(2) Adding proper amount of NaOH and Na2CO3 solution into the filtrate, and filtering to remove Mg(OH)2, CaCO3 and excessive BaCO3. Mg2+ + 2OH- =Mg(OH)2？ Ca2+ + CO32-=CaCO3？ Ba2+ + CO32- =BaCO3？
(3) using hydrochloric acid to neutralize excess NaOH and Na2Co3. Check with pH test paper to make the solution close to neutrality. H+ +OH-=H2O
2H++CO2 = CO2? + H2O
(4) evaporating, concentrating and structuring the solution, filtering out NaCl and separating the mother liquor containing K+.
B:
First, the experimental purpose
According to the knowledge learned, on the basis of summarizing the experiments of alkali metals and alkaline earth metals, the crude salt was purified according to the scheme designed by myself.
Second, the experimental principle
1. Raw salt contains insoluble impurities such as mud and sand and soluble impurities such as calcium, magnesium, potassium and sulfate ions.
2. When considering removing impurities such as calcium, magnesium, sulfate ion, etc., we should first consult the solubility data of its insoluble salts, and choose the precipitant to remove the above ions on the principle that no new impurities are introduced or the introduced impurities can be removed in the next operation.
Third, the experimental content
1. Design the experimental scheme (including the detailed steps and specific conditions of each operation) and start the experiment with the consent of the instructor.
2. Weigh 20g of raw salt, dissolve it with as little water as possible and purify it.
Fourth, product inspection.
1. Determination of sodium chloride content
Weigh about 0.2 g of dry sample to 0.0002 g, dissolve it in 70 ml of water in conical flask, add 3-5 drops of 0.5% fluorescent red alcohol solution and 10 ml non-oxidized 0.5% starch solution, then use 0. 1 mol? Titrate Dm-3 AgNO3 standard solution until the color of turbid liquid suddenly changes (pink).
The percentage (x) of NaCl content is calculated according to the following formula:
x = M？ V×0.05845× 100/ g = 5.845 mv/g.
In the formula, the dosage of V-AgNO3 standard solution (cm3), the substance concentration of M-AgNO3 standard solution, g- grams of sample, 0.05845g per millimole of NaCl, and the NaCl content should be not less than 99.8%.
2. Inspection of raw salt and product purity
Take a small amount of crude salt and refined sodium chloride, dissolve them in a small amount of distilled water, and check and compare their purity by the following methods.
(1) sulfate ion inspection.
Add a few drops of 0.2 mol solution into two test tubes (two test solutions) containing crude salt and pure NaCl solution. Dm-3 BaCl2 solution, observe whether there is precipitation of BaSO4.
(2) Calcium ion test
Add a few drops of saturated sodium oxalate solution to the two test solutions to observe whether calcium oxalate precipitates.
(3) Inspection of magnesium ions
Drop 6 mol into the two test solutions respectively. Dm-3 NaOH solution makes it alkaline. Add a few drops of magnesium reagent solution. When the solution is blue, it means that magnesium ions exist.