Objective of Salt Preparation: To produce a pure, dry sample of a salt.
Definition of a Salt
A salt is an ionic compound that can be produced from the reaction between an acid and a base.
Solubility Rules
One cannot overemphasize how necessary solubility rules are to understanding salt preparation. These rules allow you to quickly determine whether a substance is soluble or not, without the need to conduct an experiment. The most commonly used solubility rules are listed below:
1. All group I salts are soluble.
2. All ammonium salts are soluble.
3. All nitrates are soluble.
4. All oxides are insoluble except for group I oxides.
5. All sulfates are soluble except for barium sulfate, lead (II) sulfate, and calcium sulfate which is slightly soluble.
6. All carbonates are insoluble except for group I and ammonium carbonates.
7. All phosphates are insoluble except for group I and ammonium phosphates.
8. All halide salts are soluble except for silver halides, lead (II) halides, and mercury halides.
Salt Preparation Procedures
There are three methods to produce salts. A brief outline of each method is set out below:
Outline of the Titration Method
Extract a fixed volume of acid using a pipette and place it in a conical flask.
Place a significant volume of an alkali in a burette. Record the initial volume of alkali in the burette.
Add a suitable indicator (such as methyl orange) to the acid in the conical flask.
Add the alkali in the burette drop-wise into the acid in the conical flask. Swirl the mixture in the conical flask after each drop to ensure that the reaction goes to completion.
Stop adding the alkali immediately once the indicator changes colour.
Record the volume of alkali remaining in the burette. Calculate the volume of alkali added by subtracting the remaining volume of alkali from the initial volume of alkali.
Repeat steps 1 to 4, except step 3. Stop adding the alkali immediately once the volume of alkali added reaches the volume calculated in step 5.
Carry out evaporation till dryness or crystallization on the salt solution in the conical flask to obtain crystals of the salt.
(Note: It is also possible to have the alkali in the conical flask and the acid in the burette as well. In such a case suitable indicators such as phenolphthalein would become appropriate while methyl orange would become inappropriate.)
Outline of the Excess Method
Add a fixed volume of acid into a beaker. Take note of the concentration of the acid.
Using mole concepts and stoichiometry, calculate the minimum mass of insoluble reagent to be added to the acid such that all the acid is used up.
Add this mass of insoluble reagent to the acid. Then, add three spatulas worth of the same insoluble reagent to the acid so as to ensure that the insoluble reagent is in excess. Stir the mixture to ensure that the reaction takes place to its full extent.
Carry out filtration on the mixture. The unreacted insoluble reagent would be collected as the residue, while the salt solution would be collected as the filtrate.
Carry out evaporation till dryness or crystallization on the salt solution to obtain crystals of the salt.
Outline of the Precipitation Method
After selecting two soluble salt solutions as reagents, pour them into a beaker. Stir the mixture to ensure that the reaction takes place to its full extent.
A precipitate should have formed. Carry out filtration on the mixture to obtain the precipitate as the residue.
Wash the residue with distilled water and dry it between two pieces of filter paper.
Choosing between the separation techniques of Evaporation till Dryness and Crystallization
Both evaporation to dryness and crystallization separate a solute from its solvent without retaining the solvent.
However, evaporation till dryness can only be used for heat stable salts. If used on non-heat stable salts, the salt will decompose due to the high heat involved in this separation technique.
A salt is not heat stable when it contains a polyatomic ion. The most common examples of polyatomic ions in salts are: ammonium, nitrate, sulfate, and phosphate.
Crystallization can be used without a problem for both heat stable and non-heat stable salts.
Choosing the Appropriate Salt Preparation Method
A salt preparation method is only appropriate when it can meet the objective stated above, which is to produce a pure, dry sample of the salt. An appropriate salt preparation method for one salt cannot be assumed to be appropriate for another. Likewise, an inappropriate salt preparation method for one salt could very well be appropriate for another.
In determining the appropriate salt preparation method, the flowchart below is a useful guide:
If a salt is insoluble in water, the appropriate salt preparation method is the precipitation method. This is because any excess reagents contaminating the salt can be easily separated from the insoluble salt using filtration and washing with distilled water, allowing a pure sample of the salt to be obtained.
If a salt is soluble in water, the choice of salt preparation method becomes tricky. You would have to accurately choose between the titration method and the excess method. First, deduce the cation that the salt contains. Second, come up with bases that contain that particular cation. Next, use solubility rules to determine if that base is soluble. If soluble, the titration method is appropriate. If insoluble, the excess method is appropriate.
1st Example: Suppose the salt to be prepared is Copper (ll) Sulfate. It is soluble in water. It contains the Cu2+ cation. Bases which contain the Cu2+ cation are Copper (ll) Oxide and Copper (ll) Hydroxide. Both of these bases are insoluble in water. Hence, the Excess Method is appropriate for producing Copper (ll) Sulfate.
2nd Example: Suppose the salt to be prepared is Sodium Chloride. It is soluble in water. It contains the Na+ cation. Bases which contain the Na+ cation are Sodium Oxide and Sodium Hydroxide. Both of these bases are soluble in water. Hence, the Titration Method is appropriate for producing Sodium Chloride.
A shortcut to the thinking process above is to consider whether the salt contains a cation of a Group l element or the ammonium cation. If yes, then the Titration Method is appropriate. If no, then the Excess Method is appropriate.
Choosing Suitable Reagents
The table below can be a useful guide in choosing suitable reagents to produce a desired salt. Do note that its accuracy depends on the correct salt preparation method being chosen beforehand.
For titration, all substances involved are soluble (i.e. both reagents and the salt to be produced), the reaction will always be a neutralization reaction, and the by-product will always be water.
For the excess method, an acid will be added to an insoluble reagent to produce an insoluble salt. There can be up to four suitable insoluble reagents to produce a salt using the excess method. There is some variety in the by-products that can be produced, as the by-product produced depends on the insoluble reagent chosen.
For the precipitation method, there is a choice between mixing two salt solutions together, or mixing a salt solution with an acid. If the former is chosen, the by-product will be another salt solution. If the latter is chosen, the by-product will be another acid.
Comments