Purification of Sodium Chloride

The crude salt contains not only insoluble impurities such as sediment, but also soluble impurities such as K⁺, Ca²⁺, Mg²⁺, and SO₄^2-. Insoluble impurities can be removed by filtration, while Ca²⁺, Mg²⁺, and SO₄^2- in soluble impurities can be removed by adding BaCl₂, NaOH and Na₂CO₃ solutions to form insoluble sulfate, carbonate and basic carbonate precipitates, or by adding BaCO₃ solids and NaOH solutions to remove them as follows:

Experimental Procedure

1. Purification of Crude Sal

(1) Weighing and dissolution of crude salt

Weigh 8g crude salt on the platform scale, put it into the 150mL beaker, add about 30mL water, heat and stir to dissolve it.

(2) Removal of SO₄^2-

In the boiling saline solution, about 2mL of 1mol/L BaCl₂ solution was added dropwise while stirring. The method to check whether the SO₄^2- ion is completely precipitated: remove the alcohol lamp, and after the precipitate sinks, drop 1~2 drops of BaCl₂ solution into the supernatant, and observe whether the solution is turbid. If the upper layer is clear and there is no turbidity, it proves that the SO₄^2- has completely precipitated. If it becomes turbid, continue to add BaCl₂ solution dropwise until the precipitation is complete. Then heat with low fire for 5 minutes to promote the growth of precipitated particles and facilitate filtration. Filter with a common funnel, keep the filtrate in a beaker, and discard the precipitate.

(3) Removal of Ca²⁺, Mg²⁺ and excessive Ba²⁺

Add an appropriate amount (about 1mL) of 2mol/L NaOH solution and 3mL of 1mol/L Na₂CO₃ solution into the filtrate to check whether the Ca²⁺, Mg²⁺ and Ba²⁺ have been completely precipitated. Continue to heat the filtrate with low fire and boil it for 5min. Filter it with an ordinary funnel. Keep the filtrate in the beaker and discard the precipitation.

(4) Adjust the pH value of the solution

Add 2mol/L HCl solution drop by drop into the filtrate, add while stirring, and dip the filtrate with a glass rod on the pH test paper on the watch glass until the solution is slightly acidic (pH=4~5).

(5) Evaporation and concentration

Transfer the solution to an evaporating dish, heat on low fire, evaporate and concentrate until a layer of crystal film appears on the surface of the solution, and then form a gruel, and the solution must not be evaporated to dryness.

(6) Crystallization, vacuum filtration and drying

Cool the concentrate to room temperature. Filter with a Buchner funnel under reduced pressure and drain as much as possible. The crystal is then transferred to an evaporating dish, placed on an asbestos mesh, heated and stirred with a small fire, dried, cooled, weighed, and calculated the yield.

2. Product Quality Inspection

About 1g of crude salt and purified product were respectively dissolved in about 5mL distilled water, then divided into 3 parts and filled in 3 test tubes. Their purity is compared and tested as follows.

(1) Test of SO₄^2-

2 drops of 1mol/L BaCl₂ solution were added to observe the formation of white BaSO₄ precipitation.

(2) Test of Ca²⁺

Add 2 drops of 0.5mol/L (NH₄) ₂C₂O₄ solution and wait a moment to see if white CaC₂O₄ precipitates are formed.

(3) Test of Mg²⁺

Add 2~3 drops of 2mol/L NaOH solution, and then add a few drops of magnesium reagent (p-nitroazo resorcinol). If there is blue precipitation, it indicates the existence of Mg²⁺.