capacity. Then, add about 20ml of water to the beaker and stir continuously. If the solution doesn’t become clear and dissolved properly, you can slightly warm the solution. Keep the solution aside.


In a second beaker of similar capacity, add 20g of powdered aluminum sulphate. In this beaker, add about 30ml of distilled water. Also add about 2ml of concentrated sulphuric acid. Stir the solution continuously with a glass rod to make a clear solution. You can also warm the solution a bit and also add a little bit of extra sulphuric acid if aluminum sulphate doesn’t dissolve properly in water.  We need a clear solution in this beaker as well.


Now, take a china dish and pour the two prepared solutions into it after passing them through a filter paper. It helps to separate the undissolved components of the solution. 


Now, set-up the Bunsen burner and place the wire gauze on a tripod stand. Place the evaporating china dish on the wire gauze. Heat the solution and stir it regularly. Keep heating the solution until it reaches the crystallization point.  To check for the crystallization point, take the stirring rod out of the china dish and blow some air on it. Formation of crystalline crust on the glass rod after blowing air is a sign of reaching crystallization point.  


Now, take the dish off from heat and place it over cold water after covering with watch glass. Wait till the alum crystals are formed in the solution. After crystals are formed, filter them from the mother liquor and wash the crystals with approximately 5 - 6 ml of ice cold distilled water. Press the washed crystals between filter papers to let them dry. 


Your alum is ready. Now, weigh the end product, and record the observations. 


The procedure follows the chemical equation as below:

K2SO4 + Al2(SO4)3.18H2O + 6H2O 🡪 K2SO4.Al2(SO4)3.24H2

Equation 1: Preparation of Potash Alum


Yield Calculation

The yield of the experiment is calculated as a ratio between experimental yield and theoretical yield.

Percent yield = \[\frac{\text{experimental yield}}{\text{theoritical yield}}\times 100\] %

Equation 2: Calculation of Percentage Yield


We know the experimental yield from the last step of procedure where we weighed the end product. To calculate theoretical yield, we first need to compute the moles of potash alum prepared during the experiment. Moles of potash alum are equal to the moles of reactants, i.e. potassium sulphate and aluminum sulphate as they combine in an equimolar ratio.  Now, we know that number of moles are equal to the ratio of actual grams to the gram molecular weight of the compound.

Number of moles = \[\frac{\text{Weight of compound in grams}}{\text{Gram molecular weight of compound}}\]

Equation 3: Number of Moles

So, we need to know the gram molecular weight of 3 compounds present in this experiment.


Compound 

Chemical Formula 

Gram Molecular Weight 

Potash Alum 

K2SO4.Al2(SO4)3.24H2O

948.76g

Aluminium sulphate 

Al2(SO4)3.18H2O

666.42g

Potassium sulphate 

K2SO4

174.25g


Using equation 3, we can calculate the number of moles as below.

Number of moles of aluminium sulphate = \[\frac{20}{666.42}\] = 0.030

Number of moles of potassium sulphate = \[\frac{5}{174.25}\] = 0.029

We started with slightly higher moles of aluminum sulphate, but since potassium sulphate will be the limiting reagent, we will theoretically get 0.029 moles of potash alum from this experiment. We can convert the number of moles into grams as follows.


Number of grams of potash alum = number of moles x gram molecular weight = 0.029 x 948.76 = 27.51 grams.


Now, using the ratio of experimental yield to theoretical yield, we can calculate the percentage yield of this experiment.


Viva Voice 

Why is sulphuric acid added in preparation of potash alum?

Diluted sulfuric acid is added in the preparation of potash alum to prevent hydrolysis of the salt (aluminium sulphate). It helps to dissolve aluminium sulphate in warm water. 


Whether potash alum is acidic or basic?

Aqueous solution of potash alum turns blue litmus paper into red, so it is acidic in nature. 


What happens when potash alum is heated?

When potash alum is heated moderately then it gets dissolved in its water of crystallization but if heating is continued for a longer time period then the water of crystallization evaporates and potash alum salt froths and swells. If potash alum is heated very strongly then at last an amorphous powder remains which contains alumina and sulfate of potash as sulfuric acid also gets removed. 


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